Background/Aims To investigate the causes of isolated fourth nerve palsy (IFNP) and the association among aetiology, prognosis and ocular deviation.

Methods A total of 126 consecutive cases of IFNP was retrospectively reviewed. According to aetiologies, all patients were classified into five groups: microvascular, congenital, decompensation of congenital, traumatic and others. We investigated the recovery rate of IFNP patients who could be followed for more than 6 months or until they recovered completely. Patients also had the magnitude of vertical and horizontal ocular deviations (prism diopter) measured in the primary eye position on the first visit.

Results Major causes of IFNP were microvascular (47%) and decompensated (33%). The rate of recovery was significantly different between microvascular IFNP and decompensated IFNP (92% vs 55%, p<0.001). There were no differences in both age of onset or mean vertical deviation between the two aetiologies (68.6±9.8 vs 65.4±13.3, 5.7±3.3 vs 7.8±7.9). However, for mean horizontal deviation, there was a significant difference between microvascular and decompensated IFNP (0.4±3.0 vs 4.9±5.6, p<0.001). Although the fourth nerve abducts the eyeball, 69 of 126 cases (55%) showed exotropia. The microvascular IFNP group included more cases of exodeviation, while the decompensated IFNP group included more cases of esodeviation (p<0.001).

Conclusions Contrary to previous thinking, the horizontal deviation of IFNP mainly showed exodeviation, and the degree of horizontal deviation is useful for making a determination between vasculopathic and decompensated IFNP. This differentiation could be critical for predicting the outcome.