Ataxia-telangiectasia is an autosomal recessive disorder caused by mutations in the ATM gene, presenting in early childhood with progressive cerebellar ataxia, oculocutaneous telangiectasias, immunodeficiency, and increased susceptibility to malignancies due to defective DNA repair.

Ataxia-Telangiectasia (A-T) — Paediatric Notes

Ataxia-telangiectasia (A-T) is an autosomal recessive, multisystem neurodegenerative disorder caused by biallelic pathogenic variants in the ATM (Ataxia-Telangiectasia Mutated) gene on chromosome 11q22.3 ^[1]^[2].

Breaking down the name:

"Ataxia" (Greek: a- = without, taxis = order) → disordered, uncoordinated movement — specifically cerebellar ataxia
"Telangiectasia" (Greek: telos = end, angeion = vessel, ektasis = dilation) → dilated small blood vessels, typically visible on the bulbar conjunctivae and skin

The condition belongs to the family of chromosomal instability / DNA repair defect syndromes and is classified as a combined (T- and B-cell) primary immunodeficiency with prominent neurological, immunological, endocrine, and oncological consequences ^[2]^[3].

Key Concept – ATM as a 'Guardian of the Genome'

ATM is a serine/threonine protein kinase that acts as a master regulator of the DNA damage response (DDR), specifically sensing double-strand breaks (DSBs). Without functional ATM, cells cannot properly:

Detect DSBs
Activate cell-cycle checkpoints (G1/S, intra-S, G2/M)
Recruit repair machinery
Trigger apoptosis of irreparably damaged cells

This single molecular defect explains the entire clinical phenotype: neurodegeneration (neurons are exquisitely sensitive to unrepaired DNA damage), immunodeficiency (lymphocyte development requires programmed DSBs for V(D)J recombination and class-switch recombination), cancer predisposition (genomic instability), and radiosensitivity.

Parameter	Detail
Incidence	~1 in 40,000–100,000 live births worldwide ^[4]
Carrier frequency	~1 in 100 in most populations (heterozygous carriers are generally healthy but have modestly increased cancer risk, especially breast cancer)
Sex predilection	None (autosomal recessive)
Age at onset	Typically 1–4 years (when cerebellar ataxia becomes apparent as the child begins to walk); telangiectasiae appear later, often by age 3–6 years
Life expectancy	Historically median survival ~25 years; with modern supportive care, many patients survive into their 30s–40s. Death most commonly from respiratory infections/bronchiectasis or malignancy ^[4]
Hong Kong context	Rare; exact local incidence data are limited, but cases are seen and managed at tertiary paediatric centres (QMH, PWH). A-T is included in the Hong Kong Primary Immunodeficiency Registry ^[2]

High Yield: A-T is the most common cause of progressive cerebellar ataxia in early childhood and one of the most common autosomal recessive ataxias. In any child with progressive ataxia + oculomotor apraxia + recurrent sinopulmonary infections, A-T must be at the top of the differential ^[1]^[2].

Since A-T is a monogenic autosomal recessive condition, the primary "risk factor" is inheritance of two pathogenic ATM alleles:

Consanguinity — increases the probability that both parents carry the same ATM variant; relevant in certain Hong Kong populations with endogamous marriage patterns
Family history of A-T or unexplained childhood ataxia/immunodeficiency/lymphoid malignancy
Known ATM carrier status in parents (carrier frequency ~1%)

There are no modifiable environmental risk factors for developing A-T itself. However, iatrogenic radiation exposure (diagnostic X-rays, CT scans, radiotherapy) dramatically worsens outcomes in A-T patients due to their extreme radiosensitivity — this is a critical point for clinical management ^[4].

4. Anatomy & Function: The ATM Protein and Relevant Systems

Feature	Detail
Gene	ATM, chromosome 11q22.3
Protein	ATM kinase — 3,056 amino acids, ~370 kDa; a member of the PI3K-related kinase (PIKK) family
Normal function	DNA double-strand break (DSB) sensor and signal transducer

How ATM works (simplified):

DSB occurs → MRN complex (MRE11-RAD50-NBS1) senses the break
→ MRN recruits ATM (which exists as an inactive dimer)
→ ATM undergoes autophosphorylation at Ser1981 → monomerises → becomes active
→ Active ATM phosphorylates downstream targets:
   • p53 → G1/S checkpoint arrest & apoptosis
   • CHK2 → cell-cycle arrest
   • BRCA1 → homologous recombination repair
   • H2AX (→ γH2AX) → chromatin remodelling at break site
   • NBS1 → intra-S checkpoint

4.2 Relevant Anatomy / Systems Affected

Purkinje cells and granular layer neurons are postmitotic and extremely long-lived. They accumulate oxidative DNA damage over time.
Without ATM, these neurons cannot repair DSBs → progressive neuronal loss → cerebellar atrophy
This explains the progressive cerebellar ataxia (the hallmark of the disease)

V(D)J recombination (which generates immunoglobulin and TCR diversity) requires programmed DSBs mediated by RAG1/RAG2 enzymes. ATM is essential for the proper resolution of these breaks.
Class-switch recombination (CSR) in B cells (e.g., IgM → IgA, IgG) also requires DSB repair involving ATM.
Loss of ATM → defective lymphocyte development and function → combined immunodeficiency (variable severity)

ATM deficiency leads to abnormal vascular endothelial homeostasis and premature senescence of endothelial cells, manifesting as telangiectasiae (dilated capillaries/venules)

Meiotic recombination requires DSBs → ATM deficiency → gonadal dysgenesis/hypogonadism, particularly in females

Growth failure is common, partly related to insulin resistance/glucose intolerance (ATM is involved in insulin signalling pathways)

5. Aetiology and Pathophysiology

Inheritance: Autosomal recessive
> 600 different pathogenic variants have been identified across the ATM gene (mostly truncating — nonsense, frameshift, splice-site mutations leading to absent protein)
Genotype–phenotype correlation:
- Classic (severe) A-T: biallelic null/truncating mutations → absent ATM protein → full clinical phenotype
- Mild/variant A-T: at least one missense or leaky splice-site mutation allowing residual kinase activity → later onset, slower progression, may lack telangiectasiae initially ^[4]

5.2 Pathophysiology — System-by-System

Neurons (especially Purkinje cells) are:

Post-mitotic — they cannot be replaced
Metabolically extremely active — high mitochondrial activity → high oxidative stress → frequent DNA damage
Long-lived — damage accumulates over a lifetime

Without ATM, unrepaired DSBs accumulate in Purkinje cells → activation of aberrant cell-cycle re-entry or apoptotic pathways → progressive neuronal death → cerebellar cortical atrophy (visible on MRI from ~age 5–7 years onwards)

Later in the disease, there is also involvement of:

Dorsal columns and spinocerebellar tracts → sensory ataxia, loss of proprioception
Anterior horn cells → peripheral neuropathy (mixed axonal/demyelinating)
Basal ganglia → extrapyramidal features (dystonia, choreoathetosis) which may eventually dominate the clinical picture in older patients ^[1]

Immune Defect	Mechanism	Clinical Consequence
Low IgA (60–80% of patients)	Defective CSR (IgM → IgA requires DSBs resolved by ATM)	Sinopulmonary infections, GI infections
Low IgG subclasses (especially IgG2, IgG4)	Defective CSR	Sinopulmonary infections
Low IgE	Defective CSR	—
Elevated IgM (sometimes)	"Stuck" at IgM stage because CSR is impaired	Mimics hyper-IgM syndrome
T-cell lymphopenia	Defective V(D)J recombination in thymus; thymic hypoplasia	Opportunistic infections, reduced T-cell repertoire
Progressive decline in immunity	Ongoing DNA damage in lymphocytes without repair → premature immune senescence	Worsening infections with age

The immunodeficiency is variable — some children have near-normal immune function early in life, while others present with severe combined immunodeficiency-like features ^[2]^[3].

Genomic instability → accumulation of chromosomal translocations, particularly involving chromosomes 7 and 14 (where the TCR and immunoglobulin heavy chain genes reside)
Common translocations: t(7;14), inv(7)(p13q35), t(14;14)
~25–30% lifetime risk of malignancy ^[4]
In children: predominantly lymphoid malignancies:
- T-cell ALL (most common childhood malignancy in A-T)
- T-cell lymphoma
- B-cell lymphoma / Hodgkin lymphoma
In adults with A-T: broader spectrum including B-cell lymphomas, breast cancer, gastric cancer
Heterozygous carriers (parents/siblings with one ATM mutation): modestly increased risk of breast cancer (~2–3× population risk) and other solid tumours — this is the basis for ATM being listed among cancer susceptibility genes in gynaecological/breast cancer genetic testing panels ^[5]

ATM in Cancer Genetics

Ataxia-telangiectasia – ATM is explicitly listed as a genetic tumour syndrome in the HKU Pathology AOS material on molecular genetic testing in gynaecological cancers ^[5]. ATM carriers have increased breast cancer risk, and ATM is now a standard gene on multi-gene cancer panels.

Ionising radiation causes DSBs as its primary mechanism of cell killing. Normal cells repair these DSBs (partly via ATM-mediated pathways). A-T cells cannot efficiently repair radiation-induced DSBs → severe and sometimes fatal reactions to standard-dose radiation therapy and increased risk from diagnostic imaging.

Clinical implication: If a child with A-T develops lymphoma and receives standard-dose radiotherapy or alkylating chemotherapy, they may develop catastrophic toxicity (severe mucositis, marrow failure, death). Modified protocols with reduced-intensity chemotherapy are essential ^[4].

ATM is involved in insulin signalling (phosphorylation of 4E-BP1 and S6K1 in the mTOR pathway)
Loss of ATM → insulin resistance → glucose intolerance / type 2 diabetes in later childhood/adolescence
Growth failure — multifactorial: chronic illness, poor nutrition from dysphagia, insulin/IGF-1 pathway disruption
Premature ovarian failure in girls due to defective meiotic DSB repair

Respiratory disease is the #1 cause of mortality in A-T, resulting from the "triple hit":

Immunodeficiency → recurrent sinopulmonary infections → bronchiectasis
Bulbar dysfunction / dysphagia (as neurodegeneration progresses) → chronic aspiration
Neuromuscular weakness → ineffective cough → mucus retention

This combination leads to progressive restrictive and obstructive lung disease, eventually respiratory failure.

6. Classification

Type	ATM Protein Level	Age of Onset	Severity
Classic A-T	Absent (null mutations on both alleles)	1–3 years	Severe; wheelchair-bound by ~10 years
Variant / Mild A-T	Residual (≤10–15% of normal; at least one missense/leaky mutation)	5–20+ years	Slower progression; longer survival; later cancer onset

Within the International Union of Immunological Societies (IUIS) Classification of Primary Immunodeficiencies, A-T falls under:

Table 2: Combined immunodeficiencies with associated or syndromic features Sub-category: DNA repair defects (alongside Nijmegen breakage syndrome, Bloom syndrome, etc.) ^[2]^[3]

A-T is a combined (T- and B-cell) primary immunodeficiency with syndromic features ^[2].

Syndrome	Gene	Inheritance	Key Features
Ataxia-telangiectasia	ATM	AR	Ataxia, telangiectasia, immunodeficiency, lymphoid cancers
Nijmegen breakage syndrome	NBS1 (NBN)	AR	Microcephaly, immunodeficiency, lymphoid cancers
Bloom syndrome	BLM	AR	Short stature, photosensitivity, immunodeficiency, cancers
Fanconi anaemia	FANC genes	AR/XL	Bone marrow failure, skeletal anomalies, cancers
Xeroderma pigmentosum	XP genes	AR	UV sensitivity, skin cancers

7. Clinical Features

7.1 Symptoms (What the child/family reports)

Symptom	Typical Age	Pathophysiological Basis
Unsteady gait / frequent falls	1–3 years (as the child begins walking)	Cerebellar Purkinje cell degeneration → truncal and gait ataxia ^[1]
Slurred speech (dysarthria)	2–5 years	Cerebellar vermis/hemisphere atrophy → incoordination of speech muscles
Difficulty with fine motor tasks (e.g., writing, using cutlery)	Progresses through childhood	Cerebellar limb ataxia → intention tremor, past-pointing, dysdiadochokinesia
Drooling / swallowing difficulties (dysphagia)	Later childhood–adolescence	Bulbar dysfunction + oropharyngeal incoordination
Involuntary movements (writhing, twisting)	Late childhood onwards	Basal ganglia/extrapyramidal involvement → choreoathetosis, dystonia
Abnormal eye movements / difficulty reading	Early childhood	Oculomotor apraxia (difficulty initiating voluntary saccades) → the child thrusts the head to shift gaze, then the eyes "catch up" — characteristic compensatory mechanism

High Yield — Oculomotor Apraxia: The child cannot voluntarily move their eyes quickly to a new target. Instead, they use a characteristic head thrust to overcome this — the head overshoots the target, the eyes are dragged along by the vestibulo-ocular reflex, and then the head slowly returns. This is a very characteristic sign of A-T and is testable on exam ^[1].

Symptom	Pathophysiological Basis
Recurrent upper respiratory tract infections (sinusitis, otitis media)	IgA deficiency → poor mucosal immunity
Recurrent lower respiratory tract infections (pneumonia, bronchitis)	IgG subclass deficiency + T-cell lymphopenia → impaired bacterial clearance
Chronic productive cough	Bronchiectasis from recurrent/chronic lower respiratory infections
Recurrent skin infections	Combined immunodeficiency

Recurrent sinopulmonary infections are the most common infectious manifestation and are present in ~60–80% of patients ^[2]^[3].

