High Yield: The afferent limb runs from retinal ganglion cells → optic nerve → optic chiasm (nasal fibres decussate) → optic tracts → pretectal nucleus (midbrain at superior colliculus level) → BILATERAL Edinger-Westphal nuclei (EWN). ^[1]

Why bilateral EWN? Because the pretectal nucleus projects to BOTH the ipsilateral and contralateral EWN. This is the anatomical basis for the consensual light reflex. When you shine a light in one eye, both pupils constrict because the signal reaches both EWNs.

High Yield: The efferent limb: EWN → preganglionic parasympathetic fibres travel along the PERIPHERY of CN3 → synapse at ciliary ganglion → postganglionic fibres via short ciliary nerve → pupillary sphincter muscle (constriction). ^[1]

Key point — PNS fibres run on the PERIPHERY of CN3. This is critical for understanding why compressive lesions (e.g., PCA aneurysm) affect the pupil first (they compress the outer fibres), while ischaemic CN3 palsy (e.g., from DM/HTN) typically spares the pupil (ischaemia affects the vasa nervorum supplying the core of the nerve, not the peripheral parasympathetic fibres).

High Yield: The accommodative reflex afferent pathway differs from the light reflex: it goes via the lateral geniculate nucleus (LGN) → visual cortex → back to EWN and CN3 nucleus. ^[1]

Why does this matter? Because the accommodative pathway bypasses the pretectal nucleus. This is the anatomical basis of light-near dissociation — if the pretectal area is damaged (as in Argyll Robertson pupil or Adie's pupil), the light reflex is impaired but the accommodation reflex is preserved, because it takes a different route.

High Yield: The efferent limb of accommodation produces THREE responses: (1) Pupillary constriction (same pathway as light: EWN → CN3 → CG → SCN), (2) Accommodation (SCN → ciliary muscle contracts → lens zonules relax → lens becomes rounder → higher refractive power), (3) Convergence (EWN + CN3 nucleus → bilateral medial recti contract). ^[1]

The near triad: Constriction + Accommodation + Convergence — all mediated via CN3/EWN.

High Yield: RAPD is detected by the swinging flashlight test. It indicates asymmetric damage to the AFFERENT pathway (retina to pretectal nucleus). The affected pupil paradoxically dilates when light is swung from the normal eye to the affected eye. ^[1]

Why does the pupil dilate? When you shine light in the normal eye, both pupils constrict (direct + consensual). When you quickly swing the light to the affected eye, the afferent signal from that eye is weaker. Both pupils perceive a relative decrease in light intensity, so both pupils dilate. You see the affected pupil dilate (instead of staying constricted) — this is the RAPD.

High Yield — Causes of RAPD (think anatomically, front to back along the afferent limb): ^[1]

1. Ganglion cells/Retina: Glaucoma, severe retinal pathology (chronic detachment, severe macular haemorrhage/oedema, severe retinitis, severe RVO)
2. Optic nerve: Optic neuritis, ischaemic optic neuropathy (AION/GCA, NAION), compressive, infiltrative, trauma
3. Optic chiasm: Compressive, infiltrative, ischaemic, infective
4. Optic tracts: Compressive, infiltrative, ischaemic, infective
5. Pretectal nucleus: Compressive, infiltrative, ischaemic, infective

Clinical pearl: Cataract does NOT cause RAPD (it reduces total light equally and diffusely — not asymmetric enough to produce an RAPD). A common exam trap.

High Yield: Before attributing anisocoria to a neurological cause, ALWAYS check for other causes: trauma, uveitis leading to posterior synechiae, medications (mydriatics for uveitis, miotics for AACG prevention). ^[1]

High Yield — Which is the abnormal eye? Compare anisocoria in bright vs dim lighting:

• Greater in bright light → the large pupil is abnormal (it cannot constrict)
• Greater in dim light → the small pupil is abnormal (it cannot dilate) ^[1]

Why? In bright light, both pupils should constrict. If one can't constrict, the difference becomes more apparent. In dim light, both should dilate. If one can't dilate, the difference becomes more apparent.

