51. Sixth Nerve Disease

History: In August of 1975 J Lawton Smith describes eight anatomical subtypes of 6th cranial nerve palsy. He also goes over the anatomical course of the 6th nerve and various disease processes that affect the 6th nerve at different anatomical locations. Anatomy: J. Lawton Smith describes the course of the 6th nerve originating from the abducens nucleus in pons and subsequently talks about 6th nerve course in proximity with the pontomedullary junction. Running in proximity with the pre-pontine cistern close to apex of the temporal bone, through the cavernous sinus in proximity with the Gruber's ligament into the superior orbital fissure and then residing in the orbit supplying the lateral rectus muscle. He talks about origins of all the cranial nerves from the brainstem. Pathology: Depends on the anatomical site at which sixth nerve is affected. Pontine lesion, pontine infarct, abducens or cranial nerve lesion, meningitis, fracture of the base of the skull, increased intracranial pressure. Disease/Diagnosis: Abducens or 6th cranial nerve palsy; pontine infarct; anterior and inferior cerebellar artery syndrome; Internuclear ophthalmoplegia; Differential diagnosis: a. Old squint. b. Duane syndrome. c. Thyroid eye disease. d. Spasm of near reflex. e. Entrapment syndrome f. Secondary fibrosis of the medial rectus. g. Diabetic ophthalmoplegia. Clinical: Forced duction test for thyroid eye disease. Pupils constrict when the patient is asked to look to one side, the eye does not abduct in the spasm of near reflex. Anisometropia will be seen in old squints.The globe retracts when the patient tries to adduct in Duane syndrome. Neuroimaging: Head x-ray, orbital venography with substraction to demonstrate cavernous sinus lesion.