Nuclear and infranuclear disorders.

Lesions of the brain stem can either affect the nuclei or the fascicles of the third, fourth or sixth cranial nerves and thus produce ocular motor disorders. Lesions of the oculomotor nuclear complex differ from lesions of the third nerve, since the motoneurones in the nucleus are specifically grouped. Similarly, a lesion of the sixth nerve nucleus results in a conjugate gaze palsy and not in an abducens palsy, because of 'interneurones' being intermingled with the abducens motoneurons. Isolated lesions of a nerve fascicle, which is the part of the cranial nerve running through the brain stem, usually cannot be distinguished clinically from lesions of the nerve outside the brain stem unless other brain stem signs are present. In the case of an isolated ocular motor nerve palsy, modern imaging techniques, particularly magnetic resonance imaging, may help to localize the lesion to the brain stem. Most often, however, brain stem lesions also involve structures surrounding the ocular motor nuclei or fascicles, sometimes leading to characteristic eponymic syndromes. In congenital eye movement disorders the pathoanatomical situation is more complex. Since the lesion takes place during intrauterine or early postnatal development, corrective misdirection of neurones occurs in addition to aplasia or hypoplasia of parts of the cranial nerves. Correspondingly, abnormal movements accompanying an attempted eye movement can be observed in some characteristic syndromes.