Brainstem Pathways for Horizontal Eye Movement: Pathologic Correlation with MR Imaging

Horizontal eye movements are conducted by the medial rectus and the lateral rectus muscles, which are innervated by the oculomotor nerve (cranial nerve III) and the abducens nerve (cranial nerve VI), respectively. The oculomotor and the abducens nuclei are interconnected by a tract in the brainstem named the medial longitudinal fasciculus (MLF). Through the MLF, the actions of the oculomotor and the abducens nuclei are coordinated, generating conjugate horizontal eye movements. The disorders of horizontal eye movement that are caused by brainstem lesions are classified into three groups: (a) lateral gaze palsy, (b) internuclear ophthalmoplegia, and (c) one-and-a-half syndrome. Lateral gaze palsy is caused by a lesion involving the paramedian pontine reticular formation (PPRF) or the abducens nucleus. Internuclear ophthalmoplegia occurs as a result of a lesion involving the MLF. One-and-a-half syndrome is a combination of lateral gaze palsy and internuclear ophthalmoplegia and is caused by a lesion involving both (a) the ipsilateral PPRF or the ipsilateral abducens nucleus and (b) the ipsilateral MLF. The pathologic lesions depicted on magnetic resonance images were topographically well correlated with the brainstem pathways and each type of horizontal eye movement disorder. Most of the lesions were tiny acute infarctions and were found in the most posterior region of the pons, which corresponded to the location of the brainstem pathways. Therefore, awareness of the brainstem pathways controlling horizontal eye movement is important to avoid missing a small pontine lesion.