[Oculomotor paralysis caused by lesion of the brain stem]

Oculomotor palsies due to lesions of the brain stem may be complete, affecting all types of eye movements in one direction; this reflects a nuclear, internuclear or infractuclear lesion. This may also be disassociated, affecting only one type of eye movements, and this suggests supranuclear lesions. Concerting lateral movements, complete paralysis of abduction, adduction (sparing convergence) or ipsilateral laterality is suggestive of damage to the radicular fibres of the abducencs nucleus (VI), to its internuclear neurons or to the nucleus itself respectively. Isolated paralysis of saccades is in favour of a lesion of the paramedian pontine reticular formation. As regards vertical movements, paralysis of the oculomotor nerve (III), either isolated or associated with complete paralysis of the contralateral rectus superior oculi muscle, suggests a lesion of the radicular fibres of III or of the nucleus itself respectively. Paralysis of downward or both downward and upward saccades indicates the presence of bilateral lesions in the region of the mesencephalic reticular formation. Paralysis of upward saccades alone reflects a unilateral lesion located near the posterior commissure.