Horizontal deviation appears useful for diagnosing isolated fourth nerve palsy

This retrospective study found that most isolated fourth nerve palsy (IFNP) cases in a mainly adult population are caused by microvascular and decompensated pathologies and that, contrary to previous thinking, horizontal deviations can be helpful in diagnosing and differentiating between these major pathologies.

Subjects were 126 consecutive cases of IFNP classified into five groups: microvascular, congenital, decompensation of congenital, traumatic and others. The authors investigated the recovery rate of all patients who were followed for more than six months or until they recovered completely and had undergone measurement of the magnitude of vertical and horizontal ocular deviations in the primary eye position on the first visit.

The most common causes of IFNP were microvascular (47 percent of cases) and decompensation (33 percent of cases). Recovery rates differed significantly between these groups: 92 percent for microvascular and 55 percent for decompensation. There was no difference in age of onset or mean vertical deviation between the two groups, but there was a significant difference in mean horizontal deviation, with the microvascular group having more cases of exodeviation and the decompensation group having more cases of esodevation. 

The authors write that the fact that more than half of decompensated IFNP cases showed some recovery may suggest that the triggers of decompensation include conditions from which one can recover, such as physical fatigue or mental fatigue.