Cyclotropia to differentiate longstanding and acute fourth cranial nerve palsy

Palsy of the fourth cranial nerve is the most common isolated cyclovertical extraocular muscle palsy, and the most common isolated cranial nerve palsy. Fourth cranial nerve palsy may be acquired or "congenital." Without a clear history of acute onset vertical diplopia in a patient without risk factors, it can be difficult to distinguish between congenital and acquired superior oblique palsy. The weight of evidence following a thorough sensorimotor exam is used to support the presumed etiology. The purpose of this study was to determine if the ratio of excyclotropia to hypertropia in primary position could be used to distinguish acquired from congenital unilateral superior oblique palsy.

A computer database search was done to identify patients with unilateral fourth cranial nerve palsy. Patients were sorted into two groups: those with documented acute fourth nerve palsy (Group A), and those with presumed congenital onset (Group B).

Although the degree of cyclotropia was not statistically different between Groups A and B, the amount of hypertropia was significantly larger in the congenital group. Group B patients had an average of 0.3°of excyclotropia per 1(Δ) of hypertropia in primary, compared to 1.16° per 1(Δ) in Group A (P < 0.001).

The degree of cyclotropia is correlated to the severity of the SO weakness in acute fourth nerve palsy, but not in presumed congenital fourth nerve palsy. The ratio of degrees of cyclotropia per prism diopter of hypertropia may be helpful in differentiating longstanding from acute fourth nerve palsy.