Elderly woman develops sudden onset of a right sided partial ptosis and a divergent squint , there is no pupil asymmetry. When asked to raise both arms and she has weakness of the left arm in comparison with the right arm. The likely cause is :
a) pontine lesion
b) lesion at cerebellpontine angle
c) Weber's syndrome
d) mid-brain vascular lesion
e) diabetic third nerve palsy
Answer:
d) mid-brain vascular lesion
Explanation
This patient has a right sided partial ptosis and a divergent squint due to pupil sparing third nerve (oculomotor) palsy�that is, there is no pupil asymmetry. The patient also has left hemiparesis .
The likely cause is a crossed hemiplegia due to a vascular lesion affecting the right mid-brain involving the right third nerve and the corticospinal tract at the level of the cerebral peduncles.
The third nerve nucleus has two components in the mid-brain: the Edinger-Westphal nucleus and the motor oculomotor nucleus.
The Edinger-Westphal nucleus is situated in the upper mid-brain and gives rise to the preganglionic parasympathetic neurones responsible for the pupilloconstrictor fibres.
The motor third nucleus is situated in the lower mid-brain and gives rise to the motor fibres supplying the extraocular muscles.
The pupilloconstrictor fibres and those innervating the levator palpebrae superioris lie superficially in the trunk of the nerve.
The sudden occurrence of the pupil sparing third nerve palsy on the right side and hemiplegia on the left side indicate a mid-brain vascular lesion for the clinical manifestations of the patient. Pupil sparing third nerve palsy could occur both intra-axially (within the mid-brain) or extra-axially, that is, in the nerve trunk. Pupil sparing third palsy is a recognised complication in some patients with diabetes mellitus, and in ischaemic damage of the nerve
The classical description of the Weber's syndrome includes ipsilateral third nerve palsy with mydriasis and crossed hemiplegia. However, the patient described here is similar to classical Weber's syndrome but the pupils were spared.