Clinical evaluation for aneurysm in patients with third cranial nerve palsy

A patient presenting with a third nerve palsy might be harboring a life-threatening aneurysm. The diagnostic evaluation of third nerve palsy has evolved in the last few years because newer, relatively noninvasive, safe, specific and sensitive neuroimaging studies have been developed that can detect an aneurysm without a traditional catheter-based angiogram. In many centers, computed tomography (CT) angiography (CTA) is rapidly becoming the initial imaging study of choice for patients with third nerve palsy in order to rule out aneurysm. Unfortunately, MRI remains superior to CT scan for the evaluation of nonaneurismal etiologies of third nerve palsy. Whether the clinician selects CT/CTA or MRI with magnetic resonance angiography as the initial study, followed by the other modality if this study is negative, remains an individual clinical decision but many centers prefer CTA first. The clinician should be confident in the capability, availability and reliability of the interpreting neuroradiologist and the imaging technology at their individual institution before relying solely upon noncatheter techniques to exclude aneurysm. Neuro-ophthalmology referral or close coordination and communication among ophthalmologists, neurologists and neurosurgeons are critical for the appropriate management of these cases. If the clinical risk of aneurysm is low or if the risk of traditional catheter angiography is high (e.g., in the elderly or individuals at high risk of renal failure or stroke), then an initial less invasive angiography study (e.g., MRI/ magnetic resonance angiography or CTA) might be sufficient. If, on the other hand, the patient has a moderate, high or uncertain risk of aneurysm (e.g., insufficient quality of imaging or unreliable interpretation), then an MRI/magnetic resonance angiography or CTA combination may not be sufficient. In these cases, my recommendation would be to still consider catheter angiography if the noninvasive study can not confidently exclude aneurysm or the post-test clinical suspicion remains high for aneurysm.