Neurological Lesion Localisation

Most fibres for skilled limb movement arise from the contralateral cerebral hemisphere, descend through the corona radiata → posterior limb of the internal capsule → cerebral peduncle → basis pontis → medullary pyramid, then cross in the pyramidal decussation. The crossed fibres descend in the lateral corticospinal tract, influencing spinal interneurons and motor neurons. The LMN then exits the anterior horn through the ventral root and travels to muscle.

Therefore, a left cerebral, pontine or rostral medullary corticospinal lesion weakens the right limbs. A right spinal cord corticospinal lesion, below the decussation, weakens the right limbs below the lesion. Smaller uncrossed/bilaterally projecting pathways contribute especially to axial control; the simple crossing rule is most useful for limb pyramidal signs.

A joint-position signal from the right great toe enters the spinal cord and ascends on the right in the fasciculus gracilis. It first relays in the gracile nucleus in the caudal medulla; second-order fibres cross as internal arcuate fibres, then ascend on the left as the medial lemniscus to the ventral posterolateral (VPL) thalamus and somatosensory cortex. Upper-limb fibres use the fasciculus cuneatus, which is present from approximately T6 upwards and lies lateral to gracilis in the cord.

So a right dorsal column lesion causes right loss of vibration/position below the lesion, while a right medial lemniscus lesion, after the sensory decussation, causes left body sensory loss. Discriminative touch also uses this system; crude touch is less useful for a clean tract localisation because it has additional pathways.

A painful stimulus in the right foot enters via the dorsal root. Small fibres may ascend or descend a short distance in Lissauer's tract, then synapse in the dorsal horn. Second-order neurons cross through the anterior white commissure, usually within roughly one or two spinal segments, and ascend in the left anterolateral tract to the thalamus.

Thus a right spinal cord spinothalamic lesion causes left pain/temperature loss, often beginning about one or two segments below the lesion. A right pontine spinothalamic lesion also causes left body pain/temperature loss: those fibres have already crossed well below the brainstem.

The face has separate trigeminal pathways. Facial pain/temperature fibres enter the pons and descend ipsilaterally in the spinal trigeminal tract to the spinal trigeminal nucleus before crossing and ascending to the ventral posteromedial (VPM) thalamus. This is why a right lateral medullary lesion can remove right facial but left body pain/temperature sensation. Anatomical foundations: UTHealth, Anatomy of the Spinal Cord; clinical sensory patterns.

Damage to primary motor cortex causes contralateral weakness. Damage to the dominant inferior frontal language network can cause non-fluent aphasia: speech is effortful and reduced, usually with relatively better comprehension than output, although complex syntax may still be difficult. A person with dysarthria knows the words but articulates them poorly; a person with aphasia has a disorder of language itself, affecting speaking, understanding, naming, repetition, reading or writing.

The frontal eye field helps drive saccades to the opposite side. A destructive hemispheric lesion can leave the eyes deviated towards the lesion, away from the weak limbs. An irritative seizure can drive the eyes the other way. Frontal association lesions can produce impaired planning, disinhibition, apathy or abulia; medial frontal lesions may impair initiation and bladder control.

Worked reasoning: right face/arm weakness, non-fluent aphasia and leftward gaze preference fit a left dominant frontal/MCA-territory lesion. Aphasia is the finding a peripheral nerve, root or cord lesion cannot explain.

The parietal cortex interprets and integrates sensation. Test stereognosis (recognising an object by touch), graphesthesia (recognising a number traced on skin) and extinction during simultaneous bilateral stimulation. These are interpretable only if basic sensation, comprehension and cooperation are adequate.

A right, usually non-dominant, parietal lesion can produce left hemispatial neglect: the patient fails to attend to the left side of space or their own body. Neglect is not simply loss of visual input. A patient with a hemianopia may actively turn to compensate; a patient with neglect may not recognise anything is missing. The two can coexist.

The dominant inferior parietal region can produce Gerstmann's tetrad: agraphia, acalculia, finger agnosia and left–right disorientation. Apraxia is inability to perform a learned action despite sufficient strength, sensation and comprehension; it implicates cortical networks rather than muscle alone.

The dominant posterior temporal language network is associated with fluent aphasia: speech may flow but be empty or contain paraphasias, and comprehension is impaired. Mesial temporal disease can impair new memory formation and cause focal seizures. The occipital cortex processes vision; a unilateral lesion typically causes a contralateral homonymous visual field defect.

Language dominance is usually left-sided but is not determined with certainty from handedness. Aphasia is a network disorder, and subcortical lesions can also produce aphasia; the named cortical syndromes are practical localisation patterns rather than rigid one-centre models.

