Fourth Nerve Palsy

The fourth cranial nerve innovates the Superior Oblique (SO) Extraocular Muscle.

The SO muscle depresses, intorts (incyclotorsion) and abducts.

Therefore:

• The eyes would sit straight or with a vertical misalignment.
• When taken into laevodepression, an underaction of the SO muscle would be seen.
• Other muscles will compensate for this imbalance and over/underact according to Herring’s and Sherrington’s Laws. This is called a muscle sequelae.

Causes of a 4th Nerve Palsy:

• Congenital
• Trauma (whiplash)
• Vascular disease
• Aneurysm or tumour (less common)
• Stroke

‘See also’ below the eye movements recorded by an Orthoptist in a right fourth nerve palsy. Note the sequelae.