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Fourth nerve palsy
Neuro-Ophthalmology
Key Points
- •CN4 is the only cranial nerve that exits dorsally and decussates completely at the anterior medullary velum - bilateral CN4 palsy after head trauma is due to contrecoup injury at this decussation point
- •The Parks 3-step test (hypertropic eye → worse in which horizontal gaze → worse on which head tilt) systematically localizes the paretic muscle - combined with Bielschowsky head tilt test this is the definitive bedside diagnostic method
- •Congenital CN4 palsy is the most common cause overall - look for head tilt on old photographs, large vertical fusional amplitudes (>3 PD), and facial asymmetry to distinguish from acquired
- •Microvascular CN4 palsy (in patients >50 with HTN/DM) has an 85–90% spontaneous recovery rate within 3–6 months - observation is appropriate; failure to recover mandates neuroimaging
- •Excyclotorsion >10 degrees on double Maddox rod test strongly suggests bilateral CN4 palsy - even if the hypertropia appears unilateral
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17 more sections are included with the Clinical plan.
- Definition
- Epidemiology
- Aetiology and causes
- Pathogenesis
- Classification
- Risk factors and associations