High Yield — History clues: ^[1]

• Binocular or monocular? Cover one eye at a time
• Persistent or transient? (diurnal variation → think MG)
• Sudden or gradual onset?
• Horizontal, vertical, diagonal, or rotational separation?
• Painless or painful?

High Yield: ^[1]

• Binocular diplopia: Image from an object focused on non-corresponding retinal locations in 2 eyes → disappears when EITHER eye is covered. This IS neurological.
• Monocular diplopia: Images from an object focused on 2 retinal locations in the SAME eye → persists despite covering the fellow eye. This is NOT neurological (e.g., uncorrected refractive error, dry eyes, corneal scar, cataract).

High Yield — CN3 supplies: ^[1]

• Upper eyelid: Levator palpebrae superioris
• Extraocular muscles: Superior rectus, medial rectus, inferior rectus, inferior oblique
• Iris: Pupillary sphincter (parasympathetic)

High Yield — Signs of CN3 palsy: ^[1]

• Ipsilateral ptosis (complete)
• Ipsilateral "down and out" / abducted and mildly depressed eye position (because CN4 → SO pulls down, and CN6 → LR abducts — these are intact and unopposed)
• If pupil involved → ipsilateral pupil dilation with impaired/absent pupillary light reflex

Why "down and out"? The SO (CN4) depresses the eye when it is adducted, and the LR (CN6) abducts the eye. With all CN3-innervated muscles paralysed, the only remaining functional muscles pull the eye down (SO) and out (LR).

High Yield — Causes of isolated CN3 palsy: ^[1]

• Ischaemic: DM, HTN, hyperlipidaemia → classic PUPIL-SPARED palsy (microischaemia affects core of nerve, sparing peripheral PNS fibres)
• Compressive: Neoplasms, ICH, high ICP, aneurysm (especially PCA compressing peripheral CN3 → PNS disrupted first → PUPIL INVOLVED)
• Traumatic
• Infiltrative: Lymphoma, sarcoid
• "Mimics": MG, thyroid eye disease, myositis, orbital fracture

High Yield: Confirm CN4 and CN6 remain intact (i.e., normal intortion and abduction) to rule out brainstem disorder. ^[1]

High Yield — CN4 supplies the superior oblique muscle, which is responsible for: ^[1]

1. Intorsion (primary action when eye is abducted)
2. Depression (especially when eye is adducted — the classic test position)
3. Abduction (minor)

High Yield — Signs of CN4 palsy: ^[1]

1. Ipsilateral eye elevation (hypertropia — because the SO cannot depress)
2. Ipsilateral extorsion
3. Contralateral head tilt (patient tilts head AWAY from the affected side to counter extorsion and minimize torsional diplopia)
4. ± Chin down position (to counter vertical diplopia from eye elevation)

The Parks-Bielschowsky three-step test is the clinical method to isolate which SO is affected:

1. Which eye is higher? → affected SO is on the higher eye side
2. Is hypertropia worse on looking left or right? → worse when looking to the opposite side (because SO depresses in adduction)
3. Is hypertropia worse with ipsilateral or contralateral head tilt? → worse with ipsilateral head tilt (because tilting toward the affected side demands intorsion by the SO, which is paretic)

High Yield: ^[1]

• Unilateral palsy frequently caused by microvascular disease (DM/HTN/hyperlipidaemia)
• Bilateral palsy frequently congenital, or caused by closed head trauma

Why bilateral CN4 from trauma? CN4 is the only cranial nerve that exits dorsally from the brainstem and decussates. Contrecoup injury to the dorsal midbrain (e.g., from a contusion against the tentorial edge) can damage both CN4s.

