Diagnosis: Complete CNIII (oculomotor) palsy of the right eye secondary to compressive PCom aneurysm
A complete, isolated CN III palsy presents as an eye which is “down and out” in primary gaze and complete ptosis of the upper eyelid. Its causes can be classified under 2 main categories: ischemia or compression. In this case, the cause was by extrinsic compression in the setting of a posterior communicating (PCom) aneurysm. Another compressive etiology for CNIII palsy is uncal herniation in the setting of intracranial hemorrhage.
A review of the neuroanatomy of the oculomotor nerve and its innervations can help to determine the etiology of a complete CNIII palsy. The oculomotor nerve can be divided into two main parts: the somatic fibers and the parasympathetic fibers. The somatic fibers run inside the nerve and innervate the superior rectus, medial rectus, inferior rectus, inferior oblique, and levator palpebrae muscles. The parasympathetic fibers run on the periphery of the nerve and control pupillary constriction, innervating the sphincter pupillae muscle. When the ocular muscles innervated by the third nerve are impaired, there is unopposed action from the superior oblique (innervated by CN IV) and lateral rectus (innervated by CN VI), resulting in the characteristic “down and out” gaze. The most common cause of damage to the somatic fibers is ischemia, with type II diabetes mellitus being a common culprit. If the parasympathetic fibers are affected, the “down and out” presentation may be accompanied by mydriasis. Because the parasympathetic fibers run along the peripheral aspect of the oculomotor nerve, they are most susceptible to insult by compressive forces, such as mass or aneurysm. It’s important to note that CNIII palsy can also be caused by other insults along its course from the brainstem, in which case it may be associated with other neurologic deficits, or incompletely affected. (Above left image modified from image by Patrick J. Lynch, medical illustrator and C. Carl Jaffe, MD, cardiologist, License: CC A 2.5 Generic).
A study led by neuro-ophthalmologist Dr. Rucker evaluated various causes of CN III palsy. Of 335 cases in the study, about 20% were caused by a Circle of Willis aneurysm (eg., PCom aneurysm). Other causes of third nerve palsy included tumor, head trauma, vascular disorders (ie., hypertension or arteriosclerosis) including the presence of diabetes mellitus, as well as encephalitis, herpes zoster ophthalmicus, migraine, temporal arteritis, and syphilis, among other etiologies. So there are many possible causes of CN III palsy! Today, research suggests that the most common cause in adults is due to nerve ischemia, in the setting of poorly controlled diabetes and hypertension. This study marked the first observations that the pupil was affected in the majority of cases due to compressive causes (90/99 affected pupils), such as tumor or aneurysm while the pupil was spared in most of the third nerve palsies due to vascular disorders (52/63 normal pupils). When treating patients, if the pupil is involved then it is assumed to be secondary to an aneurysm until proven otherwise. With that said, given that an expanding aneurysm presents a life-threatening emergency, and still ~10% (9/99) with compressive cause lacked pupillary involvement, all patients need stat neuroimaging.
It’s critical to keep in mind that Pcom aneurysms can lead to and present with subarachnoid hemorrhage. If someone presents with a third nerve palsy in the setting of an excruciating sudden onset “thunderclap headache,” this is an emergency most likely due to aneurysm rupture. Studies have suggested that about one third of patients who have a Pcom aneurysm present with a third nerve palsy. This patient with CN III palsy requires urgent workup because it indicates imminent rupture of an aneurysm. While there are several factors to take into account when a third nerve palsy presents, including associated neurologic defects, patient age and risk factors, and more, all patients should undergo neuroimaging. The neuroimaging should include CT or MRI and CTA or MRA with contrast. While cerebral angiogram is the gold standard diagnostic measure, this is not performed due to its invasive and high risk nature. Only when the imaging is not conclusive and the suspicion remains high for compressive cause of CN III palsy should angiogram be completed.
The treatment for a third nerve palsy depends on its etiology. In the case of microvascular cause and no associated neurologic defects, observation is generally appropriate with treatment focused on managing the chronic disease, such as diabetes mellitus. Ischemic motility disturbances often resolve over time. If there are residual disturbances after at least 3 months, surgical correction to realign the eyes may be considered.
When an aneurysm is present, treatment is aimed at preventing rupture which can lead to subarachnoid hemorrhage. Patients will likely undergo neurosurgical procedures such as clipping or coiling of the aneurysm. Studies have shown that most patients should have full vision recovery after timely treatment.
References and Additional Resources:
1. Ko, J. H., and Y. J. Kim. “Oculomotor nerve palsy caused by posterior communicating artery aneurysm: evaluation of symptoms after endovascular treatment.” Interventional Neuroradiology 2011;17(4): 415-419.
2. “EyeWiki: Acquired Oculmotor Nerve Palsy.” Available at: https://eyewiki.aao.org/Acquired_Oculomotor_Nerve_Palsy.
3. Rucker, CW. Paralysis of the third, fourth and sixth cranial nerves. American Journal of Ophthalmology 1958;46(69):787-794.
Neuro-oph Case 4 Index