Symptom	Pathophysiological Basis
Poor weight gain / short stature	Multifactorial: dysphagia, chronic infection, insulin/IGF pathway dysfunction
Delayed or absent puberty (girls more affected)	Premature ovarian failure; impaired meiotic DSB repair → gonadal dysgenesis
Increased thirst/polyuria (uncommon, later)	Insulin resistance → glucose intolerance/diabetes

Symptom	Pathophysiological Basis
Fatigue / decreased exercise tolerance	Combination of chronic infection, malnutrition, neuromuscular weakness
Photosensitivity (mild)	DNA repair defect; some sensitivity to UV-induced damage (less prominent than in xeroderma pigmentosum)

7.2 Signs (What you find on examination)

Sign	Details	Pathophysiological Basis
Cerebellar ataxia	Truncal > limb initially; wide-based gait, positive Romberg (later, due to proprioceptive loss as well)	Purkinje cell loss in cerebellar cortex ^[1]
Oculomotor apraxia	Inability to generate volitional saccades; head thrusting as compensatory mechanism	Loss of saccade-generating neurons in brainstem/cerebellum
Dysarthria	Slurred, scanning, explosive speech	Cerebellar speech area involvement
Intention tremor	Worsens as the hand approaches the target (finger-nose test)	Cerebellar hemisphere dysfunction
Dysdiadochokinesia	Inability to perform rapid alternating movements	Cerebellar dysfunction
Dystonia and choreoathetosis	Involuntary writhing, twisting movements; may eventually dominate over ataxia	Basal ganglia degeneration (later feature)
Hypotonia (early) → later peripheral neuropathy	Reduced reflexes, distal weakness, sensory loss (vibration, proprioception)	Anterior horn cell loss + dorsal column/spinocerebellar tract degeneration
Loss of deep tendon reflexes	Areflexia, especially ankle jerks	Peripheral neuropathy
Positive Babinski sign (variable, late)	Upper motor neuron involvement in some patients	Corticospinal tract degeneration

Mixed Neurological Picture

A-T produces a unique neurological combination of:

Cerebellar signs (ataxia, dysarthria, nystagmus)
Extrapyramidal signs (dystonia, choreoathetosis)
Lower motor neuron signs (areflexia, wasting from peripheral neuropathy)

This combination in a child should immediately raise suspicion for A-T ^[1].

Sign	Details	Pathophysiological Basis
Oculocutaneous telangiectasiae	Dilated venules on bulbar conjunctivae (most prominent), ears, nose, malar area, antecubital/popliteal fossae, dorsum of hands	Endothelial cell premature senescence and vascular instability due to ATM deficiency
Progeria / premature ageing of skin	Thinning of skin, grey hair (adolescence)	Accelerated cellular senescence from unrepaired DNA damage
Café-au-lait spots	Flat, hyperpigmented macules (variable)	DNA repair defect (shared feature with other instability syndromes)
Cutaneous granulomas (uncommon)	Non-caseating granulomas on skin	Immune dysregulation / chronic inflammation

Important: Telangiectasiae are NOT present at birth and typically appear between ages 3–6 years, sometimes later. The neurological features (ataxia) precede the telangiectasiae, so early A-T may present as "ataxia without telangiectasia" — do not dismiss the diagnosis just because you don't see telangiectasiae in a toddler ^[4].

Sign	Details	Pathophysiological Basis
Short stature / low BMI	Below 3rd centile for age (common)	Chronic disease, dysphagia, endocrine dysfunction
Absent/small thymus (imaging)	Thymic hypoplasia	Defective T-cell development (V(D)J recombination requires ATM)
Hepatosplenomegaly (uncommon unless malignancy)	Lymphoid malignancy infiltration	Genomic instability → lymphoproliferation
Wheelchair dependence	By ~age 10–12 years in classic A-T	Progressive neurodegeneration
Drooling, pooling of secretions	Oropharyngeal dysphagia	Bulbar dysfunction
Masked facies (later)	Hypomimia from extrapyramidal involvement	Basal ganglia degeneration

While the full diagnostic algorithm will follow in the next response, the following lab features are so characteristic that they are often considered clinical features of A-T:

Finding	Explanation
Elevated serum AFP (alpha-fetoprotein)	Present in > 95% of classic A-T patients. AFP is normally produced by the foetal liver and yolk sac and declines after birth. In A-T, hepatocytes have impaired regulation of AFP gene expression (mechanism not fully understood but relates to defective chromatin remodelling at the AFP locus). This is a very useful screening test ^[4]
Elevated serum CEA (variable)	Similar mechanism to AFP elevation
Low IgA, low IgG subclasses, low IgE	Defective class-switch recombination
Lymphopenia (especially CD4+ T cells)	Defective V(D)J recombination → reduced thymic output
Chromosomal instability on karyotype (7;14 translocations)	Failure to repair DSBs at TCR/Ig loci
Increased radiosensitivity on colony survival assay	Functional test; gold standard historically

AFP as a Screening Test

Elevated AFP in a child with progressive ataxia is almost pathognomonic for A-T. No other common cause of childhood ataxia causes persistently elevated AFP. If you see unexplained elevated AFP + ataxia → think A-T until proven otherwise. (Note: AFP should be interpreted with age-adjusted reference ranges, as normal neonatal AFP is very high and declines over the first year of life.)

A-T is classified as a combined immunodeficiency with syndromic features ^[2]^[3]. In the context of the GC 144 lecture (A child with recurrent infections: Primary immunodeficiencies) ^[3]:

A-T exemplifies a combined (T + B cell) immunodeficiency where the immunodeficiency is variable and progressive
It should be distinguished from:
- SCID (severe combined immunodeficiency) — presents in infancy with severe opportunistic infections; A-T immunodeficiency is usually milder
- Common variable immunodeficiency (CVID) — presents later; no neurological features ^[2]
- Hyper-IgM syndromes — A-T can mimic hyper-IgM due to defective CSR ^[2]
The recurrent sinopulmonary infections in A-T are the same pattern seen in antibody deficiency states (similar to CVID, XLA)

GC 144 Link: A-T in the PID Spectrum

In a child with recurrent infections, the presence of neurological signs (ataxia, oculomotor apraxia) or telangiectasiae should immediately point you toward A-T as the underlying primary immunodeficiency. Always perform a neurological examination in any child being worked up for PID — you might pick up A-T that was initially misdiagnosed as "clumsy child syndrome" or developmental coordination disorder ^[3].

In the context of GC 091 (Unsteady gait: cerebellar lesions; movement disorders; Parkinsonism) ^[1]:

A-T is a key paediatric cause of progressive cerebellar ataxia
The clinical examination shows a mixed picture:
- Cerebellar signs: gait ataxia, limb ataxia, dysarthria, nystagmus, dysdiadochokinesia, intention tremor
- Extrapyramidal/movement disorder signs: dystonia, choreoathetosis, myoclonus (later in disease)
- Peripheral neuropathy signs: areflexia, distal wasting, sensory loss
The combination of cerebellar ataxia + oculomotor apraxia + telangiectasiae is essentially diagnostic ^[1]

System	Key Features	Pathophysiological Basis
Neurological	Progressive cerebellar ataxia, oculomotor apraxia, dysarthria, dystonia, choreoathetosis, peripheral neuropathy	Purkinje cell loss, basal ganglia degeneration, anterior horn cell loss
Cutaneous/Vascular	Oculocutaneous telangiectasiae, progeric skin changes, café-au-lait spots	Endothelial senescence, DNA repair defect
Immunological	Recurrent sinopulmonary infections, bronchiectasis	Low IgA, IgG subclass deficiency, T-cell lymphopenia
Oncological	T-ALL, T-cell lymphoma, Hodgkin lymphoma (children); solid tumours (adults)	Chromosomal instability, translocations at 7/14
Pulmonary	Chronic lung disease, bronchiectasis, restrictive disease	Infections + aspiration + weak cough
Endocrine	Growth failure, insulin resistance, gonadal failure	ATM role in insulin signalling and meiotic DSB repair
Radiosensitivity	Severe toxicity from ionising radiation	Cannot repair radiation-induced DSBs

High Yield Summary

Ataxia-Telangiectasia (A-T) — Key Points:

Autosomal recessive; ATM gene (11q22.3) — encodes a DSB-sensing kinase critical for DNA damage response
Clinical triad: Progressive cerebellar ataxia (onset age 1–3) + oculocutaneous telangiectasiae (onset age 3–6) + recurrent sinopulmonary infections (combined immunodeficiency)
Oculomotor apraxia with head thrusting is a hallmark neurological sign
Immunodeficiency: Low IgA (most common), low IgG subclasses, low IgE, T-cell lymphopenia; progressive with age
Elevated serum AFP is present in > 95% — a key screening test; almost pathognomonic when combined with ataxia
Cancer predisposition: ~25–30% lifetime risk; predominantly lymphoid malignancies in childhood (T-ALL, T-cell lymphoma)
Extreme radiosensitivity — avoid unnecessary ionising radiation; modified chemotherapy protocols needed for malignancy
ATM heterozygous carriers have increased breast cancer risk (~2–3×) — relevant for cancer genetic counselling
Leading cause of death: Respiratory disease (bronchiectasis + aspiration + neuromuscular weakness) and malignancy
Classified as a combined immunodeficiency with syndromic features and a DNA repair / chromosomal instability syndrome
No cure — management is supportive (IVIG, prophylactic antibiotics, physiotherapy, modified cancer treatment)

Active Recall - Ataxia-Telangiectasia (Definition, Epidemiology, Pathophysiology, Clinical Features)

^[1] Lecture slides: GC 091. Unsteady gait cerebellar lesions; movement disorders; Parkinsonism.pdf ^[2] Senior notes: Jerry's immunodeficiencies.pdf ^[3] Lecture slides: GC 144. A child with recurrent infections Primary immunodeficiencies.pdf ^[4] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf; Adrian Lui Pediatrics Notes.pdf; AOS - Paeds.pdf ^[5] AOS material: AOS - Pathology.pdf (Molecular genetic testing in gynaecological cancers section)

Ataxia-Telangiectasia — Differential Diagnosis

When a child presents with the features suggestive of A-T — namely progressive cerebellar ataxia, oculocutaneous telangiectasiae, recurrent sinopulmonary infections, and/or elevated AFP — you need a systematic framework to differentiate it from other conditions that can mimic one or more of these features.

The key clinical anchors to build your differential around are:

Progressive cerebellar ataxia in childhood
Telangiectasiae / vascular skin lesions in a child
Combined immunodeficiency with syndromic features
Elevated AFP in a non-hepatic/non-oncological context
Cancer predisposition (especially lymphoid malignancies) in childhood

Let's work through each of these axes systematically.

1. Differential Diagnosis of Progressive Cerebellar Ataxia in Childhood

This is the most important clinical axis because ataxia is typically the first and most prominent symptom that brings the child to medical attention ^[1]^[6].

Clinical Reasoning Principle

When approaching a child with progressive ataxia, always ask: Is it acute, episodic, or chronic progressive? A-T causes chronic progressive ataxia. This immediately narrows the differential away from acute post-infectious cerebellitis, drug toxicity, or acute demyelination.

Condition	Gene / Inheritance	Key Distinguishing Features from A-T
Friedreich ataxia (FA)	FH/FXN gene, Chr 9q13; AR (GAA trinucleotide repeat expansion)	Most common inherited ataxia overall. Onset ~5–15 years. Pes cavus, hammer toes, kyphoscoliosis (long-standing peripheral neuropathy). Hypertrophic cardiomyopathy (leading cause of death — heart failure). Diabetes. Absent reflexes. NO telangiectasiae, NO immunodeficiency, NO elevated AFP, NO oculomotor apraxia ^[6]
Ataxia with oculomotor apraxia type 1 (AOA1)	APRATAXIN (APTX); AR	Very similar to A-T: progressive ataxia + oculomotor apraxia. BUT: NO telangiectasiae, normal AFP, NO immunodeficiency, NO cancer predisposition. Low serum albumin and high cholesterol are clues
Ataxia with oculomotor apraxia type 2 (AOA2)	SENATAXIN (SETX); AR	Ataxia + oculomotor apraxia + elevated AFP (like A-T!). BUT: NO telangiectasiae, NO immunodeficiency, later onset (~10–22 years), slower progression
Nijmegen breakage syndrome (NBS)	NBS1 (NBN); AR	DNA repair defect (same pathway as A-T). Microcephaly (hallmark — distinguishes from A-T), immunodeficiency, lymphoid malignancies, radiosensitivity. Mild/no ataxia. NO telangiectasiae ^[3]
Spinocerebellar ataxias (SCAs)	Various genes; most AD	Usually later onset (adolescence/adulthood). AD inheritance (affected parent). Variable features depending on SCA type. NO immunodeficiency, NO telangiectasiae
Wilson disease	ATP7B; AR	Can cause cerebellar ataxia, dysarthria, dystonia, tremor, choreoathetosis — overlapping extrapyramidal features with A-T. Kayser-Fleischer rings on slit-lamp (not telangiectasiae!). Liver disease is a key differentiator. Low ceruloplasmin, high 24-hour urinary copper ^[7]
Abetalipoproteinaemia (Bassen-Kornzweig)	MTTP; AR	Ataxia + peripheral neuropathy + retinitis pigmentosa + acanthocytosis. Fat malabsorption/steatorrhoea. NO telangiectasiae, NO immunodeficiency
Refsum disease	PHYH or PEX7; AR	Ataxia + peripheral neuropathy + retinitis pigmentosa + anosmia + ichthyosis. Elevated phytanic acid. NO immunodeficiency

Condition	Key Distinguishing Features
Posterior fossa tumour (medulloblastoma, ependymoma, astrocytoma)	Acute/subacute onset ataxia, often with raised ICP signs (headache, vomiting, papilloedema). NOT progressive over years. MRI diagnostic ^[8]
Acute post-infectious cerebellitis	Acute onset after viral illness (varicella, EBV). Self-limiting over weeks. NOT progressive
Drug/toxin-induced (e.g., phenytoin, carbamazepine)	History of drug exposure. Reversible on cessation
Multiple sclerosis (demyelinating disease)	Relapsing-remitting pattern; rare in young children. MRI shows white matter lesions. NO telangiectasiae ^[8]
Neuroblastoma-associated opsoclonus-myoclonus-ataxia	Acute onset; opsoclonus ("dancing eyes"); associated with occult neuroblastoma. Very different eye movement pattern from A-T oculomotor apraxia

Key Differentiator: Oculomotor Apraxia + Elevated AFP

Only three conditions commonly cause the combination of ataxia + oculomotor apraxia + elevated AFP:

Ataxia-telangiectasia (ATM mutation)
AOA2 (SETX mutation)
A-T–like disorder / ATLD (MRE11 mutation — extremely rare)

Of these, only A-T has telangiectasiae and immunodeficiency. So telangiectasiae and immune status are your definitive clinical differentiators ^[1].