High Yield — CN6 supplies the lateral rectus muscle. ^[1]

High Yield — Signs of CN6 palsy: ^[1]

1. Ipsilateral eye adducted (because the unopposed medial rectus pulls the eye in)
2. Ipsilateral face turn (patient turns face TOWARD the affected side to minimize horizontal diplopia)

High Yield: ^[1]

• Unilateral palsy frequently caused by microvascular disease (DM, HTN, hyperlipidaemia) and trauma (especially children)
• Increased ICP (e.g., tumour) is an important cause to rule out, especially if bilateral and/or associated with papilloedema — CN6 has the LONGEST intracranial track → most susceptible to ICP changes

This is why bilateral CN6 palsy is called a "false localizing sign" — it doesn't localize the lesion to the pons (where CN6 nucleus is) but instead indicates generalized raised ICP stretching the nerve along its long course.

High Yield — When to be concerned / need admission: ^[1]

• Papilloedema present or acute presentation / progressively worsening
• Young patients ( < 55): higher chance of space-occupying lesions (in HK, especially NPC)
• No gradual resolution over 3 months

High Yield: ^[1]

• Chronic autoimmune disorder, any age or gender
• Auto-antibodies to AChR → interferes with neuromuscular transmission in skeletal muscles
• 50% of patients present with ptosis and diplopia
• Characterized by muscular FATIGABILITY
• Can MIMIC nearly any oculomotor paresis (CN3, 4, 6)
• Consider MG in any patient with unexplained ptosis or ocular movement problems that do NOT obey usual patterns

Why fatigability? Because the autoantibodies destroy AChRs, there are fewer receptors available. With sustained muscle use, the readily available ACh in the synaptic cleft depletes, and with fewer receptors, transmission fails more and more → muscles weaken with repeated use and recover with rest.

High Yield — Investigations: ^[1]

• Serology: Anti-AChR antibodies (BUT 10% generalized MG and 60% ocular MG are SERONEGATIVE)
• EMG (repetitive nerve stimulation → decremental response)
• Tensilon (edrophonium) / ice test / sleep test
• Other: TFT (associated thyroid disease), CT thorax (thymoma — present in ~15% of MG patients)

Ice test: Place ice on the ptotic eyelid for 2 minutes → if ptosis improves, it supports MG (cold reduces acetylcholinesterase activity at the NMJ → more ACh available → improved transmission).

High Yield — Common causes of optic disc swelling (the four "I"s): ^[1]

1. Intracranial pressure (papilloedema)
2. Ischaemic (AAION / NAION / PION)
3. Inflammation (optic neuritis)
4. Infiltration (tumour)

Other causes: Compression (e.g., TED), Toxins (e.g., ethambutol)

High Yield — Fundal signs of optic disc swelling: ^[1]

• Elevation of optic disc (can be physiological)
• Loss of optic cup (can be physiological)
• Vascular tortuosity
• Blurred disc margin
• ± Disc hyperaemia/pallor
• ± Disc haemorrhages

Definition: Optic disc swelling caused specifically by raised intracranial pressure. It is BILATERAL.

High Yield: ^[1]

• Bilateral. Vision initially UNAFFECTED but may have recurrent transient visual obscurations (TVOs) lasting seconds — "greyout", flickering, or blurred vision
• Other symptoms of raised ICP: headache, nausea/vomiting, CN6 palsy with diplopia ("false localizing sign")

High Yield — Causes of elevated ICP: ^[1]

1. Intracranial space-occupying lesion (tumour, epidural/subdural haematoma)
2. Intracerebral oedema or haemorrhage (trauma, meningitis, encephalitis, dural sinus thrombosis)
3. Idiopathic intracranial hypertension (pseudotumour cerebri)

High Yield — Management: ^[1]

1. Immediate MRI or CT scan
2. Lumbar puncture — ONLY after a mass or venous thrombosis has been excluded — to evaluate CSF and its pressure

Chronic papilloedema can cause PERMANENT optic nerve damage if not relieved.

