A 40-year-old man presents with vertical double vision that is worst when reading and walking downstairs. He has adopted a head tilt to the left without realising it — his wife noticed. There was no trauma. He has hypertension and is a smoker. Vision is 6/6 each eye and the eyes look normal at rest.
Examination Findings
On cover testing there is a right hypertropia (right eye higher) that worsens on left gaze and on right head tilt, and improves on left head tilt — the classic three-step pattern. Ocular movements are otherwise full; there is no ptosis and pupils are normal. The head tilt is to the left, away from the affected side. Fundus normal; no disc swelling.
Investigations
Blood pressure, glucose and lipid review for microvascular risk factors. Neuroimaging is arranged when the palsy is atypical — young patient, other cranial nerves involved, or no recovery — but an isolated vasculopathic fourth palsy in a vasculopath is often observed first. Prism measurement documents the deviation for follow-up.
Questions to Think About
- Why does he tilt his head away from the affected eye?
- What is the three-step test telling us?
- When does an isolated fourth nerve palsy need a scan?
Diagnosis
Isolated right fourth (trochlear) nerve palsy, presumed microvascular.
Reasoning
Reasoning: The fourth nerve supplies the superior oblique, which depresses the eye especially in adduction — when it fails, the eye drifts up, producing vertical diplopia worst on downgaze (reading, stairs). Tilting the head away from the affected side reduces the demand on the weak muscle and fuses the images; the three-step test (hypertropia worse on contralateral gaze and ipsilateral head tilt) localises the fault to the superior oblique. In a 40-year-old vasculopath with no trauma, a microvascular cause is likely — but congenital fourth palsies commonly decompensate in adulthood, so old photographs are worth checking for a long-standing tilt. Diagnosis: isolated right fourth nerve palsy, presumed microvascular. Management: typical management includes observation with vascular risk-factor control, since most microvascular palsies recover over weeks to months; prisms or patching relieve diplopia meanwhile, and strabismus surgery is considered only if the deviation persists. He is told to return urgently for any new headache, other double-vision patterns, or failure to improve.
Differential Diagnosis
- Myasthenia gravis — considered: but the pattern is comitant-obeying and fatigability is absent; tested for if atypical.
- Thyroid eye disease — ruled out: no proptosis, lid signs or restriction pattern.
- Skew deviation — ruled out: no brainstem signs and the three-step pattern fits a peripheral palsy.
Management
Typical management includes vascular risk-factor optimisation, temporary prism or occlusion for the diplopia, and review over weeks to months expecting spontaneous recovery. If the palsy persists beyond the expected window or any atypical feature appears (pain, other nerves, bilateral signs), neuroimaging is arranged. Persistent symptomatic deviations are referred for strabismus surgery assessment.
Key Learning Points
- Fourth nerve palsy: vertical diplopia worst on downgaze, head tilt away from the affected side.
- The three-step test localises the paretic muscle — learn it, it is examined often.
- Congenital fourth palsies decompensate in adulthood — check old photos for a lifelong tilt.
- Isolated vasculopathic palsies usually recover; atypical ones get imaged.
Red Flags
- Multiple cranial nerves or bilateral signs — image urgently; not a lone microvascular palsy.
- A fourth palsy after head trauma — the commonest traumatic cause; look for it.
- No recovery over months — reconsider: myasthenia, thyroid, or a structural lesion.
Educational content only — not medical advice. Clinical decisions must be made by a qualified professional for the individual patient.