When you see dilated blood vessels in a child, consider:

Condition	Key Distinguishing Features
Hereditary haemorrhagic telangiectasia (HHT / Osler-Weber-Rendu)	AD inheritance. Telangiectasiae on lips, tongue, fingertips, nasal mucosa. Recurrent epistaxis (hallmark). Pulmonary AVMs, hepatic AVMs, cerebral AVMs. NO ataxia, NO immunodeficiency. Genes: ENG, ACVRL1 ^[9]
Bloom syndrome	AR; DNA repair defect. Telangiectasia on sun-exposed areas (facial butterfly distribution). Short stature, immunodeficiency, cancer predisposition. BUT: NO progressive ataxia ^[3]
Rothmund-Thomson syndrome	AR (RECQL4). Poikiloderma (telangiectasia + atrophy + pigmentary changes). Short stature, skeletal anomalies, cancer risk. NO ataxia
CREST / systemic sclerosis	Rare in children. Telangiectasiae on face/hands. Calcinosis, Raynaud's, sclerodactyly. NO ataxia
Normal physiological conjunctival vessels	NOT pathological telangiectasiae — important not to over-diagnose in anxious parents

A-T is classified under "CID with associated or syndromic features" in the IUIS classification ^[3]. The differential here includes other syndromic combined immunodeficiencies:

Condition	Gene / Inheritance	Distinguishing Features from A-T
Wiskott-Aldrich syndrome (WAS)	WAS; X-linked	Tetrad: eczema, micro-thrombocytopenia (small platelets), immunodeficiency, bloody diarrhoea. NO ataxia, NO telangiectasiae. Low IgM, high IgA/IgE ^[3]^[5]
Hyper-IgE syndrome (HIES)	STAT3 (AD, LOF); DOCK8 (AR)	Markedly elevated IgE, eosinophilia, recurrent cold staphylococcal abscesses, pneumatoceles on CXR. Coarse facies, scoliosis, retained primary teeth. NO ataxia, NO telangiectasiae ^[3]
DiGeorge syndrome (22q11.2 deletion)	TBX1 region; AD/de novo	T-cell deficiency (thymic hypoplasia), conotruncal cardiac defects, hypocalcaemia (absent parathyroids), characteristic facies. NO ataxia (unless associated cerebellar anomaly), NO telangiectasiae
SCID (various forms)	Multiple genes (e.g., IL2RG, RAG1/2, ADA)	Presents in infancy with severe opportunistic infections (PJP, disseminated BCG, candida). Much more severe immunodeficiency than A-T. NO ataxia at presentation ^[3]
Nijmegen breakage syndrome	NBS1; AR	Immunodeficiency + cancer predisposition + radiosensitivity (like A-T). BUT: microcephaly is the hallmark. Minimal or no ataxia ^[3]
ICF syndrome	DNMT3B; AR	Immunodeficiency, centromeric instability, facial anomalies. NO ataxia

GC 144 High Yield: Syndromic PIDs with Eczema

From the Dermatology lecture slides, when eczema is a prominent feature in a child with immunodeficiency, the differential includes: ataxia telangiectasia, Wiskott-Aldrich syndrome, hyper-IgE syndrome, DiGeorge syndrome, and SCID ^[4]^[5]. A-T patients can have eczematous skin changes (immune dysregulation), but the eczema is generally not the dominant feature — ataxia and infections are.

Persistently elevated AFP beyond infancy (after the normal neonatal decline) has a limited differential:

Cause	Context
Ataxia-telangiectasia	Progressive ataxia + immunodeficiency → AFP is a screening marker
AOA2	Progressive ataxia + oculomotor apraxia, no immunodeficiency
Hepatoblastoma	Most common liver tumour in children < 3 years. Abdominal mass. AFP very high (usually > 1000 ng/mL). Imaging diagnostic
Hepatocellular carcinoma	Rare in children unless underlying liver disease (tyrosinaemia, HBV)
Yolk sac tumour (endodermal sinus tumour)	Gonadal or extragonadal germ cell tumour. AFP elevated as tumour marker
Hereditary tyrosinaemia type 1	Liver disease + renal tubular dysfunction + porphyria-like crises. AFP markedly elevated. Succinylacetone in urine diagnostic
Normal neonatal AFP	AFP is physiologically very high at birth (up to ~100,000 ng/mL) and declines to adult levels by ~8–12 months. Must use age-adjusted reference ranges

AFP Interpretation Pitfall

Do not diagnose hepatoblastoma based on elevated AFP alone in a child with ataxia. In A-T, AFP is moderately elevated (typically 20–200 ng/mL) and persistently elevated, whereas in hepatoblastoma AFP is usually markedly elevated (often > 1000 ng/mL) and there is an abdominal mass. Always correlate with the clinical context.

When a child presents with lymphoid malignancy AND features suggesting an underlying predisposition:

Syndrome	Gene	Key Features
Ataxia-telangiectasia	ATM	Ataxia, telangiectasiae, immunodeficiency, radiosensitivity
Nijmegen breakage syndrome	NBS1	Microcephaly, immunodeficiency, radiosensitivity
Bloom syndrome	BLM	Short stature, sun-sensitive facial erythema, immunodeficiency
Li-Fraumeni syndrome	TP53	Multiple early-onset cancers (sarcoma, breast, brain, adrenal). NO immunodeficiency ^[10]
Constitutional mismatch repair deficiency	MLH1, MSH2, MSH6, PMS2	Café-au-lait spots, haematological malignancies, colorectal cancer, brain tumours
Fanconi anaemia	FANC genes; AR	Bone marrow failure, short stature, skeletal anomalies (thumb/radial ray), café-au-lait spots, cancer predisposition (AML, squamous cell carcinomas). NO ataxia ^[11]

Feature	A-T	Friedreich Ataxia	AOA2	Wilson Disease	Nijmegen BS	WAS
Ataxia	✓ Progressive	✓ Progressive	✓ Progressive	✓ Variable	Mild/absent	Absent
Telangiectasiae	✓	✗	✗	✗	✗	✗
Oculomotor apraxia	✓	✗	✓	✗	✗	✗
Immunodeficiency	✓	✗	✗	✗	✓	✓
Elevated AFP	✓	✗	✓	✗	✗	✗
Cancer risk	✓ (lymphoid)	✗	✗	✗	✓ (lymphoid)	✓ (lymphoid)
Radiosensitivity	✓	✗	✗	✗	✓	✗
Cardiomyopathy	✗	✓ (HCM)	✗	✗	✗	✗
Microcephaly	✗	✗	✗	✗	✓	✗
Liver disease	✗	✗	✗	✓	✗	✗
Thrombocytopenia	✗	✗	✗	✗	✗	✓ (micro)
Eczema	Mild/variable	✗	✗	✗	✗	✓ (severe)
Pes cavus	✗	✓	✗	✗	✗	✗

Using the VINDICATE mnemonic adapted to the paediatric differential ^[8]:

Category	Examples
Vascular	Cerebellar stroke (rare in children unless cardiac/haematological), AVM
Infection	Post-infectious cerebellitis, brain abscess, HIV encephalopathy
Neoplasm	Posterior fossa tumours (medulloblastoma, ependymoma, astrocytoma) ^[8]
Degenerative / DNA repair	A-T, Friedreich ataxia, AOA1/2, spinocerebellar ataxias
Inflammatory / Immune	MS, ADEM, opsoclonus-myoclonus, autoimmune cerebellitis
Congenital	Cerebral palsy (ataxic subtype — non-progressive) ^[12], Joubert syndrome, Dandy-Walker
Autoimmune	Anti-GAD antibody cerebellitis, coeliac disease-associated ataxia
Trauma / Toxin	Drug toxicity (phenytoin, carbamazepine), alcohol, lead
Endocrine / Metabolic	Wilson disease, abetalipoproteinaemia, mitochondrial disease, vitamin E deficiency

Don't Miss: Posterior Fossa Tumour

In any child with new-onset or rapidly progressive ataxia, the number one diagnosis to exclude is a posterior fossa tumour — especially medulloblastoma, which peaks at age 3–8 years. MRI brain with contrast is mandatory. A-T ataxia is insidious over months to years, while tumour-related ataxia typically progresses over days to weeks with signs of raised ICP ^[8].

9. Special Paediatric Considerations in the Differential

Age at Presentation	More Likely Differential	Less Likely
< 1 year	Congenital ataxia (Joubert, Dandy-Walker), metabolic (mitochondrial), ataxic CP	A-T (too early for definitive features), Friedreich ataxia
1–5 years	A-T (classic), posterior fossa tumour, post-infectious cerebellitis, opsoclonus-myoclonus	Wilson disease (rarely presents < 5 years neurologically)
5–15 years	A-T (should be clear by now), Friedreich ataxia (onset ~5–15), Wilson disease, AOA2	Congenital causes (would have been picked up earlier)
Adolescence	Friedreich ataxia, Wilson disease, AOA2, variant/mild A-T, SCAs	Classic A-T (already diagnosed by now)

Ataxic CP is non-progressive (by definition, CP is a static encephalopathy) ^[12]. A key differentiator:

CP: Ataxia present from early infancy, does NOT worsen (though may become more apparent as motor demands increase)
A-T: Ataxia is clearly progressive — the child who previously walked independently becomes wheelchair-bound

If a child initially labelled as "ataxic CP" is getting worse, reconsider the diagnosis — A-T, Friedreich ataxia, and metabolic/degenerative conditions must be re-evaluated ^[12].

Early A-T is frequently misdiagnosed as:

Developmental coordination disorder (DCD / "clumsy child")
Ataxic cerebral palsy
Benign delayed motor development

The clue that this is NOT benign: progression. A truly "clumsy" child with DCD does not get progressively worse. If motor skills are deteriorating, pursue neurological workup ^[1].

Some children with A-T are referred to immunology rather than neurology, because their recurrent infections are more prominent than their ataxia (especially in early stages or variant A-T). In this context, the differential of a child with recurrent sinopulmonary infections and low IgA includes ^[3]^[5]:

Condition	Key Differentiator from A-T
Selective IgA deficiency	Most common PID (~1:500). Isolated low IgA, no other features. NO ataxia
CVID	Low IgG + low IgA ± low IgM. Later onset (usually adolescence/adulthood). NO ataxia ^[3]
X-linked agammaglobulinaemia (XLA)	Boys only. Very low all immunoglobulins. Absent B cells. NO ataxia ^[3]
Hyper-IgM syndrome	Low IgG/IgA with elevated IgM. CD40L mutation (X-linked) most common. NO ataxia ^[3]
Cystic fibrosis	Recurrent pulmonary infections + pancreatic insufficiency + failure to thrive. Sweat test diagnostic. NO ataxia

GC 144 teaching point: In a child with recurrent infections, always perform a full neurological examination. The finding of ataxia, oculomotor apraxia, or telangiectasiae in such a child immediately redirects the diagnosis toward A-T ^[3].

High Yield Summary — Differential Diagnosis of A-T

Progressive cerebellar ataxia in childhood → DDx: Friedreich ataxia (pes cavus, HCM, no immunodeficiency), AOA1/AOA2 (oculomotor apraxia but no telangiectasiae/immunodeficiency), Wilson disease (liver disease, KF rings), posterior fossa tumour (acute/subacute, raised ICP), ataxic CP (non-progressive)
Ataxia + oculomotor apraxia + elevated AFP → Narrows to: A-T, AOA2, ATLD (extremely rare). Only A-T has telangiectasiae AND immunodeficiency.
Syndromic combined immunodeficiency → DDx: WAS (eczema, micro-thrombocytopenia), Hyper-IgE (cold abscesses, pneumatoceles), DiGeorge (cardiac defects, hypocalcaemia), NBS (microcephaly), SCID (severe, infantile onset)
Cancer predisposition with radiosensitivity → DDx: NBS, Bloom syndrome, Fanconi anaemia
Red flag: Any child labelled as "ataxic CP" or "clumsy child" who is getting progressively worse must be re-evaluated for A-T or other degenerative ataxias
Always perform neurological examination in a child being worked up for PID — you may diagnose A-T that was missed

Active Recall - Ataxia-Telangiectasia Differential Diagnosis

^[1] Lecture slides: GC 091. Unsteady gait cerebellar lesions; movement disorders; Parkinsonism.pdf ^[3] Senior notes: Jerry's immunodeficiencies.pdf ^[4] Lecture slides: Dermatology I Basics of Dermatology.pdf (Differential diagnosis of eczema — lists ataxia telangiectasia, Wiskott-Aldrich syndrome) ^[5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Section on eczema differential — genetic/metabolic/immunodeficiency syndromes featuring atopic eczema) ^[6] Senior notes: Maksim Medicine Notes.pdf (Friedreich ataxia and Ataxia-telangiectasia sections) ^[7] Senior notes: Block A - Patients with non-viral chronic liver diseases.pdf (Wilson disease); Ryan Ho GI.pdf (Wilson disease clinical features) ^[8] Lecture slides: CFB_Neuro clinical skills demonstration_01.08.22_file to students.pdf (VINDICATE approach to neurological differential) ^[9] Senior notes: Introduction to Clinical pharmacology (I) (Pharmaco-Genomics, Precision Medicine).pdf (Hereditary haemorrhagic telangiectasia case) ^[10] AOS material: AOS - Pathology.pdf (Cancer predisposition syndromes including ATM) ^[11] Senior notes: Block A - Family history of anaemia_ inherited causes of anaemia; haemolytic anaemia; aplastic anaemia.pdf (Fanconi anaemia) ^[12] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (Cerebral palsy differential — metabolic, neurodegenerative)

Diagnostic Criteria for Ataxia-Telangiectasia

Unlike many conditions that have published consensus diagnostic criteria (e.g., Jones criteria for rheumatic fever, McDonald criteria for MS), ataxia-telangiectasia does not have a single universally adopted set of formal diagnostic criteria. Instead, diagnosis is based on a constellation of clinical features supported by characteristic laboratory findings and confirmed by genetic testing ^[6]^[13].

