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A 62 year old woman with new diplopia is found to have the clinical features of a left third nerve palsy. She also mentions that her right hand seems newly weak. Your examination confirms that fine finger movements of the right hand are relatively poor, but she has normal coordination in all four limbs. The right nasolabial fold is depressed and the right lower facial muscles are relatively weak. Speech is distinct, but she has circumduction of her right leg when she walks. Deep tendon reflexes are accentuated in the right extremities relative to the left extremities.
Where is the lesion?
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This is a classic “Weber syndrome” of third nerve palsy and contralateral hemiparesis. It arises from a lesion in the left third nerve fascicles as they
pass through the cerebral peduncle, which carries corticopontine fibers that cross in the rostral pons and corticospinal fibers that cross in the medullary
pyramids and innervate muscular on the opposite side of the body.
Although most third nerve palsies have no accompanying neurologic abnormalities, you should recognize this particular “third nerve palsy plus” syndrome because it affects the nerve in its midbrain segment. The cause is usually an infarction in the domain of an occluded paramedian arterial perforator, but inflammations and cancers can also do this.
Aneurysm is not a consideration, but rarely Weber syndrome may be an early sign of rostral basilar (basilar apex) thrombosis, a condition sometimes called “top of the basilar syndrome.” Prompt vascular imaging is necessary. If it shows a thrombus, endovascular thrombectomy may have to be deployed emergently. More often, the basilar and posterior cerebral arteries are patent, and MRI shows only restricted diffusion in the ventral midbrain
Equally often the stroke is too small to show up on MRI. Even if MRI is negative, assume there is a stroke and direct attention to future stroke prevention
caused by disorders affecting small arterial vessels.
Although most third nerve palsies have no accompanying neurologic abnormalities, you should recognize this particular “third nerve palsy plus” syndrome because it affects the nerve in its midbrain segment. The cause is usually an infarction in the domain of an occluded paramedian arterial perforator, but inflammations and cancers can also do this.
Aneurysm is not a consideration, but rarely Weber syndrome may be an early sign of rostral basilar (basilar apex) thrombosis, a condition sometimes called “top of the basilar syndrome.” Prompt vascular imaging is necessary. If it shows a thrombus, endovascular thrombectomy may have to be deployed emergently. More often, the basilar and posterior cerebral arteries are patent, and MRI shows only restricted diffusion in the ventral midbrain
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