Vascular boundaries and collateral flow vary. An anatomical pattern suggests a territory but cannot establish the occluded vessel by itself. See the Ischaemic Stroke article for stroke assessment and management.

The genu carries important corticobulbar fibres; the posterior limb carries corticospinal fibres and ascending somatosensory projections. Since these fibres are densely packed, a small lesion can produce contralateral face, arm and leg weakness without aphasia, neglect or a cortical sensory deficit.

Pure motor hemiparesis therefore suggests a deep motor pathway lesion, often the internal capsule, but also the corona radiata or ventral pons. Absence of cortical signs does not prove an internal capsular lesion. Pure motor, pure sensory, sensorimotor, ataxic hemiparesis and dysarthria–clumsy hand patterns are lacunar clinical syndromes; imaging determines the actual lesion.

The VPL relays body sensation and the VPM relays face sensation. A right thalamic lesion can therefore cause left face–arm–leg sensory loss without the crossed face/body pattern of a lateral lower brainstem lesion. Painful dysaesthesia or allodynia may develop after a thalamic stroke, sometimes delayed. Associated weakness suggests involvement of adjacent internal capsule or wider networks; not every thalamic lesion is purely sensory.

Bradykinesia, rigidity, rest tremor, chorea and dystonia suggest basal ganglia circuits. A patient can move slowly despite near-normal power on testing; do not automatically label bradykinesia as paresis. Contralateral hemiballismus classically suggests a lesion in the subthalamic region, although other network lesions can cause it. Dense UMN weakness in a “basal ganglia stroke” often reflects adjacent internal capsule involvement. Motor-system explanations: UTHealth.

“Medial” is not the same as “all adjacent in a tiny spot”. The corticospinal tract is ventral, while the MLF is dorsal and paramedian. A small ventral lesion need not produce INO. A lesion also cannot affect the XII nucleus in the pons, because that nucleus is in the medulla. The mnemonic tells you what to inspect, not what every medial lesion must contain.

The first S is a useful umbrella for cerebellar pathways, rather than a claim that one continuous spinocerebellar bundle runs unchanged through every level. The fourth S explains why a small pupil and mild ptosis can belong to a brainstem syndrome even though the eye muscles are not paralysed. Horner ptosis reflects sympathetic loss to Müller’s muscle; a III palsy can produce much greater ptosis from levator weakness.

The spinal trigeminal nucleus extends from the pons through the medulla into upper cervical cord, and vestibular nuclei span levels. Thus facial pain loss or vertigo alone does not uniquely identify the pons.

Excluding I and II, the numbers that divide 12 evenly are III, IV, VI and XII. These identify the relatively medial somatic motor nuclei. V and VII also have motor nuclei, but sit more laterally. A nucleus can lie laterally while its exiting fascicle passes through another compartment: this distinction is essential for facial palsy from a ventral pontine lesion.

The main exceptions worth stating in an exam are:

• Trochlear fibres cross before leaving the dorsal midbrain. A right IV nuclear/predecussation lesion weakens the left superior oblique; a right IV nerve lesion after the crossing weakens the right superior oblique.
• A III nuclear complex lesion can produce bilateral ptosis and contralateral superior rectus weakness because of its subnuclear organisation. A typical III fascicular/nerve lesion gives the more familiar ipsilateral pattern.
• A VI nucleus lesion causes ipsilateral conjugate horizontal gaze palsy, whereas a VI fascicle/nerve lesion causes ipsilateral abduction weakness only.
• Not every named syndrome contains all four Ms or all four Ss. Work from the actual deficits and use the eponym last.

The facial motor nucleus lies in the caudal pontine tegmentum. Its fibres first loop dorsomedially around the abducens nucleus, forming the internal genu of VII and contributing to the facial colliculus, then travel ventrolaterally to exit near the pontomedullary junction.

Damage to the right facial nucleus, intrapontine fascicle or peripheral facial nerve can all weaken the whole right face, including forehead wrinkling and forceful eye closure. This is an LMN pattern, even when the lesion is anatomically inside the CNS. “LMN” describes which motor neuron is affected; it does not mean “outside the skull”.

In contrast, a left supranuclear corticobulbar lesion classically weakens mainly the right lower face because the upper facial motor neurons receive bilateral cortical input. Forehead sparing is useful, but not absolute; central lesions can cause some upper facial weakness. Look especially for limb, gaze, sensory and other cranial nerve findings.

Whole right facial weakness + left limb UMN weakness is therefore a right pontine pattern. Right lower facial weakness + right limb UMN weakness is more suggestive of a left supranuclear lesion. An isolated peripheral VII lesion cannot explain contralateral limb pyramidal signs.