However, the European Society for Immunodeficiencies (ESID) and expert consensus guidelines provide a structured diagnostic framework that can be conceptualised as follows:

Working Diagnostic Framework for A-T

A child should be suspected of having A-T when ≥ 2 of the following cardinal features are present:

Cardinal Feature	Explanation
Progressive cerebellar ataxia beginning in early childhood (age 1–5 years)	The earliest and most consistent feature; truncal ataxia → gait ataxia → limb ataxia
Oculocutaneous telangiectasiae	Typically bulbar conjunctivae; onset age 3–6 years. May be absent early
Oculomotor apraxia	Inability to initiate voluntary saccades; compensatory head thrusting
Recurrent sinopulmonary infections suggesting immunodeficiency	Reflecting IgA deficiency, IgG subclass deficiency, and/or T-cell lymphopenia
Family history of A-T (affected sibling) or consanguinity	Autosomal recessive — may have affected siblings

Finding	Sensitivity	Specificity
Elevated serum AFP (alpha-fetoprotein) for age	> 95% in classic A-T	High (very few other childhood ataxias cause elevated AFP)
Low IgA	~60–80%	Low (many PIDs have low IgA)
Low IgG subclasses (especially IgG2, IgG4)	~50–70%	Low
T-cell lymphopenia (especially CD4+)	~60–70%	Low
Chromosomal instability — spontaneous breaks/translocations involving chromosomes 7 and 14 on karyotype of peripheral blood lymphocytes	High when present	High
Increased radiosensitivity on colony survival assay (CSA)	~100% in classic A-T	High

Test	What It Confirms
Absent or markedly reduced ATM protein on Western blot / immunoblot of patient lymphoblastoid cell line or fibroblasts	Functional absence of ATM — present in > 90% of classic A-T
Absent ATM kinase activity (functional assay)	ATM cannot phosphorylate downstream targets (e.g., p53, CHK2)
Biallelic pathogenic variants in ATM gene on molecular genetic testing (Sanger sequencing or next-generation sequencing / NGS)	Gold standard definitive diagnosis

Key Diagnostic Principle

In practice, the diagnosis of A-T is made when a child has progressive cerebellar ataxia + elevated serum AFP + supportive immunological abnormalities, confirmed by ATM protein analysis and/or ATM gene sequencing. You do NOT need to wait for telangiectasiae to appear — they may be absent in young children and in variant A-T. Elevated AFP in a child with ataxia is almost pathognomonic and should trigger definitive testing immediately ^[6]^[13].

The European Society for Immunodeficiencies (ESID) Registry uses the following tiered approach for A-T registration, which serves as a practical diagnostic framework ^[13]:

Definitive diagnosis requires:

Progressive cerebellar ataxia AND
At least ONE of the following:
- Oculocutaneous telangiectasiae
- Elevated serum AFP (age-adjusted)
- Chromosomal instability (7;14 translocations)
- Increased cellular radiosensitivity
PLUS confirmatory molecular or protein evidence:
- Biallelic pathogenic ATM variants on genetic testing, OR
- Absent/markedly reduced ATM protein on immunoblot, OR
- Absent ATM kinase activity

Probable diagnosis (when genetic/protein testing is unavailable):

Progressive cerebellar ataxia AND
At least THREE of: telangiectasiae, elevated AFP, low IgA, chromosomal instability, increased radiosensitivity AND
Exclusion of alternative diagnoses (Friedreich ataxia, AOA1/2, etc.)

Step-by-Step Clinical Approach

Investigation Modalities — Detailed Interpretation

A. First-Line Screening Investigations

Parameter	Details
What it measures	AFP is an oncofetal protein normally produced by the foetal liver and yolk sac
Normal range	Age-dependent: neonatal AFP can be > 100,000 ng/mL at birth; it gradually declines to adult levels (< 10 ng/mL) by ~8–12 months of age
Finding in A-T	Persistently elevated — typically 20–200 ng/mL in classic A-T, but can be higher
Sensitivity	> 95% in classic A-T (the single most sensitive screening test)
Why it is elevated	Incompletely understood. ATM is involved in chromatin remodelling and transcriptional regulation. Without ATM, hepatocytes fail to properly silence the AFP gene after foetal life → persistent AFP production. Also reflects ongoing hepatocyte DNA damage and regeneration
Interpretation pitfalls	(1) Must use age-adjusted reference ranges — do not compare to adult normals in infants. (2) Also elevated in hepatoblastoma, yolk sac tumours, hereditary tyrosinaemia — context matters. (3) AFP may be normal in variant/mild A-T with residual ATM function (rare)

Clinical Pearl — AFP in A-T

AFP is the best single screening test for A-T. It is cheap, widely available, and has > 95% sensitivity. In any child with unexplained progressive ataxia, order an AFP. If it comes back elevated (and the child is > 12 months old), A-T is the most likely diagnosis until proven otherwise. This test alone can save years of diagnostic delay ^[6]^[13].

Immunoglobulin	Typical Finding in A-T	Frequency	Pathophysiological Basis
IgA	Low or absent	60–80%	Defective class-switch recombination (CSR) — IgM → IgA requires ATM-dependent DSB resolution
IgG subclasses (especially IgG2, IgG4)	Low	50–70%	Defective CSR
IgM	Normal or elevated	Variable	"Stuck" at IgM stage because CSR is impaired — can mimic hyper-IgM phenotype
IgE	Low or absent	~80%	Defective CSR (IgM → IgE)
Total IgG	Normal or low	Variable	IgG1 may compensate for low IgG2/IgG4

Interpretation: The combination of low IgA + low IgE + low IgG2 in a child with ataxia is highly suggestive of A-T. However, some A-T patients have normal immunoglobulins, especially early in the disease — so normal Ig levels do not exclude A-T ^[3]^[13].

Important paediatric note: Always compare immunoglobulin levels to age-matched reference ranges. Normal values for IgG, IgA, IgM change dramatically with age (e.g., IgA is physiologically very low in infants < 1 year).

Parameter	Finding in A-T	Explanation
Total lymphocyte count	Often low (lymphopenia)	Defective V(D)J recombination → reduced T-cell and B-cell production
CD3+ T cells	Low (especially CD4+)	Thymic hypoplasia + defective TCR recombination
CD4+ T cells	Disproportionately low	Particularly affected by defective thymic selection
CD8+ T cells	Variable	May be relatively preserved
CD19+ B cells	Low-normal	Less consistently affected than T cells
CD16/56+ NK cells	Usually normal	NK cells do not undergo V(D)J recombination
Naive T cells (CD45RA+)	Low	Reduced thymic output of new T cells
Haemoglobin, platelets	Usually normal	Unless malignancy develops

Why this matters: The pattern of T-cell lymphopenia with relatively preserved NK cells, low naive T cells, and low IgA is the immunological "fingerprint" of A-T. It helps distinguish from SCID (where T cells are severely depleted or absent and the child is critically ill) ^[3].

B. Confirmatory Investigations

Parameter	Details
Method	Western blot of lymphoblastoid cell lines (LCLs) or peripheral blood mononuclear cells (PBMCs) using anti-ATM antibodies; also possible by flow cytometry (intracellular ATM staining — faster, requires less sample)
Finding in classic A-T	Absent ATM protein (both alleles produce no functional protein)
Finding in variant A-T	Reduced but detectable ATM protein (residual expression from leaky/missense mutations)
Advantage	Faster and cheaper than full gene sequencing; can be performed in specialised immunology labs
Limitation	Does not identify the specific mutation; some missense mutations produce a protein of normal size but with no kinase activity → may appear falsely "normal"

Parameter	Details
Method	Sanger sequencing of all 66 exons of the ATM gene, OR targeted next-generation sequencing (NGS) gene panel for ataxia/PID genes, OR whole-exome sequencing (WES)
What it detects	Biallelic pathogenic variants: nonsense, frameshift, splice-site, missense, large deletions
Gold standard	This is the definitive diagnostic test — identifies the exact mutations on both alleles
Clinical utility	(1) Confirms diagnosis. (2) Enables carrier testing for family members (parents, siblings). (3) Enables prenatal diagnosis in future pregnancies. (4) Genotype-phenotype correlation: truncating mutations → classic severe A-T; missense/leaky splice → variant/mild A-T
Turnaround time	Weeks to months depending on lab; NGS panels have shortened this considerably
Hong Kong context	Available at tertiary genetics laboratories (QMH, PWH); can also be sent to reference laboratories

Genetic Counselling — Essential

Genetic testing in A-T must always be accompanied by genetic counselling for the family. Key points to discuss:

Autosomal recessive inheritance → 25% recurrence risk for each subsequent pregnancy if both parents are carriers
Carrier detection for siblings (2/3 chance of being a carrier if unaffected)
ATM heterozygous carriers have increased breast cancer risk (~2–3×) — female relatives should be offered cancer genetics referral ^[10]
Prenatal/preimplantation genetic testing is possible once family mutations are identified

Parameter	Details
Method	Measure ATM-dependent phosphorylation of downstream substrates (e.g., p53 at Ser15, CHK2 at Thr68, or H2AX at Ser139 → γH2AX) after irradiation of patient's cells
Finding in A-T	Absent or severely reduced phosphorylation of substrates after radiation — cells cannot mount a proper DNA damage response
When used	When immunoblot shows present ATM protein but clinical suspicion remains high (rare missense mutations producing inactive protein)
Availability	Research/specialised labs only

C. Supportive / Functional Investigations

Parameter	Details
Method	Phytohaemagglutinin (PHA)-stimulated peripheral blood lymphocyte culture → G-banded karyotype analysis; specifically looking for spontaneous chromosomal breaks and translocations
Characteristic findings in A-T	Increased chromosomal breakage rate and characteristic translocations involving chromosomes 7 and 14: t(7;14)(p13;q11), inv(7)(p13q35), t(14;14)(q11;q32)
Why chromosomes 7 and 14?	These chromosomes harbour the T-cell receptor (TCR) genes (chr 7: TCRβ and TCRγ; chr 14: TCRα/δ) and the immunoglobulin heavy chain (IgH) gene (chr 14q32). V(D)J recombination at these loci creates programmed DSBs, which require ATM for proper resolution. Without ATM → illegitimate translocations
Sensitivity	~50–70% (not all A-T patients show translocations on a single sample)
Specificity	High for DNA repair defect syndromes (A-T, NBS, Bloom)
Clinical utility	(1) Supportive evidence for A-T diagnosis. (2) Cancer surveillance — clonal expansion of cells with these translocations may herald lymphoid malignancy

Parameter	Details
Method	Patient's fibroblasts or lymphoblastoid cells are irradiated with graded doses of ionising radiation, then cultured. Colony-forming ability is compared to normal controls
Finding in A-T	Markedly increased radiosensitivity — A-T cells show dramatically reduced survival compared to controls at the same radiation dose (typically 3–4× more sensitive)
Historical gold standard	This was the classic functional diagnostic test before molecular genetics became widely available
Limitations	Time-consuming (weeks), requires specialised lab, not widely available in routine clinical settings
Current role	Used when genetic testing is inconclusive or to confirm functional significance of novel ATM variants of uncertain significance (VUS)

Parameter	Details
Method	Irradiate patient cells → assess the G2/M checkpoint by flow cytometry (measure proportion of cells entering mitosis after radiation)
Finding in A-T	Radioresistant DNA synthesis (RDS) — A-T cells fail to arrest at the G2/M checkpoint after radiation → continue to enter mitosis with unrepaired DNA damage → increased mitotic index after radiation
Significance	Demonstrates failure of ATM-dependent cell-cycle checkpoint activation

D. Baseline Assessment Investigations (Once Diagnosis is Confirmed)

These investigations are performed not for diagnosis but to stage the disease and guide ongoing management:

Finding	Timing	Significance
Normal in early disease (< 5 years)	Early	Ataxia precedes visible atrophy
Cerebellar atrophy — particularly vermis and hemispheres	Progressive, usually visible by age 5–7+ years	Purkinje cell and granular neuron loss
Thinning of cerebellar folia	Later	Advanced neurodegeneration
No white matter lesions (unlike leukodystrophies)	Throughout	Helps exclude demyelinating conditions
No mass lesions	Throughout	Excludes posterior fossa tumour

MRI brain is useful for excluding other causes of ataxia (especially posterior fossa tumours) and for documenting baseline cerebellar volume for monitoring progression.

Parameter	Expected Finding	Significance
FVC (Forced Vital Capacity)	Reduced (restrictive pattern)	Respiratory muscle weakness + chronic lung disease
FEV1	Reduced proportionally	Mixed restrictive/obstructive in bronchiectasis
DLCO	May be reduced	Parenchymal lung damage

Baseline PFTs should be obtained as soon as the child can cooperate (usually ~age 5–6 years)
Serial PFTs every 6–12 months monitor respiratory decline
Spirometry may be difficult in A-T patients due to poor coordination and weakness → may need to use adapted techniques

Finding	Significance
Bronchiectasis (lower lobes predominantly)	Chronic/recurrent infection
Ground-glass opacities	Active infection or lymphoid interstitial pneumonitis
Air trapping	Small airways disease

Obtained at baseline and when there is clinical suspicion of deterioration
Minimise radiation exposure — use low-dose protocols given the extreme radiosensitivity of A-T patients ^[13]

Radiation Caution

Every radiological investigation involving ionising radiation must be carefully justified in A-T patients. Use the lowest possible dose, prefer MRI and ultrasound over CT where clinically feasible, and avoid unnecessary repeat imaging. Diagnostic CT and X-rays are NOT absolutely contraindicated (the risk is from therapeutic-dose radiation, not individual diagnostic studies), but the principle of ALARA (As Low As Reasonably Achievable) must be strictly applied. NEVER give standard-dose radiotherapy to an A-T patient without a modified protocol ^[13].

