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Down and Out!


The third cranial nerve/ oculomotor nerve supplies the extraocular muscles of the eye (SR, LR, MR, IO) except for the LR and SO, the LPS which supplies the upper eyelid and the sphincter pupillae. The third nerve can be involved anywhere along its course from its nucleus in the midbrain to the extraocular muscles.

Causes of 3rd nerve palsy:

  • Idiopathic

  • Vascular- Diabetes, Hypertension, Hyperlipidemia etc

  • Aneurysm- Posterior communicating artery or its junction with internal carotid artery

  • Trauma- subdural hematoma with uncal herniation, base of skull fracture

  • Inflammation/infection- linked to cavernous sinus thrombosis

  • Intracranial tumours

  • Autoimmune/ Demyelinating diseases etc. Third nerve palsy can be partial/complete. Patient presents with complaints of drooping of eyelid and binocular double vision.

Clinical signs:

  • Ptosis

  • Abduction present

  • Limitation of elevation

  • Limitation of depression

  • Limitation of adduction

  • Position of eye- Down and out (Exotropia and Hypotropia)

‘Out’ because the lateral rectus (innervated by the 6th nerve) maintains muscle tone in comparison to the paralyzed medial rectus.

‘Down’ because the superior oblique (innervated by the 4th nerve), is unantagonized by the paralyzed superior rectusinferior rectus and inferior oblique.

Examination of PUPIL is very important in 3rd nerve palsy.

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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