High Yield — "Typical" (demyelinating) optic neuritis: ^[1]

• Young females (age 18-45)
• Unilateral, with periorbital discomfort and/or pain with eye movement
• Features of optic neuropathy: RAPD, impaired VA, impaired colour vision, VF defects
• Acute (hours) to subacute (days) onset, progresses to peak severity at 1-2 weeks, followed by spontaneous gradual recovery over 1-2 months
• Frequently associated with MS, but can occur in isolation

High Yield — "Atypical" optic neuritis: ^[1]

• Does NOT follow all the above features
• Occurs with NMO (neuromyelitis optica), syphilis, HIV, SLE, RA, post-viral infection (children)

Why "typical" vs "atypical" matters: Typical optic neuritis associated with MS has a good visual prognosis (most recover well). Atypical optic neuritis, especially NMO-associated, tends to cause more severe and irreversible visual loss and requires different immunosuppressive treatment. The distinction guides investigation and prognosis.

High Yield — Management of typical optic neuritis: ^[1]

• MRI brain to look for white matter lesions related to risk of MS development
• Acute treatment:
  • Observation
  • Steroid: IV (3 days) then oral (11 days) — hastens recovery within first 4-6 weeks but NO difference in long-term outcome
  • ORAL STEROID ALONE IS CONTRA-INDICATED as it may INCREASE recurrences
• Long-term (MS progression modifying): ocrelizumab, beta interferons, glatiramer, etc.

High Yield: Acute, PAINLESS visual loss in adults from microvascular infarction of the optic nerve. ^[1]

Arteritic AION (AAION) — Giant Cell Arteritis

High Yield: ^[1]

• Initially unilateral but may rapidly become BILATERAL
• Elderly ( > 60 years old)
• Associated with GCA/temporal arteritis
• Other symptoms of GCA: headache, scalp tenderness (e.g., hair combing), jaw claudication (pain with chewing), proximal muscle pain and stiffness (polymyalgia rheumatica), constitutional symptoms (fever, malaise, anorexia, weight loss)

High Yield — Signs and investigations: ^[1]

• RAPD, altitudinal VF loss, swollen optic disc
• Suspect arteritic AION if: elderly, visual loss severe, optic disc very pale
• Investigations: ESR, CRP, platelets (ALL raised), temporal artery biopsy (within 2 weeks of starting steroid if possible)

High Yield — Treatment: ^[1]

• Immediately start HIGH-DOSE STEROIDS (can prevent other eye involvement)

How to distinguish AAION from NAION:

• Pupils: Asymmetry noted? Trauma? Medications? Pain?
• Diplopia: Binocular or monocular? Onset? Direction? Diurnal variation? Pain?
• Visual loss: Acute or gradual? Painful or painless? Unilateral or bilateral? TVOs?
• Systemic: DM, HTN, hyperlipidaemia, smoking, headaches, jaw claudication, muscle pain, constitutional symptoms, recent surgery/anaesthesia

• VA, VF, colour vision (Ishihara plates)
• Pupils: RAPD (swinging torch), anisocoria in bright vs dim light, light-near dissociation
• Eyelids: Ptosis — partial (Horner's, MG) vs complete (CN3)
• EOM: 6 cardinal positions, note limitation, direction of diplopia, head position
• Fundoscopy: Disc swelling, pallor, haemorrhages, cupping

1. RAPD does NOT cause anisocoria at rest — it is a dynamic finding on swinging torch test only
2. Cataract does NOT cause RAPD — it affects light transmission diffusely, not asymmetrically
3. Pupil-sparing CN3 palsy = ischaemic (DM/HTN); pupil-involving = compressive (aneurysm) → urgent imaging
4. CN6 palsy + papilloedema = raised ICP (false localizing sign), not pontine lesion
5. Oral steroid alone in optic neuritis increases recurrence — always IV first or observe
6. Horner's ptosis is partial; CN3 ptosis is complete
7. MG can mimic any cranial nerve palsy — if the pattern doesn't fit, think MG
8. 60% of ocular MG is seronegative for anti-AChR
9. Bilateral disc swelling in malignant HTN ≠ papilloedema (not due to raised ICP)
10. GCA: start steroids before biopsy — biopsy can wait 2 weeks; the other eye cannot