Assessment	Purpose
Clinical bedside swallowing assessment	Identify overt aspiration risk
Videofluoroscopic swallowing study (VFSS)	Gold standard for aspiration assessment; demonstrates pharyngeal incoordination
Fibreoptic endoscopic evaluation of swallowing (FEES)	Alternative to VFSS; no radiation exposure

Aspiration is a major contributor to progressive lung disease in A-T
Should be assessed at baseline and serially as neurological function declines

Test	Rationale
Fasting glucose, HbA1c, oral glucose tolerance test	Screen for insulin resistance / diabetes (ATM involved in insulin signalling)
Growth parameters plotted on growth chart	Monitor for growth failure
IGF-1, IGFBP-3	Assess growth hormone axis if growth failure is disproportionate
FSH, LH, oestradiol/testosterone (at pubertal age)	Assess gonadal function — premature ovarian failure in girls
Thyroid function (TSH, fT4)	Exclude concurrent thyroid dysfunction

Investigation	Purpose
CBC with differential (6–12 monthly)	Monitor for cytopenias suggesting leukaemia
Peripheral blood smear	Look for blasts, atypical lymphocytes
LDH	Non-specific tumour marker; rising LDH may suggest lymphoproliferative disease
Physical examination	Lymphadenopathy, hepatosplenomegaly
Low threshold for further workup (CT, bone marrow biopsy) if clinical concern	~25–30% lifetime malignancy risk

There is no standardised cancer screening protocol for A-T, but regular clinical surveillance with a low threshold for investigation is recommended.

Test	Purpose	Method
Targeted ATM sequencing of family members	Identify heterozygous carriers (parents, siblings)	Sequence for the known familial mutations
Prenatal diagnosis	For at-risk pregnancies (25% recurrence risk)	Chorionic villus sampling (CVS) at 10–12 weeks or amniocentesis at 15–18 weeks → test foetal DNA for known familial ATM mutations
Preimplantation genetic testing (PGT-M)	Avoid affected pregnancy via IVF	Test embryos for ATM mutations before transfer
Newborn screening	NOT currently included in routine newborn screening panels. However, SCID newborn screening (via TREC) may incidentally detect A-T — low TREC levels in A-T neonates may trigger further workup ^[3]

TREC-Based Newborn Screening May Incidentally Detect A-T

T-cell receptor excision circles (TRECs) are used for SCID newborn screening. TRECs are by-products of V(D)J recombination in the thymus. In A-T, defective V(D)J recombination → low TRECs at birth. Some A-T babies are flagged by SCID newborn screening programmes and then found to have A-T rather than SCID on further workup. This represents an opportunity for early diagnosis before clinical symptoms develop ^[3]^[13].

Tier	Investigation	Key Finding	Purpose
Screening	Serum AFP	Elevated for age (> 95% sensitivity)	Best single screening test
Screening	Immunoglobulins (IgA, IgG, IgM, IgE, IgG subclasses)	Low IgA, low IgG2/4, low IgE	Supports immunodeficiency
Screening	CBC + lymphocyte subsets	T-cell lymphopenia (especially CD4+)	Supports combined immunodeficiency
Confirmatory	ATM gene sequencing	Biallelic pathogenic variants	Gold standard definitive diagnosis
Confirmatory	ATM protein immunoblot	Absent or reduced ATM protein	Confirms functional deficiency
Supportive	Chromosomal breakage analysis	7;14 translocations, increased breaks	Supports DNA repair defect
Supportive	Colony survival assay / radiosensitivity testing	Increased radiosensitivity	Historical gold standard functional test
Baseline staging	MRI brain	Cerebellar atrophy	Excludes other causes; documents baseline
Baseline staging	PFTs + HRCT chest (low-dose)	Restrictive defect; bronchiectasis	Documents respiratory baseline
Baseline staging	Swallowing assessment	Aspiration risk	Guides feeding management
Baseline staging	Endocrine workup	Insulin resistance, growth failure, gonadal dysfunction	Guides endocrine management
Family	Targeted ATM testing of relatives	Carrier detection	Genetic counselling, prenatal diagnosis

High Yield Summary — Diagnosis of A-T

Elevated serum AFP (age-adjusted) is the single best screening test — > 95% sensitivity in classic A-T; cheap and widely available
Immunoglobulin panel showing low IgA + low IgE + low IgG2 supports the diagnosis; T-cell lymphopenia (CD4+ predominantly) on lymphocyte subsets
ATM gene sequencing identifying biallelic pathogenic variants is the gold standard definitive diagnostic test
ATM protein immunoblot showing absent/reduced ATM protein is a faster confirmatory method
Chromosomal breakage analysis showing 7;14 translocations and colony survival assay showing increased radiosensitivity are supportive functional tests
MRI brain shows progressive cerebellar atrophy (may be normal early); essential to exclude posterior fossa tumour
TREC-based newborn screening for SCID may incidentally identify A-T neonates (low TRECs from defective V(D)J recombination)
Minimise ionising radiation exposure in all investigations — use ALARA principle strictly
Genetic counselling is mandatory — enable carrier testing of family, prenatal diagnosis for future pregnancies, and breast cancer risk counselling for female ATM carriers
There are no formal published "diagnostic criteria" per se — diagnosis rests on clinical constellation + AFP + immunological profile + molecular/protein confirmation

Active Recall - Diagnostic Criteria, Algorithm, and Investigations for A-T

^[3] Senior notes: Jerry's immunodeficiencies.pdf ^[6] Senior notes: Maksim Medicine Notes.pdf (Ataxia-telangiectasia section) ^[10] AOS material: AOS - Pathology.pdf (Molecular genetic testing — ATM as cancer susceptibility gene) ^[13] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf; AOS - Paeds.pdf (A-T diagnostic approach and immunodeficiency classification)

Management of Ataxia-Telangiectasia

There is no cure for A-T. Management is supportive, multidisciplinary, and lifelong ^[14]. The goals are:

Prevent and treat infections → the leading cause of morbidity and mortality
Preserve and optimise neurological function → slow functional decline, maintain independence as long as possible
Prevent pulmonary deterioration → address the "triple hit" of infection + aspiration + weak cough
Surveillance for and modified treatment of malignancy → radiosensitivity means standard protocols are lethal
Monitor and manage endocrine complications → growth, puberty, insulin resistance
Support the child and family → psychosocial, educational, genetic counselling

No Disease-Modifying Therapy — Yet

As of 2026, there is no approved disease-modifying therapy for A-T. Several experimental approaches are in preclinical or early clinical trials (gene therapy, antisense oligonucleotides, antioxidants, small-molecule ATM activators), but none has reached standard clinical use. Management remains entirely supportive. This makes the multidisciplinary team absolutely essential ^[14].

Treatment Modalities — Detailed

1. Immunological Management

Parameter	Details
Indication	Significant antibody deficiency manifesting as: (1) Low IgG (or IgG subclasses) with poor specific antibody responses to vaccines, AND (2) Recurrent or severe sinopulmonary infections despite prophylactic antibiotics ^[3]^[14]
Not all A-T patients need Ig replacement	If immunoglobulins are normal or only mildly low AND infections are well-controlled, Ig replacement is not routinely started
Formulations

IVIG (Intravenous Immunoglobulin):

Dose: 400–600 mg/kg every 3–4 weeks (paediatric dosing same as for other PIDs)
Administered in day-care unit or infusion centre
Advantages: established, reliable trough levels
Disadvantages: requires IV access each time, infusion reactions possible, time-consuming

SCIG (Subcutaneous Immunoglobulin):

Dose: equivalent weekly dose calculated from IVIG dose (typically 100–150 mg/kg/week)
Self-administered at home by trained parents/caregivers using a subcutaneous infusion pump
Advantages: better suited for paediatric/home use, avoids IV access, steadier serum IgG levels (no peak-trough fluctuation), fewer systemic side effects
Disadvantages: local site reactions (redness, swelling), requires training

SCIG — Preferred in Paediatric PID

For children with A-T requiring long-term immunoglobulin replacement, SCIG is often preferred because it can be given at home, is well-tolerated, maintains more stable IgG levels, and avoids repeated hospital visits for IV infusions. This is particularly important for A-T children who have progressive mobility limitations ^[3]^[14].

Contraindications for Ig replacement	Details
IgA deficiency with anti-IgA antibodies	Rare but important — anaphylaxis risk. Use IgA-depleted preparations if needed. Most A-T patients with low IgA do NOT have anti-IgA antibodies, but it should be checked before first infusion
Selective IgA deficiency without significant infections or IgG deficiency	Ig replacement not indicated if only IgA is low and infections are mild

Monitoring on Ig replacement:

Trough IgG levels (pre-infusion) — target trough > 5–8 g/L (age-adjusted)
Clinical response — reduction in infection frequency, severity, antibiotic use
Liver function, renal function periodically

Parameter	Details
Indication	Recurrent sinopulmonary infections, especially if immunoglobulin replacement alone is insufficient, or while awaiting Ig replacement
Common regimen	Co-trimoxazole (trimethoprim-sulfamethoxazole / Septrin) — 5 mg/kg/day TMP component, given once daily or on alternate days
Rationale	Broad-spectrum prophylaxis against common respiratory pathogens (Strep. pneumoniae, H. influenzae, Moraxella); also provides PJP prophylaxis (relevant given T-cell lymphopenia)
Alternative	Azithromycin 10 mg/kg once weekly or 3 days per week — also has anti-inflammatory properties for bronchiectasis (similar to use in cystic fibrosis)
Side effects	Co-trimoxazole: bone marrow suppression (monitor CBC), rash, Stevens-Johnson syndrome (rare). Azithromycin: GI upset, QTc prolongation, microbial resistance

Recommendation	Rationale
All inactivated vaccines should be given as per schedule	Safe and important; responses may be suboptimal but still beneficial
LIVE vaccines are generally CONTRAINDICATED	A-T patients have combined immunodeficiency (T-cell + B-cell). Live vaccines (BCG, MMR, varicella, rotavirus, live influenza) carry a risk of vaccine-derived infection in immunocompromised hosts ^[3]
Exception — case-by-case	Some patients with milder immunodeficiency (variant A-T with near-normal T-cells) may receive live vaccines after careful immunological assessment by a specialist. This is a paediatric immunologist decision
Annual inactivated influenza vaccine	Recommended for all A-T patients
Pneumococcal vaccines (PCV13 + PPSV23)	Important given high risk of sinopulmonary infections; check specific antibody responses post-vaccination to guide Ig replacement decisions
COVID-19 vaccination	Inactivated/mRNA vaccines are safe and recommended

Live Vaccine Caution

Never give live vaccines to a child with A-T without consulting a paediatric immunologist. BCG, for example, can cause disseminated BCGosis in severely T-cell-deficient patients. Since many A-T children are diagnosed after infancy, they may have already received BCG and MMR — ask about any adverse reactions to prior vaccines ^[3].

2. Respiratory Management

Pulmonary disease is the leading cause of death in A-T (the "triple hit" of infection + aspiration + weak cough) ^[14]. Proactive management is critical.

Technique	Details
Chest physiotherapy	Postural drainage, percussion, vibration — daily, intensified during infections
Positive expiratory pressure (PEP) devices	Flutter valve, Acapella — help mobilise secretions
Cough-assist devices (mechanical insufflation-exsufflation)	For children with weak cough (neuromuscular component) — simulates a forceful cough to clear secretions
Exercise	As tolerated — even wheelchair-bound children benefit from upper body exercises to maintain respiratory muscle strength

Intervention	When	Details
Dietary modification	When dysphagia is identified on swallowing assessment	Thickened fluids, soft diet, appropriate food textures guided by speech therapist
Postural manoeuvres during feeding	Ongoing	Upright positioning, chin tuck, small boluses
Nasogastric (NG) tube feeding	Acute aspiration crises or temporary support	Short-term bridge; not ideal long-term
Gastrostomy (PEG/surgical)	When oral feeding becomes unsafe or nutritionally inadequate	Often needed by late childhood/adolescence in classic A-T; provides safe enteral nutrition and medication delivery; reduces aspiration risk. Discuss early with family as a proactive measure, not a "last resort"
Fundoplication	If concurrent gastro-oesophageal reflux disease (GORD) contributing to aspiration	Often combined with gastrostomy

Principle	Details
Low threshold for treating respiratory infections	Start antibiotics early and empirically for sinopulmonary infections
Sputum/cough swab culture when possible	Guide antibiotic choice; chronic infections may harbour resistant organisms (Pseudomonas, MRSA)
IV antibiotics for severe pneumonia	Hospital admission for IV antibiotics (e.g., co-amoxiclav, ceftriaxone, meropenem depending on local resistance patterns and culture results)
Long-term azithromycin	Anti-inflammatory and antimicrobial properties for established bronchiectasis

Investigation	Frequency
Pulmonary function tests	Every 6–12 months (when child can cooperate, typically from age ~5–6)
Low-dose HRCT chest	Baseline; repeat every 2–3 years or as clinically indicated; minimise radiation — use ALARA protocols
Swallowing assessment	Annually, or more frequently if clinical concern
Overnight oximetry / sleep study	If respiratory failure suspected (hypoventilation, sleep-disordered breathing)

3. Neurological Management

There is no treatment that can halt or reverse the neurodegeneration in A-T. Management focuses on maintaining function, managing symptoms, and preventing secondary complications ^[14].

Domain	Goals	Interventions
Gross motor	Maintain ambulatory ability as long as possible; optimise wheelchair mobility when needed	Gait training, balance exercises, core strengthening; timely provision of walking aids → wheelchair → powered wheelchair
Fine motor	Maintain hand function for ADLs (eating, writing, using devices)	Hand coordination exercises, adaptive utensils, assistive technology (tablet/computer with eye-tracking or adapted controls)
Posture	Prevent/manage scoliosis, contractures	Seating assessments, orthotic supports, stretching programmes
Respiratory muscles	See respiratory section above	Breathing exercises, incentive spirometry

Timing of wheelchair provision: Most children with classic A-T become wheelchair-dependent by approximately age 10–12 years. The transition should be planned and positive — framed as gaining freedom and energy conservation rather than "giving up walking" ^[14].