To look right, the right paramedian pontine reticular formation (PPRF) activates the right VI nucleus. That nucleus contains motor neurons for the right lateral rectus and interneurons whose axons cross and ascend in the left MLF to the left III medial rectus subnucleus. Right abduction and left adduction occur together.

Convergence can be preserved in INO because its control differs from conjugate horizontal gaze, but this is not invariable. Preserved pupils and absence of ptosis support INO over a complete III palsy. An INO localises the MLF on the side of the adduction deficit, usually in the pons or midbrain; it is not an abducens nerve palsy. MLF anatomy and imaging.

Weber syndrome combines a right III fascicular palsy with left UMN hemiparesis. The III fascicle passes towards the ventral surface beside the descending motor fibres in the cerebral peduncle. The eye may be “down and out”, with ptosis and possibly a dilated pupil; the left limbs are weak because corticospinal fibres have not crossed yet. This is a ventral midbrain localisation, often involving paramedian branches of the PCA or basilar apex region.

Claude syndrome combines III palsy with contralateral ataxia, implicating midbrain tegmental cerebellar connections around the red nucleus/superior cerebellar peduncle region. Benedikt syndrome is commonly used for III palsy with contralateral tremor or other involuntary movement, ataxia and sometimes weakness, reflecting a larger overlapping tegmental lesion. Definitions overlap across sources: state the structures and findings before the eponym.

Dorsal midbrain (Parinaud) syndrome suggests pretectal/posterior commissural disease: impaired upward gaze, convergence-retraction movements on attempted upgaze, pupillary light–near dissociation and sometimes lid retraction. A pineal-region mass or hydrocephalus is a classic cause. This is an example of a clinically important syndrome that the simple medial/lateral rule of 4s does not fully describe.

Ventral caudal pontine / Millard-Gubler pattern: right VII fascicular involvement produces whole-face weakness; right corticospinal involvement produces left UMN hemiparesis. Right VI fascicular involvement may additionally cause abduction weakness. Definitions differ over whether VI involvement is required, so a stem lacking diplopia or an abduction deficit is best answered first as “right ventral caudal pons involving VII fibres and corticospinal tract”. StatPearls: Millard-Gubler syndrome; an imaging-supported clinical example.

Dorsal caudal pontine / Foville pattern: right conjugate horizontal gaze palsy plus right LMN facial weakness identifies the VI nucleus/PPRF and nearby looping VII fibres. Contralateral hemiparesis requires extension to motor pathways; it is not an automatic consequence of a tiny facial-colliculus lesion. A complete gaze palsy and an isolated abduction palsy are therefore not interchangeable clues.

Lateral pontine syndrome, often AICA territory: right facial weakness, right facial pain/temperature loss, left body pain/temperature loss, right ataxia, vertigo/nystagmus and sometimes right hearing loss. The facial and vestibulocochlear structures identify the level; the crossed sensory findings and ataxia identify the lateral compartment. Facial palsy and hearing loss favour lateral pons over lateral medulla. Hearing loss is helpful when present but not mandatory; the labyrinthine artery has variable origin.

Locked-in syndrome: a large bilateral ventral pontine lesion can interrupt both corticospinal and corticobulbar pathways, producing quadriplegia and anarthria while awareness is preserved. Vertical eye movements and blinking are often retained and may provide communication. Preserved consciousness reflects relative sparing of arousal networks; incomplete or extensive lesions vary. Do not equate absence of speech or limb movement with coma.

Medial medullary (Dejerine) syndrome is a particularly clean application of the medial structures:

1. Right XII nucleus/fascicle → right tongue weakness; the protruded tongue deviates right, towards the weak side. Wasting/fasciculations may develop with denervation.
2. Right pyramid → left UMN limb weakness, since this is above the pyramidal decussation.
3. Right medial lemniscus → left body vibration/position loss, since these sensory fibres have already crossed.

Typical supply involves paramedian branches of the vertebral/anterior spinal arterial system. The face is usually spared in the classic motor pattern because the facial nucleus/fascicle is in the pons.

Lateral medullary (Wallenberg) syndrome combines right spinal trigeminal, spinothalamic, cerebellar and sympathetic pathway deficits: right facial pain/temperature loss, left body pain/temperature loss, right ataxia and right Horner syndrome. Involvement of nucleus ambiguus and its IX/X connections adds dysphagia, dysphonia/hoarseness and ipsilateral palatal weakness; vestibular connections add vertigo and nystagmus.

