Oculo-motor cranail nerves, anatomy, physiology, examination, syndromes
Anatomy: The oculomotor cranial nerves control eye movements and maintain proper eye position.
CN III – Oculomotor Nerve → Motor and parasympathetic fibers.
Innervates: Superior rectus. Inferior rectus. Medial rectus. Inferior oblique. Levator palpebrae superioris (elevates eyelid)
Oculomotor Nucleus: Origin point for voluntary motor commands; located in the midbrain. Edinger-Westphal Nucleus: Origin point for Parasympathetic fibers and it's located just posterior to the main nucleus
Trochlear Nerve Nucleus Location: Originates in the caudal midbrain at the level of the inferior colliculus, within the periaqueductal gray matter. Decussation: Nerve fibers decussate (cross to the opposite side) within the midbrain before exiting the brainstem. Therefore, the nucleus on the left controls the right eye's muscle.
Abducens Nerve: Nucleus Location: caudal pons.
Parasympathetic fibers:
- Sphincter pupillae → pupillary constriction
- Ciliary muscle → accommodation
CN IV – Trochlear Nerve Motor
Innervates: Superior oblique muscle
Function: Depression and intorsion of the eye, especially when adducted
CN VI – Abducens Nerve Motor
Innervates: Lateral rectus muscle
Function: Abduction of the eye
Physiology
Functions
- Conjugate eye movements
- Maintenance of binocular vision
- Eyelid elevation (CN III)
- Pupillary constriction (CN III)
- Accommodation for near vision (CN III)
Examination
- 1. Inspection
- Observe: Eye position. Ptosis. Strabismus. Pupil size and symmetry
- 2. Eye Movements: Ask the patient to follow a target through the 6 cardinal positions of gaze.
- Assesses: CN III. CN IV. CN VI
- Look for: Limitation of movement. Diplopia. Nystagmus
- 3. Pupillary Reflexes
- Light Reflex: Light in one eye should cause Direct constriction. Consensual constriction
- Accommodation Reflex: Patient focuses from a distant to a near object.
- Normal response: Convergence. Pupillary constriction. Lens accommodation
Syndromes:
1.Oculomotor Nerve (CN III) Palsy
Causes: Diabetes mellitus. Posterior communicating artery aneurysm. Brainstem lesions. Increased intracranial pressure
Clinical Features: Ptosis. Dilated pupil (mydriasis). Loss of accommodation. Eye deviated "down and out". Diplopia
Reason: Only lateral rectus (VI) and superior oblique (IV) remain functional.
2.Trochlear Nerve (CN IV) Palsy
Clinical Features: Vertical diplopia. Difficulty looking down. Problems walking downstairs or reading. Head tilted away from affected side
3.Abducens Nerve (CN VI) Palsy
Clinical Features: Inability to abduct the eye. Medial deviation (esotropia). Horizontal diplopia
Causes: Raised intracranial pressure. Diabetes. Brainstem lesions
4.Internuclear Ophthalmoplegia (INO)
Cause: Lesion of the medial longitudinal fasciculus (MLF).
Features: Impaired adduction of the affected eye. Nystagmus of the abducting eye. Convergence preserved
Common Cause: Multiple sclerosis. Brainstem stroke