As A-T progresses, dystonia and choreoathetosis may become the dominant neurological features and cause significant distress ^[1].

Drug	Mechanism	Indication	Paediatric Dosing Notes	Side Effects
Trihexyphenidyl (Artane)	Anticholinergic — blocks muscarinic receptors in basal ganglia	Dystonia	Start low (1 mg/day), titrate slowly up to 6–15 mg/day in divided doses	Dry mouth, constipation, urinary retention, cognitive impairment, blurred vision
Baclofen	GABA-B receptor agonist — reduces spasticity and some dystonia	Dystonia, spasticity	0.5–1 mg/kg/day in 3 divided doses; can also be given intrathecally for severe cases	Sedation, hypotonia, withdrawal seizures if stopped abruptly
Gabapentin	Modulates calcium channels; mechanism in movement disorders incompletely understood	Tremor, dystonia (adjunctive)	10–15 mg/kg/day in 3 divided doses, titrate up	Sedation, dizziness, behavioural changes
Botulinum toxin (Botox)	Blocks acetylcholine release at neuromuscular junction → focal muscle relaxation	Focal dystonia (e.g., cervical dystonia, limb dystonia) causing pain or functional limitation	Injected into affected muscles by specialist; dose depends on muscle size and severity	Local weakness, pain at injection site, rare systemic spread
Levodopa	Dopamine precursor	Trial for dystonia (some A-T patients have dopamine-responsive elements)	Low-dose trial: 1–3 mg/kg/day of levodopa component	Nausea, dyskinesias — response is generally poor in A-T (unlike DOPA-responsive dystonia)
Clonazepam	Benzodiazepine — enhances GABA-A inhibition	Myoclonus, tremor	0.01–0.05 mg/kg/day, given at night or in divided doses	Sedation, dependence, tolerance

Reality check: Response to these medications is often modest and inconsistent in A-T. The dystonia/choreoathetosis is caused by irreversible neuronal loss, not a neurotransmitter imbalance, so pharmacotherapy is palliative rather than curative. A trial-and-error approach is standard ^[1]^[14].

There is no effective pharmacotherapy for the oculomotor apraxia
Compensatory strategies are taught: the child learns to use head movements efficiently to redirect gaze
Educational accommodations: enlarged print, audio learning materials, reduced requirement for scanning/tracking tasks

4. Cancer Surveillance and Modified Treatment

Measure	Frequency	Rationale
Clinical examination (lymphadenopathy, hepatosplenomegaly, bruising, pallor)	Every 3–6 months	~25–30% lifetime cancer risk; predominantly lymphoid malignancies in childhood
CBC with differential	Every 6–12 months	Screen for cytopenias, circulating blasts
LDH	With CBC	Non-specific marker of lymphoproliferative disease
Low threshold for bone marrow biopsy	If clinical or laboratory concern	Early diagnosis of leukaemia

There are no validated cancer screening imaging protocols for A-T. Surveillance relies on clinical examination and basic bloods. Do NOT perform routine CT/PET scanning for cancer screening — the radiation exposure is harmful in A-T ^[14].

This is one of the most critical aspects of A-T management.

CRITICAL — Radiosensitivity and Cancer Treatment

Standard-dose radiotherapy and standard doses of radiomimetic alkylating agents (e.g., cyclophosphamide, ifosfamide) and topoisomerase II inhibitors (e.g., etoposide) are POTENTIALLY FATAL in A-T patients. These agents cause DNA double-strand breaks as their mechanism of action. A-T cells cannot repair these breaks → catastrophic toxicity (severe mucositis, prolonged pancytopenia, sepsis, organ failure, death) ^[14].

Principle	Details
NO standard-dose radiotherapy	Radiotherapy is contraindicated at standard doses. If absolutely necessary (rare), use dramatically reduced doses with close monitoring
Reduced-intensity chemotherapy	Modified protocols developed by specialist centres (e.g., UK A-T Clinical Centre, Frankfurt A-T Centre). Doses of alkylating agents, topoisomerase inhibitors, and bleomycin are typically reduced by 25–50%
Agents that are better tolerated	Vincristine (not a DSB-inducing agent — works via microtubule inhibition), corticosteroids (dexamethasone/prednisolone), L-asparaginase — these form the backbone of modified ALL protocols
Agents to use with extreme caution	Cyclophosphamide, doxorubicin, etoposide, bleomycin — all require dose reduction
Haematopoietic stem cell transplant (HSCT)	Can be considered for certain haematological malignancies, BUT carries very high risk in A-T due to conditioning regimen toxicity. Reduced-intensity conditioning (RIC) protocols are essential. Outcomes are variable and this decision requires expert centre involvement ^[3]
Targeted therapies / immunotherapy	Increasingly explored: anti-CD20 (rituximab) for B-cell lymphomas, PD-1 inhibitors — may offer efficacy with less DNA-damaging toxicity

Practical approach when malignancy is diagnosed in an A-T child:

Confirm the diagnosis promptly (bone marrow biopsy, imaging, immunophenotyping)
Contact an A-T specialist centre (e.g., UK A-T Clinical Centre at Nottingham, or relevant international expert)
Use a modified chemotherapy protocol with reduced doses of DNA-damaging agents
Avoid radiotherapy unless absolutely life-saving and at reduced dose
Meticulous supportive care — infection prophylaxis, growth factor support, transfusion support

Complication	Management
Growth failure	Nutritional optimisation (often via gastrostomy); growth hormone has been trialled but evidence is limited in A-T — consider referral to paediatric endocrinologist if growth hormone axis is abnormal
Insulin resistance / glucose intolerance / diabetes	Lifestyle measures, metformin if needed (insulin sensitiser; suitable for older children/adolescents), insulin if diabetes develops
Premature ovarian failure	Hormone replacement therapy (oestrogen + progesterone) at pubertal age for girls with delayed/absent puberty — important for bone health and secondary sexual characteristics
Delayed puberty in boys	Less common than in girls; testosterone replacement if indicated
Osteoporosis risk	Multifactorial (immobility, endocrine dysfunction, chronic disease). Ensure adequate calcium + vitamin D supplementation. Weight-bearing activities as tolerated

Issue	Approach
Caloric insufficiency (dysphagia, chronic illness)	Caloric supplementation (energy-dense feeds), dietary review by paediatric dietitian
Dysphagia progression	Graded approach: texture modification → supplemental NG feeds → gastrostomy (see respiratory section)
Micronutrient deficiencies	Regular monitoring (iron, zinc, vitamin D, B12, folate); supplement as needed
Antioxidant supplementation	Theoretically appealing (ATM deficiency → oxidative stress), but no clinical trial evidence supports routine antioxidant supplementation in A-T to date

This is a mandatory component of A-T management.

Topic	Details
Recurrence risk counselling	Both parents are obligate carriers (heterozygous); 25% recurrence risk for each subsequent pregnancy
Carrier testing for siblings	Unaffected siblings have a 2/3 probability of being carriers — offer targeted ATM testing
Cancer risk for ATM carriers	Female heterozygous carriers have ~2–3× increased breast cancer risk. All female carriers (mothers, sisters, aunts) should be informed and offered referral to cancer genetics / breast screening programme ^[10]
Prenatal diagnosis	Available via CVS or amniocentesis if familial mutations are known
Preimplantation genetic testing (PGT-M)	Offered via IVF for families wanting to avoid affected pregnancies
Newborn screening implications	SCID newborn screening (TREC-based) may identify future affected siblings before symptom onset

Domain	Interventions
Schooling	Individualised education plan (IEP); classroom accommodations (wheelchair access, enlarged print, voice-to-text technology, extra time for written work, audiobooks). Cognitive function is generally preserved in A-T — these children are intellectually normal and should be encouraged to participate fully in education
Psychological support	Living with a progressive, life-limiting condition causes significant psychological burden on both the child and family. Regular psychological input, peer support groups (international A-T societies), and age-appropriate discussions about the condition are essential
Transition planning	As A-T patients survive into young adulthood, transition from paediatric to adult services must be planned — involve adult neurology, immunology, respiratory medicine, and primary care
Palliative care	Introduce palliative care early as a supportive and symptom-management resource, not as "end-of-life care". Advanced care planning should be discussed sensitively with the family and the young person (when appropriate) as the disease progresses

Therapy	Status (2026)	Mechanism	Notes
Antisense oligonucleotides (ASOs)	Phase I/II clinical trials	Correct specific ATM splice-site mutations to restore partial ATM expression	Only applicable to patients with specific splice-site mutations (~10–15% of A-T patients); highly personalised
Gene therapy	Preclinical	Deliver functional ATM gene via viral vector to affected cells	Challenges: ATM cDNA is very large (~9.2 kb), pushing the limits of AAV vector capacity; targeting CNS is difficult
Small-molecule read-through agents	Preclinical / early phase	Promote read-through of premature stop codons (for nonsense mutations)	Ataluren (PTC124) explored but limited efficacy in A-T
Antioxidants (NAC, EPI-743, idebenone)	Preclinical / compassionate use	Reduce oxidative damage to neurons	Theoretical rationale strong; clinical evidence lacking
Glucocorticoids (betamethasone)	Observational evidence	May transiently improve ataxia symptoms (mechanism unclear; possibly anti-inflammatory or altered alternative splicing of ATM)	Some families report improvement in neurological function during steroid use; studies ongoing; not routine
Erythropoietin	Early phase	Possible neuroprotective and anti-inflammatory effects	Very preliminary data

Assessment	Frequency	Performed By
Clinical review (neurological, respiratory, growth)	Every 3–6 months	Paediatrician / multidisciplinary clinic
Immunoglobulins + CBC + lymphocyte subsets	Every 6–12 months	Immunologist
Serum AFP	Annually (for monitoring, not diagnostic once established)	Haematologist / oncologist
Pulmonary function tests	Every 6–12 months	Respiratory physician
Swallowing assessment	Annually or as needed	Speech and language therapist
Endocrine workup (glucose, HbA1c, growth, puberty)	Annually	Endocrinologist
MRI brain	Baseline; repeat only if clinical change suggests alternative pathology	Neurologist
Low-dose HRCT chest	Baseline; every 2–3 years or as needed	Respiratory physician
Physiotherapy + OT + educational review	Ongoing	Therapists + school
Psychological support	Ongoing	Psychologist
Genetic counselling	At diagnosis and at key life transitions	Clinical geneticist

High Yield Summary — Management of A-T

No cure exists — management is entirely supportive and multidisciplinary
Immunological: IVIG or SCIG replacement for significant antibody deficiency + recurrent infections; prophylactic co-trimoxazole (also covers PJP); avoid live vaccines unless immunologist-approved ^[3]
Respiratory (leading cause of death): Aggressive airway clearance (chest physio, cough-assist devices), aspiration prevention (dietary modification → gastrostomy when needed), prompt antibiotic treatment of infections, long-term azithromycin for bronchiectasis
Neurological: Physiotherapy to maintain function; adaptive equipment (wheelchair by ~age 10–12 in classic A-T); symptomatic pharmacotherapy for dystonia/choreoathetosis (trihexyphenidyl, baclofen, botulinum toxin) — response is often modest
Cancer: Modified reduced-intensity chemotherapy is essential; standard-dose radiotherapy is CONTRAINDICATED due to extreme radiosensitivity; reduce doses of alkylating agents and topoisomerase inhibitors; contact specialist A-T centre ^[14]
Endocrine: Monitor growth, insulin resistance, pubertal development; gastrostomy for nutrition; hormone replacement for gonadal failure
Genetic counselling: 25% recurrence risk; carrier testing for siblings; female ATM carriers have 2–3× breast cancer risk — offer cancer genetics referral ^[10]
Education: Cognitive function is preserved — provide full educational support with accommodations for motor disability
Palliative care: Introduce early as supportive symptom management; advance care planning in adolescence/young adulthood

Active Recall - Management of Ataxia-Telangiectasia

^[1] Lecture slides: GC 091. Unsteady gait cerebellar lesions; movement disorders; Parkinsonism.pdf ^[3] Senior notes: Jerry's immunodeficiencies.pdf ^[10] AOS material: AOS - Pathology.pdf (Molecular genetic testing — ATM as cancer susceptibility gene) ^[14] Senior notes: Adrian Lui Pediatrics Notes.pdf (Hereditary ataxia — Ataxia telangiectasia section)

Complications of Ataxia-Telangiectasia

A-T is a multisystem progressive disorder, and its complications arise directly from the underlying molecular defect — loss of ATM kinase function and consequent failure of DNA double-strand break repair. Complications can be organised into system-based categories, each logically flowing from the pathophysiology described in earlier sections.

The two leading causes of death in A-T are:

Chronic lung disease / respiratory failure (~one-third of deaths)
Malignancy (~one-third of deaths)

The remaining deaths are attributable to a combination of infections, aspiration, and multi-organ progressive decline ^[14].

1. Pulmonary Complications — The Leading Cause of Morbidity and Mortality

Feature	Details
Pathophysiology	Recurrent sinopulmonary infections (due to antibody deficiency — low IgA, low IgG subclasses) → chronic airway inflammation → irreversible bronchial wall damage → permanent dilatation of bronchi. This is exactly the same pathogenic sequence as in cystic fibrosis or any chronic suppurative lung disease
Pattern	Predominantly lower lobe, bilateral; progresses from mild cylindrical to varicose to cystic bronchiectasis
Clinical features	Chronic productive cough, purulent sputum, recurrent exacerbations with fevers, progressive dyspnoea, halitosis, clubbing (late)
Organisms	Initially: Haemophilus influenzae, Streptococcus pneumoniae, Moraxella catarrhalis. Later: Pseudomonas aeruginosa colonisation (associated with accelerated decline, similar to CF), Staphylococcus aureus
Monitoring	Regular sputum cultures; low-dose HRCT every 2–3 years; serial PFTs
Management	Immunoglobulin replacement, prophylactic antibiotics, airway clearance techniques, prompt treatment of exacerbations (see Management section)

Bronchiectasis Cycle in A-T

The vicious cycle is: Immunodeficiency → infection → inflammation → structural airway damage → impaired mucociliary clearance → further infection → more damage. Superimposed chronic aspiration (from bulbar dysfunction) and ineffective cough (from neuromuscular weakness) fuel this cycle relentlessly. Breaking any part of this cycle — with Ig replacement, antibiotics, chest physiotherapy, and aspiration prevention — is the cornerstone of A-T respiratory management.