Why is there often no major limb weakness? The pyramidal tract lies medially and is spared by the classic lateral lesion. Why is vibration often preserved? The medial lemniscus is also medial. Hoarseness/palatal weakness favour lateral medulla; LMN facial palsy/hearing loss favour lateral pons. Vertebral artery or PICA disease are typical causes, so “PICA syndrome” does not prove isolated PICA occlusion.

A lesion affecting the full cross-section causes bilateral weakness, loss of multiple sensory modalities below a level, and autonomic/sphincter dysfunction. Acute severe lesions can initially be flaccid and areflexic because of spinal shock; spasticity and hyperreflexia develop later if the segmental arcs remain intact. “Transverse syndrome” describes the pattern; it does not prove transverse myelitis. Compression, infarction, trauma and inflammation can all produce it.

The anterior/lateral cord contains corticospinal and anterolateral pathways, whereas dorsal columns are posterior. An anterior cord lesion therefore produces bilateral motor weakness and pain/temperature loss with relative preservation of vibration and position. At-level anterior horn injury can add LMN signs; autonomic pathways may also be affected.

The classic vascular explanation is anterior spinal arterial territory ischaemia, although trauma or compression can produce a similar syndrome. Preservation of dorsal-column sensation is the discriminating examination clue, not simply the presence of paraplegia.

A small central syrinx first damages the crossing pain/temperature fibres in the anterior white commissure. At a cervical level this produces bilateral suspended or cape-like pain/temperature loss, often over shoulders/upper limbs, while vibration and position remain relatively intact. “Suspended” means affected segments with relatively spared sensation above and below, rather than everything below a horizontal level.

As the cavity expands into anterior horns, the hands may develop wasting, weakness and reduced reflexes. Further extension into lateral corticospinal tracts can produce UMN signs in the legs. Dissociated sensory loss plus wasted hands plus spastic legs is therefore anatomically coherent. Not every syrinx has a perfectly symmetrical cape distribution.

Traumatic central cord syndrome usually means disproportionately greater weakness in the arms than the legs, often after cervical hyperextension in a narrowed canal, with variable sensory and bladder findings. Its mechanism is more complex than a precise “arm fibres lie centrally” diagram; do not treat the traditional concentric corticospinal map as established human anatomy or equate every central cord syndrome with a syrinx.

A posterior column lesion impairs vibration and position, causing sensory ataxia, a positive Romberg when the patient is otherwise able to stand, and sometimes pseudoathetosis: wandering finger movements when visual guidance is removed. Pain and temperature are relatively preserved if the lesion is restricted to posterior columns.

Subacute combined degeneration affects posterior columns and corticospinal pathways, classically in vitamin B12 deficiency and also with functional B12 inactivation from nitrous oxide; copper deficiency can produce a similar myelopathy. Thus sensory ataxia and UMN signs coexist. A concurrent peripheral neuropathy can reduce ankle jerks despite corticospinal disease: absent ankle jerks do not cancel an extensor plantar response. Clinical dorsal-column patterns.

The entries below emphasise commonly tested dominant roots; they are not exclusive innervations.

Do not use the first dorsal web space as an interchangeable label for “L5” and “deep fibular nerve”: it is an autonomous peripheral nerve territory often used alongside a root examination, and the complete motor pattern distinguishes the two.

Most compressive radiculopathies lie proximal to the DRG. Distal sensory axons remain connected to their cell bodies, so the sensory nerve action potential (SNAP) measured in the limb is usually preserved despite sensory symptoms. In a plexus or peripheral nerve lesion, damage is generally distal to the DRG, so affected SNAPs may be reduced.

Needle EMG can reveal denervation in muscles sharing a root but supplied by different nerves; paraspinal involvement supports a root process. These are probabilistic clues: timing, lesion severity, DRG position and technical factors matter, and a normal early study does not exclude disease. Electrodiagnostic evaluation of cervical radiculopathy.

Both can weaken ankle dorsiflexion and great-toe extension. Add two movements supplied through different nerves:

• Inversion by tibialis posterior uses the tibial nerve, mainly L4–5. It is usually spared in an isolated common fibular lesion, but can be weak in L5 radiculopathy.
• Hip abduction by gluteus medius uses the superior gluteal nerve, importantly L5. Weakness supports a more proximal L5/root/plexus process.

Thus foot drop after prolonged leg crossing, with weak eversion but preserved inversion and hip abduction, suggests common fibular compression at the fibular head. Back pain radiating down the leg plus weak inversion and hip abduction favours L5 radiculopathy. Neither discriminator is perfect in a partial lesion; consider sciatic neuropathy, plexopathy and a small central lesion if the full pattern does not fit.

