A 26-year-old man had a motorcycle accident three years ago. Since then he has had vertical double vision attributed to a right superior oblique palsy. He was offered surgery on the right inferior oblique. Before agreeing, he seeks a second opinion because ‘the double vision is worst looking down at my phone, and I hold my chin down all day’. On examination the right hypertropia is clear — but tilting his head to the left makes the LEFT eye hypertropic instead.
Examination Findings
The right hypertropia in primary position suggests a right superior oblique palsy on the three-step test. But careful testing reveals the trap: on left head tilt the LEFT eye becomes hypertropic (the three-step test then localises to the left superior oblique), and in downgaze there is a marked esotropia with the patient adopting a chin-down posture. Bilateral inferior oblique overaction is present, worse on one side. Fundus examination shows bilateral extorsion. There is a history of significant head trauma years ago — the classic setting for bilateral superior oblique palsy. Visual acuity is 6/6 each eye.
Investigations
The three-step test performed with head tilt to EACH side — the essential manoeuvre that unmasks bilaterality. Measurement in downgaze (esotropia suggests bilateral involvement). Fundus examination for bilateral extorsion. Old trauma history taken explicitly — bilateral superior oblique palsy is characteristically post-traumatic. Assessment of torsion (double Maddox rod) to quantify bilateral cyclodeviation.
Questions to Think About
- What findings unmask the second superior oblique palsy?
- Why is head trauma the classic history here?
- What goes wrong if only the apparently affected side is operated?
Diagnosis
Masked bilateral superior oblique palsy — fixing the apparent unilateral palsy surgically would unmask the second side; management is bilateral surgery planned from the start.
Reasoning
Reasoning: the primary-position right hypertropia suggests a right superior oblique palsy, but three findings betray bilateral disease: the hypertropia alternates to the left eye on left head tilt (the three-step test localising to each side in turn), there is a marked esotropia in downgaze, and he uses a chin-down posture to avoid downgaze — the textbook triad of masked bilateral superior oblique palsy, with bilateral extorsion on funduscopy as objective confirmation. Bilateral trochlear involvement is characteristically post-traumatic, matching his motorcycle accident. The danger is the proposed unilateral surgery: weakening only the right inferior oblique would unmask the left palsy, trading one hypertropia for another. Diagnosis: bilateral superior oblique palsy, asymmetric, post-traumatic, previously masked as unilateral. Management: typical management is a bilateral surgical plan from the outset — commonly bilateral inferior oblique weakening tailored to the asymmetry — with counselling that the second side was always present and that only a bilateral approach resolves the downgaze diplopia and the chin-down posture.
Differential Diagnosis
- True unilateral superior oblique palsy — ruled out as the complete diagnosis: the three-step test is positive on alternating sides depending on head position, and downgaze esotropia with chin-down posture betrays the second palsy.
- Isolated inferior oblique overaction — ruled out: the overaction is bilateral and asymmetric because both superior obliques are weak; weakening one inferior oblique alone unbalances the system.
- Decompensated congenital unilateral palsy — close, but the alternating hypertropia and marked downgaze esotropia mark bilaterality.
- Skew deviation — ruled out: the pattern follows the three-step test on each side and there are no brainstem signs.
Management
Referral to a strabismus surgeon with the bilateral diagnosis declared: typical plan addresses both sides (e.g. bilateral inferior oblique weakening, with the surgical dose weighted to the asymmetry; some surgeons add superior oblique tucking on the worse side per individual assessment). Counselling that a single-sided operation would have unmasked the second palsy. Post-operative orthoptic follow-up including torsion and downgaze alignment.
Key Learning Points
- Bilateral superior oblique palsy hides behind a unilateral mask: the more paretic side dominates in primary position while the second side waits.
- The unmasking signs: hypertropia that alternates with head tilt direction, marked esotropia in downgaze, chin-down posture, and bilateral extorsion.
- Head trauma is the classic cause — the trochlear nerves are vulnerable where they cross near the midbrain.
- Operating only the apparent side is the trap: it unmasks the fellow palsy and the patient returns with a new hypertropia.
- Bilateral disease needs a bilateral plan from the start (commonly bilateral inferior oblique weakening, tailored to the pattern).
Red Flags
- A ‘unilateral’ superior oblique palsy with marked downgaze esotropia — suspect the second side.
- Chin-down posture — the patient is avoiding downgaze where bilateral disease is worst.
- Bilateral extorsion on fundus examination — objective evidence of bilateral cyclovertical disease.
- Previous unilateral surgery with a ‘new’ contralateral hypertropia — the classic unmasking presentation; the second side was there all along.
Educational content only — not medical advice. Clinical decisions must be made by a qualified professional for the individual patient.