THIRD FOURTH AND SIXTH CRANIAL NERVES

THIRD FOURTH AND SIXTH CRANIAL NERVES

THIRD, FOURTH AND SIXTH CRANIAL NERVES are called oculomotor nerves as they supply the extraocular muscles. These are separately known as oculomotor (3rd), trochlear (4th) and abducent (6th) nerves.

CAUSES OF LESION

General

Acute infective polyneuritis, diabetic neuropathy, polyneuritis cranialis, multiple sclerosis, encephalitis, neurosyphilis, head injury, increased intracranial pressure, sarcoidosis, myasthenia, dysthyroid ocular disease.

Brain stem

Cerebrovascular accidents, space occupying lesion, uncal or tentorial herniation due to supratentorial mass lesion, syringobulbia.

Base of brain

Meningitis, neoplasm, fracture base of skull, aneurysm of circle of Willis particularly of posterior communicating artery.

Orbital and periorbital

Cavernous sinus thrombosis, aneurysm of internal carotid artery, retro-orbital tumour, meningioma, malignant exophthalmos.

EFFECTS OF LESIONS

Third Cranial Nerve

1. Ptosis or dropping of the upper eyelid.

2. Inability to move the eyeball upwards, downwards and medially (external ophthalmoplegia).

3. External strabismus or squint.

4. Dilated pupil (internal ophthalmoplegia).

5. Loss of light reflex.

6. Loss of accommodation reflex

7. Diplopia is not usually a complaint but develops during elevation of the paralysed upper eyelid. Pupillary involvement, e.g. dilated pupil which does not react to light or accommodation is an important sign differentiating surgical from medical causes of isolated 3rd nerve palsy.

Fourth Cranial nerve

Isolated lesion is rare, usually it develops in association with 3rd and 6th nerve palsy.

1. Diplopia, particularly when the patient tries to look in the direction of action of superior oblique muscle.

There is characteristic vertical diplopia which becomes apparent on attempted reading or descending stairs. When there is bilateral 4th nerve palsy, trauma is the major cause.

2. Inability to look downwards and inwards.

3. Internal strabismus or squint.

4. Ocular posture-head is inclined forwards and towards the shoulder of the healthy side to avoid giddiness.

Sixth Cranial Nerve

1. Horizontal Diplopia, particularly when the patient tries to look externally to the affected side and on looking into the distance.

2. Inability to look outwards .

3. Internal strabismus or squint.

4. Ocular posture-head is turned towards the affected side. Bilateral 6th nerve palsy is an important sign of raised intracranial tension particularly in children.

It is a false localising sign for the site of lesion. CT or MRI will be helpful for diagnosing the site and cause of any occulomotor palsy.

TREATMENT OF THIRD, FOURTH AND SIXTH CRANIAL NERVES

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The underlying cause should be treated. To overcome diplopia frosted glass may be used in one eye.