Why is it important to look for pupil involvement in 3rd nerve palsy?
-The pupillomotor parasympathetic fibres are located superficially in the superomedial part of the 3rd nerve.
-They derive their blood supply from the pial blood vessels.
-The main interior nerve trunk is supplied by the vasa nervorum.
-Dilated fixed pupil with defective accommodation is the usual finding.
PUPIL INVOLVING- ‘Surgical’ causes such as aneurysms, trauma and uncal herniation characteristically involve the pupil by compressing the pial blood vessels and the superficially located pupillary fibres.
PUPIL SPARING- ‘Medical/ Ischemic’ causes such as occur in hypertension and diabetes usually spare the pupil. This is because the microangiopathy associated with medical lesions involves the vasa nervorum, causing ischaemia of the main trunk of the nerve, leaving the superficial pupillary fibres intact.
-Age > 50 years with pupil sparing: Rule out ischemic causes (Blood sugar, BP, Lipid profile etc).
-Age < 50 years or pupil involving cases: CT/MRI, Cerebral angiography to r/o surgical lesions.
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