Stem (paraphrased faithfully): A 69-year-old gentleman, chronic smoker for 30 years with hyperlipidaemia on statin. Noticed drooping of left eyelid, double vision and unsteady gait upon waking up yesterday. Symptoms persisted for a day. Examination: normal VA, left partial ptosis with reactive and equal pupils, impairment of left eye adduction, upward and downward gaze with diplopia. No wasting/fasciculation, normal tone but grade 4/5 weakness of right upper and lower limbs, hyperreflexia on right side.

(a) What are the cranial nerves responsible for extraocular eye movements? (3 marks) Answer: CN3 (oculomotor), CN4 (trochlear), CN6 (abducens)

(b) Which cranial nerve function is abnormal? (1 mark) Answer: CN3 (left oculomotor nerve) — left ptosis, impaired adduction, upward and downward gaze all point to CN3. Pupils are reactive and equal → pupil-sparing.

(c) What type of motor neuron lesion accounts for his right-sided weakness? (2 marks) Answer: Upper motor neuron lesion (hyperreflexia, grade 4/5 weakness)

(d) Where is the site of lesion in the neuroaxis? (3 marks) Answer: Left midbrain (cerebral peduncle) — ipsilateral CN3 palsy + contralateral UMN weakness = Weber syndrome

(e) What is the MOST LIKELY diagnosis? (1 mark) Answer: Left midbrain infarct (stroke) / Weber syndrome

Options given: A. Facial asymmetry on smiling B. Impaired left eye abduction C. Impaired left eye vertical gaze D. Impaired right eye adduction E. Loss of sensation over right face F. Nystagmus of both eyes on horizontal gaze G. Partial ptosis of left eye with small left pupil H. Partial ptosis of right eye with normal pupil size and light reflex I. Weakness of right pterygoid muscle J. Weakness of right temporalis muscle

Q1: Right cerebellar infarction → Answer: F (Nystagmus of both eyes on horizontal gaze — cerebellar lesions cause nystagmus)

Q2: Left abducens nerve palsy → Answer: B (Impaired left eye abduction — CN6 supplies LR)

Q3: Left Horner syndrome → Answer: G (Partial ptosis of left eye with small left pupil — classic Horner's triad minus anhydrosis)

Q4: Right oculomotor nerve palsy → Answer: D (Impaired right eye adduction — CN3 supplies MR. Note: option H describes partial ptosis with normal pupil, which could suggest MG or partial CN3, but the most specific sign for CN3 palsy is impaired adduction among the options given. However, re-examining: the stem asks for the MOST LIKELY sign. Right CN3 palsy → impaired right adduction, upgaze, downgaze, ptosis. Option D says "impaired right eye adduction" — this fits. Option H says "partial ptosis of right eye with normal pupil size and light reflex" — this could be MG. The answer is H for CN3 palsy if pupil-sparing (ischaemic), but examining the options more carefully, D specifically tests MR function of CN3. Most likely intended answer: H for oculomotor palsy with normal pupil = pupil-sparing CN3 palsy. But looking again — the question pairs specifically: "Right oculomotor nerve palsy" should show right ptosis + right eye movement deficits. Option H shows "partial ptosis of right eye with normal pupil size and light reflex" — this fits pupil-sparing CN3 palsy. Option D shows "impaired right eye adduction" which is also CN3. The best discriminator: H includes ptosis which is a hallmark. Answer: H (partial ptosis of right eye with normal pupil — representing a pupil-sparing CN3 palsy, classic microvascular aetiology).

Q5: Autoimmune myasthenia gravis → Answer: H ... but H was already used for CN3. Re-reading the instruction: "Each option may be used once, more than once, or not at all." So H could be used again. However, the more classic answer for MG would be something showing fatigability. Among the options, H (partial ptosis with normal pupil) fits MG well. But if H is the answer for CN3, the discriminator here is subtle. Given that MG mimics CN3/4/6 palsies and the hallmark is ptosis with normal pupils: Answer: H (partial ptosis of right eye with normal pupil size and light reflex — MG can present identically to partial CN3 palsy). Options can be reused.

