Misdirected regeneration of abducens nerve neurons into the parasympathetic pupillary pathway.

A 16-year-old girl suffered a severe head trauma with paralyses of the right oculomotor, trochlear, and abducens nerves. Three years later, only the abducens nerve had recovered. The pupil did not react to light or near. However, the diameter of the pupil correlated with horizontal gaze: with the gaze to the left, the diameter of the pupil was 8 mm, with the gaze straight ahead, 6 mm, and with the gaze to the right, 4 mm. With saccades to the right, the pupil started to constrict immediately and reached its smallest diameter after about 2 s. We conclude that the constriction of the pupil was due to misdirected regeneration of abducens nerve neurons into the parasympathetic pathway of the oculomotor nerve.