Feature	Details
Pathophysiology	Progressive cerebellar and bulbar degeneration → oropharyngeal dysphagia → incoordination of swallowing → silent and overt aspiration of food, saliva, and gastric contents
Why "silent"?	The cough reflex may also be impaired (both central and peripheral components), so aspiration can occur without overt choking — hence "silent aspiration"
Consequence	Recurrent aspiration pneumonia → lipoid pneumonia → accelerated bronchiectasis → progressive parenchymal lung damage
Detection	Videofluoroscopic swallowing study (VFSS) or fibreoptic endoscopic evaluation of swallowing (FEES) — may reveal aspiration even in children who appear to swallow normally
Management	Dietary modification → NG tube → gastrostomy when oral feeding becomes unsafe

Feature	Details
Pathophysiology	Three converging factors: (1) Parenchymal destruction from infection/aspiration (2) Chest wall/scoliosis deformity from neuromuscular weakness and immobility (3) Respiratory muscle weakness (progressive peripheral neuropathy + anterior horn cell involvement)
Pattern on PFTs	Declining FVC (restrictive pattern), mixed restrictive-obstructive in established bronchiectasis
Clinical progression	Nocturnal hypoventilation → daytime hypercapnia → cor pulmonale → death
Management	Non-invasive ventilation (NIV / BiPAP) for nocturnal hypoventilation; aggressive infection management; advance care planning discussions

Feature	Details
Pathophysiology	Immune dysregulation in A-T can cause lymphocytic infiltration of the lung parenchyma → lymphoid interstitial pneumonitis (LIP). This is related to aberrant immune responses rather than infection
Imaging	Ground-glass opacities, reticulonodular pattern on HRCT — must be distinguished from infection
Frequency	Uncommon but recognised; contributes to restrictive physiology
Management	May respond to immunosuppression (steroids) — paradoxical given the immunodeficiency; requires careful balance

2. Malignant Complications — The Other Major Cause of Death

Feature	Details
Lifetime malignancy risk	~25–30% ^[14]
Mechanism	Genomic instability from absent ATM → defective DNA DSB repair → accumulation of somatic mutations and chromosomal translocations (especially at TCR/Ig loci on chromosomes 7 and 14) → clonal expansion → malignancy

Malignancy	Frequency	Details
T-cell acute lymphoblastic leukaemia (T-ALL)	Most common childhood malignancy in A-T	Arises from accumulation of translocations in T-cell precursors; typically presents with lymphadenopathy, mediastinal mass, high WCC, hepatosplenomegaly
T-cell lymphoma	Common	Non-Hodgkin lymphoma; may present as nodal or extranodal disease
Hodgkin lymphoma	Less common than T-cell malignancies	Classical Hodgkin lymphoma presentations; may respond to modified chemotherapy
B-cell lymphoma / diffuse large B-cell lymphoma	Less common in children, increases with age	Reflects ongoing B-cell genomic instability

Malignancy	Details
B-cell lymphomas	More common than T-cell malignancies in adults with A-T
Breast cancer	In female A-T patients (homozygous); also in heterozygous female carriers (~2–3× population risk) ^[10]
Gastric cancer	Increased risk, mechanism unclear but likely related to genomic instability in GI epithelium
Other solid tumours	Brain tumours (rare), hepatocellular carcinoma — described in case series

Treatment Complications of Malignancy in A-T

The treatment of malignancy is itself a major source of complications in A-T:

Standard-dose radiotherapy → catastrophic radiotoxicity (severe mucositis, prolonged pancytopenia, multi-organ failure, death). ATM-deficient cells cannot repair radiation-induced DSBs.
Standard-dose alkylating agents and topoisomerase inhibitors → excessive myelosuppression and organ toxicity, because these drugs also cause DSBs as their mechanism of action.
Even with modified reduced-intensity protocols, treatment-related mortality is higher in A-T than in immunocompetent children with the same malignancy. This is the fundamental therapeutic dilemma in A-T oncology ^[14].

Feature	Details
T-cell clonal expansions	Abnormal T-cell clones with chromosomal translocations (especially involving TCRα/δ on 14q11) can be detected in peripheral blood by flow cytometry or T-cell receptor rearrangement studies. These may represent pre-leukaemic clones
Significance	Presence of clonal populations increases surveillance urgency but does NOT mandate treatment in the absence of frank malignancy
Monitoring	Serial CBC, clinical examination, consideration of periodic TCR clonality studies in specialised centres

3. Neurological Complications

Stage	Approximate Age (Classic A-T)	Features
Early	1–5 years	Unsteady gait, frequent falls; child can still walk independently
Intermediate	5–10 years	Increasing dependence on walking aids; deteriorating fine motor skills; dysarthria worsens; handwriting becomes illegible
Wheelchair dependence	~10–12 years	Unable to walk independently; truncal ataxia too severe for safe ambulation ^[14]
Late	Adolescence–adulthood	Dystonia and choreoathetosis may dominate over ataxia; severe dysarthria; dysphagia; may develop akinetic-rigid phenotype

Feature	Details
Pathophysiology	Progressive axonal degeneration (and some demyelination) of peripheral nerves — dorsal root ganglia neurons and anterior horn cells are affected
Clinical features	Distal sensory loss (vibration > proprioception > pain/temperature), areflexia (especially ankle jerks first, then generalised), distal muscle wasting and weakness
Consequence	Contributes to falls, impaired hand function, and respiratory muscle weakness
EMG/NCS findings	Mixed axonal and demyelinating neuropathy; reduced sensory nerve action potentials, reduced motor conduction velocities

Feature	Details
Dystonia	Sustained abnormal postures; may be focal (cervical dystonia, hand dystonia) or generalised; can be painful
Choreoathetosis	Involuntary writhing, flowing movements; may become more prominent than ataxia in older patients
Myoclonus	Brief, shock-like involuntary movements; can impair function
Pathophysiology	Degeneration of basal ganglia neurons (caudate, putamen, substantia nigra) due to accumulated unrepaired DNA damage — same mechanism as cerebellar degeneration

Feature	Details
Oculomotor apraxia	Inability to initiate voluntary saccades; compensatory head thrusting; interferes with reading, visual tracking, and academic performance
Strabismus	May develop secondary to disrupted eye movement control
Nystagmus	Gaze-evoked nystagmus reflecting cerebellar involvement

Educational impact: The combination of oculomotor apraxia + fine motor impairment + dysarthria creates significant barriers to learning despite preserved cognition. Adaptive technology (speech-to-text, eye-tracking devices, audiobooks) is essential.

4. Immunological Complications

Feature	Details
Pathophysiology	The immunodeficiency in A-T is progressive — ongoing DNA damage in lymphocytes without adequate repair → premature immune senescence → declining T-cell and B-cell function over time
Clinical consequence	Infections that were well-controlled in childhood may become more severe and frequent in adolescence/adulthood
Monitoring	Serial immunoglobulin levels and lymphocyte subsets (every 6–12 months) to guide escalation of immunological support

Feature	Details
Pathophysiology	Immune dysregulation (not just deficiency) — defective tolerance mechanisms → autoreactive lymphocyte escape → autoimmunity
Examples	ITP (immune thrombocytopenia), autoimmune haemolytic anaemia (AIHA), vitiligo, thyroiditis, juvenile idiopathic arthritis-like presentations
Frequency	~10–25% of A-T patients develop autoimmune phenomena
Management challenge	Treating autoimmunity requires immunosuppression, but the patient is already immunodeficient — careful balance required; use targeted therapies where possible

Feature	Details
Cutaneous granulomas	Non-caseating granulomas on skin; can be disfiguring; may be mistaken for sarcoidosis
Systemic granulomatous disease	Can affect liver, lungs, other organs
Pathophysiology	Aberrant inflammatory response — possibly related to defective clearance of intracellular pathogens and immune dysregulation
Management	Topical/systemic steroids, TNF-alpha inhibitors in severe cases (use cautiously given immunodeficiency)

5. Endocrine Complications

Feature	Details
Pathophysiology	ATM is involved in insulin signalling — it phosphorylates components of the insulin/mTOR pathway (4E-BP1, S6K1). Without ATM → impaired insulin signal transduction → peripheral insulin resistance
Progression	Normal glucose → impaired glucose tolerance → frank type 2 diabetes mellitus (typically in adolescence/young adulthood)
Frequency	Up to 30–40% of A-T patients develop glucose intolerance; frank diabetes is less common
Monitoring	Annual fasting glucose, HbA1c; OGTT if borderline results
Management	Lifestyle modification (limited by disability), metformin, insulin if needed

Feature	Details
Pathophysiology	Multifactorial: (1) Chronic illness and caloric deficit (dysphagia, recurrent infections) (2) IGF-1/growth hormone axis disruption (ATM affects mTOR signalling which interacts with GH/IGF-1 pathway) (3) Pubertal delay
Typical pattern	Progressive decline in height and weight centiles; often below 3rd centile by mid-childhood
Monitoring	Plot on growth charts at every visit; assess pubertal staging regularly
Management	Nutritional optimisation (gastrostomy if needed); GH therapy is controversial and not standard

Feature	Details
Pathophysiology	Meiotic recombination requires programmed DSBs → ATM needed for their resolution → without ATM, germ cells undergo apoptosis → gonadal dysgenesis, particularly premature ovarian failure in girls
Clinical features	Delayed puberty (absent breast development, absent menarche in girls; delayed testicular growth in boys); primary or secondary amenorrhoea; infertility
Monitoring	FSH, LH, oestradiol/testosterone at pubertal age; bone age if appropriate
Management	Hormone replacement therapy (oestrogen/progesterone for girls; testosterone for boys) to induce secondary sexual characteristics, maintain bone health, and support psychosocial wellbeing

Feature	Details
Pathophysiology	Multifactorial: (1) Immobility → reduced weight-bearing (2) Gonadal failure → low sex steroids (3) Nutritional deficiency (4) Possible direct effect of ATM deficiency on osteoblast function
Consequence	Increased fracture risk, particularly pathological fractures from minimal trauma
Management	Calcium + vitamin D supplementation; weight-bearing activity as tolerated; hormone replacement; bisphosphonates may be considered in severe cases (limited paediatric data)

6. Dermatological / Vascular Complications

Feature	Details
Pathophysiology	Premature senescence of vascular endothelial cells due to accumulated unrepaired DNA damage → capillary/venular dilatation
Progression	Start on bulbar conjunctivae (age 3–6 years) → spread to ears, nose, malar area, antecubital/popliteal fossae, dorsum of hands
Complications	Primarily cosmetic concern; rarely cause significant bleeding. However, they are a source of psychological distress for the child and family
Management	Mainly reassurance and psychological support; cosmetic camouflage if desired; laser therapy for prominent lesions (limited evidence in A-T)

Feature	Details
Premature ageing	Thin, atrophic skin; grey hair in adolescence; loss of subcutaneous fat
Pathophysiology	Accumulated DNA damage in dermal cells → premature cellular senescence → features resembling premature ageing

Feature	Details
Hepatic steatosis / non-alcoholic fatty liver disease	Increasingly recognised in A-T; related to insulin resistance and metabolic syndrome
Hepatic fibrosis	Can progress to fibrosis; mechanism partly related to insulin resistance, partly to direct effects of ATM deficiency on hepatic stellate cell regulation
Hepatic granulomas	Part of the systemic granulomatous disease that can occur in A-T
Monitoring	Periodic liver function tests; liver ultrasound; transient elastography (FibroScan) if concerns about fibrosis

Complication	Details
Depression and anxiety	Common in adolescents who are cognitively intact but progressively physically disabled; awareness of shortened life expectancy
Social isolation	Physical disability, wheelchair dependence, dysarthria, and cosmetic impact of telangiectasiae can lead to social withdrawal and bullying
Family impact	Enormous burden on caregivers; guilt (autosomal recessive inheritance); sibling impact; financial strain from medical costs and adaptive equipment
Grief	Anticipatory grief in families; bereavement support when needed
Management	Regular psychological review; peer support groups (international A-T societies, e.g., A-T Society UK, A-T Children's Project); family therapy; palliative care team involvement for holistic support

These are complications arising from medical interventions in A-T patients — critically important because they are preventable.

Iatrogenic Risk	Mechanism	Prevention
Radiation injury from standard-dose radiotherapy	ATM-deficient cells cannot repair radiation-induced DSBs → catastrophic cell death, mucositis, pancytopenia, multi-organ failure	NEVER use standard-dose radiotherapy; use modified reduced-dose protocols only ^[14]
Excessive toxicity from standard chemotherapy	Alkylating agents and topoisomerase inhibitors cause DSBs → same repair defect → exaggerated myelosuppression, organ damage	Use reduced-intensity chemotherapy protocols; avoid unnecessary DNA-damaging agents
Cumulative radiation from diagnostic imaging	Each CT scan contributes to radiation burden; A-T cells are more sensitive to even diagnostic-level radiation	Apply ALARA principle strictly; prefer MRI and ultrasound; use low-dose CT protocols when CT is essential
Live vaccine complications	Disseminated infection from live attenuated vaccines (BCG, MMR, varicella) in immunocompromised patients	Avoid live vaccines unless specifically cleared by paediatric immunologist ^[3]
Adverse reactions to immunosuppression for autoimmune complications	Immunosuppressive drugs (steroids, rituximab) in an already immunodeficient patient → increased infection risk	Use targeted therapies, minimise immunosuppression duration, provide antimicrobial prophylaxis

The Most Preventable Catastrophe

The single most important iatrogenic complication to prevent is radiation injury. A child with undiagnosed A-T who develops lymphoma and receives standard-dose radiotherapy may die from treatment toxicity rather than the cancer itself. This is why early diagnosis of A-T is so important — it changes the entire cancer treatment approach. Always consider A-T in any child with lymphoid malignancy and concurrent neurological features or immunodeficiency.