Stem: A 42-year-old woman presented with double vision. Past health good. Neurological examination revealed normal first and second cranial nerves, normal eye movements in ALL directions, normal other CNs, grade 5/5 power, normal reflexes, downgoing plantars, normal coordination and sensation. Nevertheless, she noted double vision upon looking DOWN AND TO THE LEFT; the upper image disappeared when her RIGHT eye was covered, and the lower image disappeared when her LEFT eye was covered. CT brain normal.

Which extraocular muscle is affected? A. Left inferior oblique B. Left inferior rectus C. Right medial rectus D. Right superior oblique

Answer: D. Right superior oblique

Rationale: Looking down and to the left requires the RIGHT eye to depress in adduction — this is the function of the RIGHT superior oblique (SO, innervated by CN4). The image from the higher eye (right eye, because the right SO is weak and the right eye cannot depress → hypertropia of right eye) is the upper image. When the right eye is covered, the upper image disappears — confirming the right eye is higher. The affected muscle is the right SO. This is a classic Parks three-step test question.

Stem: A 29-year-old woman with good past health. Presented with right-sided drooping of eyelid for about 3 weeks. Drooping is more severe in the evening. Double vision of bilateral horizontal gaze on reading for > 10 minutes. Increased tiredness in recent 1 month with mild shoulder weakness on washing hair. Physical examination: coherent speech, mild right partial ptosis with normal pupil size and reaction to light, mild impairment of bilateral eye abduction; motor power grade 5/5 with normal tendon reflexes over 4 limbs. No cerebellar ataxia, steady tandem gait.

(a) What is the MOST LIKELY clinical diagnosis? (2 marks) Answer: Myasthenia Gravis (ocular presentation with systemic features — fatigability, diurnal variation, ptosis worse in evening, bilateral abduction impairment mimicking bilateral CN6)

(b) Give one important differential diagnosis that can cause ophthalmoparesis but not ptosis. (2 marks) Answer: Internuclear ophthalmoplegia (INO) — caused by MLF lesion (e.g., MS, brainstem stroke). INO causes impaired adduction of the ipsilateral eye with nystagmus of the abducting eye, but does NOT cause ptosis.

(c) Name three important investigations which help to confirm the diagnosis. (6 marks) Answer:

1. Anti-AChR antibodies (serology) — positive in ~40% of ocular MG
2. Repetitive nerve stimulation EMG — shows decremental response
3. CT thorax — to look for thymoma (Also acceptable: anti-MuSK antibodies, ice test/sleep test, edrophonium test, TFT)

Stem: A 25-year-old man presented with high fever, severe headache and confusion. Past health good. Neck stiffness. Fundal examination revealed bilateral papilloedema. Plain CT brain was normal. You planned to perform LP. Labs: Hb 12 g/dL, WCC 14×10⁹/L, platelets 100×10⁹/L (low), PT 10s, APTT 30s.

Why is LP contraindicated? A. Increased intracranial pressure B. Lack of patient's consent C. Low platelet count D. Short prothrombin time

Answer: C. Low platelet count (100×10⁹/L — below the safe threshold for LP which is generally > 50-100×10⁹/L, and more conservatively > 100×10⁹/L for elective LP).

Rationale: Despite bilateral papilloedema suggesting raised ICP, the CT brain is normal (no mass lesion), so ICP from meningitis alone is not an absolute contraindication per se (the LP is therapeutic in this context). The key contraindication here is the thrombocytopenia (plt 100), which increases bleeding risk during LP. Note: This is debatable — some guidelines consider > 50×10⁹/L acceptable. However, in the context of this MCQ with papilloedema AND low platelets, the examiners likely intended C as the best answer.