Canis ISSN: 2398-2942

Oculomotor neuropathies

Synonym(s): Oculomotor neuritis, cranial nerve III

Contributor(s): Rodney Bagley


  • Anatomy: oculomotor nerve supplies extrinsic muscles of eye the levator palpebrae muscle (not - lateral rectus muscle, dorsal oblique muscle and retractor bulbi muscle), and carries the parasympathetic innervation to ciliary muscle and constrictor of the pupil.
  • Cause: lesions can occur at any point in the course of the nerve, eg inflammation (encephalitis/meningitis), compression (space occupying lesions or increased intracranial pressure).
  • Signs: ventrolateral strabismus (squint), inability to move eyeball towards nose, paralysis of upper eyelid (ptosis), narrow palpebral fissure; if unilateral → dilated unresponsive pupil → inequality in resting pupil diameter.
  • Diagnosis: signs, CT, MRI.
  • Prognosis: guarded.




Anatomy and function
  • The oculomotor nerve serves two functions:
    • Movement of the eye.
    • Pupillary constriction.
  • These functions are controlled by differing nuclear areas.
  • The extra-ocular motor nucleus of cranial nerve (CN) III is located in the rostral mesencephalon at the level of the rostral colliculus.
  • Axons pass ventrally through the tegmentum, exit the brain stem on the lateral side, course rostrally lateral to the hypophysis in the cavernous sinus to exit the skull in the orbital fissure.
  • CN III innervates the dorsal ventral and medial rectus, the ventral oblique (extorts the globe) and the levator palpebrae muscles.
  • The afferent pathway for pupillary constriction during light stimulation is the same as for vision from the globe to the lateral geniculate body.
  • Prior to synapse in this region, the fibers responsible for the PLR leave the visual pathway and project to the pretectal nucleus in the midbrain.
  • Here these fibers project bilaterally to the parasympathetic nucleus of CN III (PSN CN III) (Edinger Westfall nucleus in human beings).
  • This nucleus is located rostral to the motor nucleus of III at the level of the rostral part of the rostral calliculus and the pretectal area.
  • The axons course ventrally and leave the brainstem with the motor component of III medial to the cres cerebri in the lateral part of the intercural fossa.
  • The parasympathetic portion of this nerve can usually be visualized on the medial side of CN III where it is susceptible to compression during brain herniation.
  • The nerve next courses in the cavernous sinus and leaves the skull in the orbital fissure.
  • Behind the globe, ventral to the optic nerve, the pre-ganglionic fibers synapse in the ciliary ganglion.
  • The post-ganglionic fibers pass by way of the short ciliary nerves along the optic nerve to innervate the ciliary muscle and constrictor of the pupil. In dogs there are 5-8 short ciliary nerves.
  • The PSN of CN III is excited bilaterally, with impulses traveling in this nerve to the eye.
  • The contralateral projection is functionally greater than the ipsilateral projection for this reflex, making the direct PLR stronger than the indirect.
    Clinical abnormalities of CNs III, IV and VI combined
  • Paralysis of all muscles responsible for eye movement is termedcomplete ophthalmoplegia.
  • Cranial nerves III, IV and VI, the sympathetic innervation and CN V (ophthalmic branch) lie ventrally in the skull in the cavernous sinus (venous system which encircles the pituitary fossa).
  • These nerves all exit the skull through the orbital fissure.
  • Lesions on the floor of the skull in this area may damage this combination of nerves and result in thecavernous sinus syndrome.
  • Inflammation or degeneration of the oculomotor nerve → loss of oculomotor nerve function → loss of tone in extrinsic eye muscles except for lateral rectus and dorsal oblique muscles → ventrolateral squint.
  • Paralysis of levator palpebrae muscle → small palpebral fissure (ptosis).
  • Loss of parasympathetic supply to ciliary muscle and constrictor of pupil → dilated pupil which is unresponsive to light → when light is shone in the eye with the dilated pupil, the pupil of the opposite eye constricts.


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Further Reading


Refereed papers