A radial lesion weakens extensors but should spare median/ulnar-innervated wrist flexors. A C7 radiculopathy can weaken elbow extension and wrist flexion across nerve boundaries, with middle-finger sensory symptoms and a depressed triceps reflex. A spiral-groove radial lesion often spares triceps; a posterior interosseous lesion spares cutaneous sensation. Check the actual level before saying “radial palsy”.

Reduced effective postsynaptic receptor function means repeated activity can progressively fail to activate enough muscle fibres. The characteristic pattern is fatigable/fluctuating weakness, often ptosis, diplopia, facial/bulbar symptoms, neck weakness or proximal limb weakness. Rest can improve function. Sensation and pupils are usually normal, tendon reflexes are usually preserved, and UMN signs are absent.

Different eye muscles may be weak at different times; the pattern need not fit one cranial nerve. This separates MG from a fixed III, IV or VI palsy. “Tired all the time” is not the same as demonstrable fatigability. Antibody testing and appropriate electrophysiology help confirm the diagnosis; an isolated bedside observation is not definitive.

Impaired presynaptic calcium-channel function reduces acetylcholine release. The pattern often emphasises proximal leg weakness, reduced reflexes and autonomic symptoms such as dry mouth. Brief maximal activation can transiently increase strength/reflexes because presynaptic calcium accumulation improves release. This facilitation contrasts with the usual clinical fatigability of MG, although symptoms are not perfectly binary. Sensation is preserved.

Blocked acetylcholine release can cause cranial/bulbar symptoms followed by descending flaccid weakness, with autonomic dysfunction and sometimes poorly reactive/dilated pupils. Pupillary involvement favours a presynaptic/autonomic disorder over typical MG, but is not required. Rapid bulbar or respiratory deterioration is urgent regardless of the exact junctional diagnosis.

1. Right LMN-pattern facial weakness involving the upper and lower face. Failure of forceful eye closure supports orbicularis oculi weakness, while the intact left facial muscles pull the mouth left. Test forehead wrinkling explicitly; inability to close the eye is not itself a direct observation of forehead movement.
2. Left upper-limb weakness, expected to be UMN in this crossed pontine pattern.
3. Left lower-limb weakness, together with the arm deficit constituting left hemiparesis, expected to be UMN.
4. Dysarthria and dysphagia: impaired articulation and swallowing. These are the deficits directly established by slurred speech and choking.

The supplied marking guide separates upper- and lower-limb weakness into two points; preserve that distinction when writing a four-part answer. The history establishes weakness; the predicted UMN physiology is confirmed by examination.

Qualification: dysarthria and dysphagia can be described as bulbar symptoms, but do not by themselves prove a bulbar LMN palsy or pseudobulbar palsy. True pseudobulbar palsy usually requires bilateral corticobulbar dysfunction and supporting findings. Do not invent tongue fasciculations, palatal weakness or a brisk jaw jerk when they were not provided.

Right caudal pons, involving the intrapontine facial nerve fibres and descending corticospinal tract; a ventral pontine pattern.

• The right VII LMN-pattern palsy places the lesion on the right at the facial nucleus/fascicle or nerve. The facial motor nucleus is pontine.
• The left arm and leg weakness requires involvement of the right corticospinal pathway above the medullary motor crossing.
• A single lesion in the right pons can explain both. In a ventral pontine localisation, the VII fascicle, rather than the dorsally placed nucleus itself, is the important structure.
• Descending corticobulbar/swallowing networks may account for dysarthria/dysphagia, but the exact extent requires examination and imaging.

This is consistent with a Millard-Gubler / ventral caudal pontine pattern. Some descriptions include ipsilateral VI palsy, while others use VII palsy plus contralateral hemiparesis. An anatomical answer is more secure than insisting on an eponym when no abduction deficit was given. Sudden onset makes a vascular cause likely; the stem alone does not prove infarction rather than haemorrhage, an exact vessel, or the full lesion boundary. Millard-Gubler anatomy.

The right facial LMN lesion does not lower right arm or leg reflexes: their reflex arcs do not run through VII. In the first hours or days of a severe acute central lesion, left-sided tone and reflexes may initially be reduced; mention that qualification while stating the expected established pattern. An extensor plantar response supports corticospinal dysfunction, but every acute examination finding need not be present immediately.

If facial reflexes are specifically requested: the corneal reflex has V1 afferent and VII efferent limbs. With right VII efferent failure and otherwise intact pathways, the right eye fails to blink whichever cornea is stimulated, while the left eye can blink with stimulation of either cornea. A right V1 afferent lesion instead prevents both eyes blinking when the right cornea is stimulated, with bilateral blinking when the left is stimulated. This is separate from the limb tendon reflex answer.