Unilateral light-near dissociation in lesions of the rostral midbrain.

Examination of the pupillary response to light and accommodation can provide precise information concerning the localization of a lesion within the sympathetic and parasympathetic pathways, the anterior visual pathway, or the brainstem (Table 1). Classical teaching has associated all lesions of the rostral midbrain with bilateral lightnear dissociation (LND), defined as attenuation of the pupil light reflex (PLR) with relative sparing of the near response, as one component of Parinaud syndrome. We describe 2 patients with rostral midbrain lesions in whom LND was clinically present only in the eye ipsilateral to the lesion. To our knowledge, this clinical phenomenon has not been previously reported. We provide an explanation for this observation and its implications for our understanding of the anatomy of the pupil light pathway. Report of Cases. Case 1. A 19-yearold Asian woman had symptoms of headache and diplopia. Oculomotor examination findings demonstrated a vertical gaze palsy with convergence-retraction nystagmus, noncomitant skew deviation, and convergence insufficiency but preservation of all other eye movements. Examination of the pupils revealed moderate anisocoria, with the right pupil being 0.50 mm larger than the left in both bright light and in the dark (Table 2). The light response (LR) and the near response in the left eye were equally brisk and normal; however, in the right eye there was attenuation of the light reflex but preservation of a normal brisk near response (LND). These findings were confirmed by formal measurement of the amplitudes of the LR and near response in each eye, recorded separately under monocular conditions, using infrared video pupillometry. In the left eye both the LR and the near response were normal wi th no LND (Figure 1A). Slitlamp examination showed no evidence of a tonic pupil (eg, irregular shape, sector palsy, tonic near response, delayed redilation on looking back into the distance) or anterior segment pathologic conditions, nor were there any clinical signs of a third cranial nerve palsy. Her visual acuity and color vision were normal. Magnetic resonance imaging of the brain showed a hyperintense lesion at the level of the rostral midbrain immediately to the right of the aqueduct of Sylvius, within the periaqueductal gray (Figure 2A and B). Case 2. A 31-year-old man had a sudden-onset headache, vomiting, and poor balance. He reported vertical diplopia and difficulty focusing on near targets. Oculomotor examination findings revealed a noncomitant skew deviation with right hypertropia on right gaze but full horizontal eye movements. Paresis of vertical eye movements and an up-gaze saccadic palsy with convergence-retraction nystagmus was present. Visual acuity and color vision were normal. Examination of the pupils under resting conditions showed a small degree of anisoco-