Fourth Cranial Nerve Palsy

Trochlear nerve palsy paralyses the superior oblique muscle, causing vertical and torsional diplopia, worst in downgaze (reading, stairs).

• It is the commonest isolated cyclovertical palsy. The nerve has the longest intracranial course and is the only one to decussate fully and exit dorsally, making it vulnerable to closed head injury.
• Causes: congenital (decompensated in adulthood), trauma (often bilateral), microvascular, tumour, hydrocephalus, post-neurosurgical, and idiopathic.

• Vertical or oblique diplopia, torsion (images tilted), improved with head tilt away from the affected side and chin down.
• Hypertropia of the affected eye, worse in adduction and downgaze, and worse on head tilt to the same side.
• Excyclotorsion; in bilateral palsy more than 10 degrees, with a V-pattern esotropia, a reversing hypertropia on side gaze, and a large chin down posture.
• Old photographs show a long standing head tilt in congenital cases, with facial asymmetry and large vertical fusional amplitudes.

• Three step test: (1) which eye is hypertropic in primary position; (2) is the hypertropia worse in left or right gaze; (3) is it worse in left or right head tilt. A right hypertropia worse in left gaze and worse on right head tilt confirms a right superior oblique palsy.
• Double Maddox rod test to quantify torsion; prism cover test in nine positions; Hess or Lees chart.
• Neuroimaging: MRI brain with thin cuts through the midbrain for atypical features, bilateral palsy with no trauma, or any additional neurological sign. Isolated vasculopathic palsy in age over 50 can be observed.
• Exclude myasthenia, thyroid eye disease (forced duction, CT or MRI orbits), skew deviation (upright supine test) and orbital trauma.
• Blood pressure, glucose, ESR and CRP in elderly patients.

• Observe for 3 to 6 months in microvascular and traumatic palsy; most improve.
• Symptom relief: occlusion, temporary Fresnel prism; permanent ground in vertical prism for small deviations (up to 8 to 10 prism dioptres).
• Surgery after 6 months of stability, selected by the deviation in the field of maximum action:
  • Hypertropia greater in downgaze: contralateral inferior rectus recession of about 3 to 5 mm.
  • Hypertropia greater in upgaze with inferior oblique overaction: inferior oblique weakening (recession or anteriorisation).
  • Torsion above 10 degrees: Harada-Ito procedure (anterior and lateral transposition of anterior superior oblique tendon fibres).
  • Superior oblique tendon tuck for lax tendon (in congenital cases).
  • Contralateral inferior rectus recession (with adjustable suture) when ipsilateral surgery risks overcorrection.
• Bilateral palsy: usually requires surgery on both sides, planned for the torsional component.
• Treat underlying cause; counsel on risk of induced Brown syndrome after superior oblique tuck.