System	Complication	Frequency	Pathophysiological Basis	Leading Cause of Death?
Pulmonary	Bronchiectasis, aspiration pneumonia, restrictive lung disease, respiratory failure	Very common (> 80%)	Immunodeficiency + aspiration + weak cough	Yes (~1/3 of deaths)
Oncological	T-ALL, T-cell lymphoma, Hodgkin lymphoma, B-cell lymphoma	~25–30% lifetime risk	Genomic instability, chromosomal translocations	Yes (~1/3 of deaths)
Neurological	Progressive ataxia, wheelchair dependence, dystonia, peripheral neuropathy	Universal (100%)	Cerebellar/basal ganglia/spinal cord neuronal loss	Indirect (contributes to aspiration/respiratory failure)
Immunological	Progressive immunodeficiency, autoimmune disease, granulomatous disease	Very common	Defective V(D)J/CSR + immune dysregulation	Contributes to infection deaths
Endocrine	Insulin resistance, diabetes, growth failure, gonadal failure, osteoporosis	Common (30–50%)	ATM role in insulin/mTOR signalling + meiotic DSB repair	No (but impairs quality of life)
Dermatological	Progressive telangiectasiae, progeric skin	Universal	Endothelial senescence	No
Hepatic	Steatosis, fibrosis, granulomas	Increasingly recognised	Insulin resistance + ATM deficiency in hepatic cells	Rarely
Psychological	Depression, anxiety, social isolation, family burden	Very common	Chronic progressive disability with preserved cognition	No (but profoundly impacts QoL)
Iatrogenic	Radiation injury, chemotherapy toxicity, live vaccine complications	Preventable	Radiosensitivity, immunodeficiency	Potentially fatal if not prevented

High Yield Summary — Complications of A-T

Two leading causes of death: Chronic lung disease / respiratory failure and malignancy (each accounts for ~1/3 of deaths)
Pulmonary: Bronchiectasis (vicious cycle of infection + aspiration + weak cough), chronic aspiration, restrictive lung disease, respiratory failure
Malignancy: T-ALL (most common childhood malignancy in A-T), T-cell lymphoma, Hodgkin lymphoma; solid tumours in adults. Treatment is complicated by extreme radiosensitivity — standard radiotherapy is contraindicated
Neurological: Progressive disability → wheelchair dependence by ~age 10–12; dystonia/choreoathetosis may dominate later; peripheral neuropathy contributes to respiratory muscle weakness
Immunological: Progressive immunodeficiency worsens with age; autoimmune phenomena (ITP, AIHA) occur in 10–25%
Endocrine: Insulin resistance (ATM involved in insulin signalling), gonadal failure (especially premature ovarian failure), growth failure, osteoporosis
Iatrogenic — PREVENTABLE: Radiation injury from standard-dose RT is potentially fatal — always use modified protocols; minimise diagnostic radiation (ALARA); avoid live vaccines
Psychological: Preserved cognition with progressive physical disability creates enormous psychological burden; regular support essential
Female heterozygous ATM carriers (mothers, sisters) have ~2–3× increased breast cancer risk — a complication for the family, not just the patient ^[10]

Active Recall - Complications of Ataxia-Telangiectasia

References

^[3] Senior notes: Jerry's immunodeficiencies.pdf ^[10] AOS material: AOS - Pathology.pdf (ATM as cancer susceptibility gene; carrier breast cancer risk) ^[14] Senior notes: Adrian Lui Pediatrics Notes.pdf (Hereditary ataxia — Ataxia telangiectasia section)

High Yield Summary

Ataxia-Telangiectasia (A-T) — Key Points:

Autosomal recessive; ATM gene (11q22.3) — encodes a DSB-sensing kinase critical for DNA damage response
Clinical triad: Progressive cerebellar ataxia (onset age 1–3) + oculocutaneous telangiectasiae (onset age 3–6) + recurrent sinopulmonary infections (combined immunodeficiency)
Oculomotor apraxia with head thrusting is a hallmark neurological sign
Immunodeficiency: Low IgA (most common), low IgG subclasses, low IgE, T-cell lymphopenia; progressive with age
Elevated serum AFP is present in > 95% — a key screening test; almost pathognomonic when combined with ataxia
Cancer predisposition: ~25–30% lifetime risk; predominantly lymphoid malignancies in childhood (T-ALL, T-cell lymphoma)
Extreme radiosensitivity — avoid unnecessary ionising radiation; modified chemotherapy protocols needed for malignancy
ATM heterozygous carriers have increased breast cancer risk (~2–3×) — relevant for cancer genetic counselling
Leading cause of death: Respiratory disease (bronchiectasis + aspiration + neuromuscular weakness) and malignancy
Classified as a combined immunodeficiency with syndromic features and a DNA repair / chromosomal instability syndrome
No cure — management is supportive (IVIG, prophylactic antibiotics, physiotherapy, modified cancer treatment)

High Yield Summary — Differential Diagnosis of A-T

Progressive cerebellar ataxia in childhood → DDx: Friedreich ataxia (pes cavus, HCM, no immunodeficiency), AOA1/AOA2 (oculomotor apraxia but no telangiectasiae/immunodeficiency), Wilson disease (liver disease, KF rings), posterior fossa tumour (acute/subacute, raised ICP), ataxic CP (non-progressive)
Ataxia + oculomotor apraxia + elevated AFP → Narrows to: A-T, AOA2, ATLD (extremely rare). Only A-T has telangiectasiae AND immunodeficiency.
Syndromic combined immunodeficiency → DDx: WAS (eczema, micro-thrombocytopenia), Hyper-IgE (cold abscesses, pneumatoceles), DiGeorge (cardiac defects, hypocalcaemia), NBS (microcephaly), SCID (severe, infantile onset)
Cancer predisposition with radiosensitivity → DDx: NBS, Bloom syndrome, Fanconi anaemia
Red flag: Any child labelled as "ataxic CP" or "clumsy child" who is getting progressively worse must be re-evaluated for A-T or other degenerative ataxias
Always perform neurological examination in a child being worked up for PID — you may diagnose A-T that was missed

High Yield Summary — Diagnosis of A-T

Elevated serum AFP (age-adjusted) is the single best screening test — > 95% sensitivity in classic A-T; cheap and widely available
Immunoglobulin panel showing low IgA + low IgE + low IgG2 supports the diagnosis; T-cell lymphopenia (CD4+ predominantly) on lymphocyte subsets
ATM gene sequencing identifying biallelic pathogenic variants is the gold standard definitive diagnostic test
ATM protein immunoblot showing absent/reduced ATM protein is a faster confirmatory method
Chromosomal breakage analysis showing 7;14 translocations and colony survival assay showing increased radiosensitivity are supportive functional tests
MRI brain shows progressive cerebellar atrophy (may be normal early); essential to exclude posterior fossa tumour
TREC-based newborn screening for SCID may incidentally identify A-T neonates (low TRECs from defective V(D)J recombination)
Minimise ionising radiation exposure in all investigations — use ALARA principle strictly
Genetic counselling is mandatory — enable carrier testing of family, prenatal diagnosis for future pregnancies, and breast cancer risk counselling for female ATM carriers
There are no formal published "diagnostic criteria" per se — diagnosis rests on clinical constellation + AFP + immunological profile + molecular/protein confirmation

High Yield Summary — Management of A-T

No cure exists — management is entirely supportive and multidisciplinary
Immunological: IVIG or SCIG replacement for significant antibody deficiency + recurrent infections; prophylactic co-trimoxazole (also covers PJP); avoid live vaccines unless immunologist-approved ^[3]
Respiratory (leading cause of death): Aggressive airway clearance (chest physio, cough-assist devices), aspiration prevention (dietary modification → gastrostomy when needed), prompt antibiotic treatment of infections, long-term azithromycin for bronchiectasis
Neurological: Physiotherapy to maintain function; adaptive equipment (wheelchair by ~age 10–12 in classic A-T); symptomatic pharmacotherapy for dystonia/choreoathetosis (trihexyphenidyl, baclofen, botulinum toxin) — response is often modest
Cancer: Modified reduced-intensity chemotherapy is essential; standard-dose radiotherapy is CONTRAINDICATED due to extreme radiosensitivity; reduce doses of alkylating agents and topoisomerase inhibitors; contact specialist A-T centre ^[14]
Endocrine: Monitor growth, insulin resistance, pubertal development; gastrostomy for nutrition; hormone replacement for gonadal failure
Genetic counselling: 25% recurrence risk; carrier testing for siblings; female ATM carriers have 2–3× breast cancer risk — offer cancer genetics referral ^[10]
Education: Cognitive function is preserved — provide full educational support with accommodations for motor disability
Palliative care: Introduce early as supportive symptom management; advance care planning in adolescence/young adulthood

High Yield Summary — Complications of A-T

Two leading causes of death: Chronic lung disease / respiratory failure and malignancy (each accounts for ~1/3 of deaths)
Pulmonary: Bronchiectasis (vicious cycle of infection + aspiration + weak cough), chronic aspiration, restrictive lung disease, respiratory failure
Malignancy: T-ALL (most common childhood malignancy in A-T), T-cell lymphoma, Hodgkin lymphoma; solid tumours in adults. Treatment is complicated by extreme radiosensitivity — standard radiotherapy is contraindicated
Neurological: Progressive disability → wheelchair dependence by ~age 10–12; dystonia/choreoathetosis may dominate later; peripheral neuropathy contributes to respiratory muscle weakness
Immunological: Progressive immunodeficiency worsens with age; autoimmune phenomena (ITP, AIHA) occur in 10–25%
Endocrine: Insulin resistance (ATM involved in insulin signalling), gonadal failure (especially premature ovarian failure), growth failure, osteoporosis
Iatrogenic — PREVENTABLE: Radiation injury from standard-dose RT is potentially fatal — always use modified protocols; minimise diagnostic radiation (ALARA); avoid live vaccines
Psychological: Preserved cognition with progressive physical disability creates enormous psychological burden; regular support essential
Female heterozygous ATM carriers (mothers, sisters) have ~2–3× increased breast cancer risk — a complication for the family, not just the patient ^[10]

Ataxia-telangiectasia

Ataxia-Telangiectasia (A-T) — Paediatric Notes

1. Definition

2. Epidemiology

3. Risk Factors

4. Anatomy & Function: The ATM Protein and Relevant Systems

4.1 The ATM Gene and Protein

4.2 Relevant Anatomy / Systems Affected

Cerebellum

Immune System

Vasculature

Gonads

Endocrine — Growth

5. Aetiology and Pathophysiology

5.1 Genetic Basis

5.2 Pathophysiology — System-by-System

5.2.1 Neurological — Why Does Cerebellar Degeneration Occur?

5.2.2 Immunological — Why Combined Immunodeficiency?

5.2.3 Oncological — Why Cancer Predisposition?

5.2.4 Radiosensitivity — Why Can't These Patients Tolerate Radiation?

5.2.5 Endocrine — Growth Failure and Insulin Resistance

5.2.6 Pulmonary — The Leading Cause of Death

6. Classification

6.1 Clinical Classification

6.2 Immunological Classification (IUIS 2024)

6.3 Classification Among DNA Repair / Chromosomal Instability Syndromes

7. Clinical Features

7.1 Symptoms (What the child/family reports)

A. Neurological Symptoms

B. Immunological Symptoms

C. Growth and Endocrine Symptoms

D. Other Symptoms

7.2 Signs (What you find on examination)

A. Neurological Signs

B. Cutaneous Signs

C. Other Examination Findings

7.3 Natural History / Timeline of Classic A-T

8. Laboratory Hallmarks (Mentioned Here as They Are Pathognomonic / Diagnostic Clues)

9. Connections to Primary Immunodeficiency Classification

10. Connections to Movement Disorders / Cerebellar Lesions

11. Summary Table: Clinical Features by System

Active Recall - Ataxia-Telangiectasia (Definition, Epidemiology, Pathophysiology, Clinical Features)

References

Ataxia-Telangiectasia — Differential Diagnosis

Approach to the Differential

1. Differential Diagnosis of Progressive Cerebellar Ataxia in Childhood

A. Inherited / Genetic Causes of Progressive Ataxia

B. Acquired Causes of Ataxia (Must Exclude)

2. Differential Diagnosis of Telangiectasiae in Childhood

3. Differential Diagnosis of Combined Immunodeficiency with Syndromic Features

4. Differential Diagnosis of Elevated AFP in Childhood

5. Differential Diagnosis of Cancer Predisposition Syndromes in Childhood

6. Distinguishing A-T from Key Mimics: A Comparison Table

7. Diagnostic Decision Tree (Mermaid Diagram)

8. Clinical Approach Framework: VINDICATE for Childhood Ataxia

9. Special Paediatric Considerations in the Differential

9.1 Age-Based Framing

9.2 Ataxic Cerebral Palsy vs. A-T

9.3 The "Clumsy Child" Misdiagnosis

10. When A-T Presents Primarily as Immunodeficiency

Active Recall - Ataxia-Telangiectasia Differential Diagnosis

References

Diagnostic Criteria for Ataxia-Telangiectasia

Preliminary Note: There Are No Formal "Diagnostic Criteria" — Diagnosis is Clinical + Laboratory

Working Diagnostic Framework for A-T

Level 1: Clinical Suspicion ("Probable A-T")

Level 2: Supportive Laboratory Evidence ("Highly Probable A-T")

Level 3: Confirmatory Diagnosis ("Definite A-T")

ESID Diagnostic Criteria Framework (Simplified for Clinical Use)

Diagnostic Algorithm

Step-by-Step Clinical Approach

Investigation Modalities — Detailed Interpretation

A. First-Line Screening Investigations

1. Serum Alpha-Fetoprotein (AFP)

2. Immunoglobulin Panel (IgG, IgA, IgM, IgE, IgG Subclasses)

3. Complete Blood Count (CBC) with Differential + Lymphocyte Subsets

B. Confirmatory Investigations

4. ATM Protein Analysis (Immunoblot / Western Blot)

5. ATM Gene Sequencing (Molecular Genetic Testing)

6. ATM Kinase Activity Assay