What is the best diagnostic approach for a patient with 3rd (oculomotor) and 6th (abducens) nerve palsy, pupil sparing, decreased vision (visual impairment) and diplopia (double vision)?

Diagnostic Approach for Combined 3rd and 6th Nerve Palsy with Pupil Sparing

The lesion is most likely located in the cavernous sinus or orbital apex, and MRI with contrast is required for proper evaluation of this clinical presentation.

Localization of the Lesion

• Multiple ipsilateral cranial nerve palsies affecting cranial nerves III and VI suggest a lesion at the cavernous sinus or orbital apex 1
• The combination of decreased vision (6/60 in left eye) with diplopia and cranial nerve palsies further supports localization to the cavernous sinus or orbital apex, as pathology in these locations can affect both ocular motility and visual function 1
• Pupil-sparing in a third nerve palsy typically suggests a microvascular etiology, but when combined with 6th nerve palsy and vision loss, a compressive, infiltrative, or inflammatory lesion is more likely 1, 2

Diagnostic Imaging Recommendations

• MRI with contrast is the preferred imaging modality for this presentation as it provides superior evaluation of the cranial nerves, cavernous sinus, and orbital apex 1, 2
• Plain MRI without contrast is insufficient for proper evaluation of this clinical scenario, as contrast enhancement is essential to detect inflammatory, infiltrative, or neoplastic processes affecting the cranial nerves 1, 2
• High-resolution T2-weighted images focused on the cranial nerves should be included in the imaging protocol to better visualize the nuclear, cisternal, and skull-base cranial nerve segments 1

Additional Imaging Considerations

• Vascular imaging with MRA should be considered as a complement to the contrast-enhanced MRI to evaluate for possible vascular compression, particularly if aneurysm is suspected 1
• If a mass lesion is identified near the cavernous sinus, additional MRV (magnetic resonance venography) may be indicated to assess the integrity of the dural venous sinus 1
• CT with contrast is an alternative but less preferred option compared to MRI for evaluating the cavernous sinus and orbital apex 1, 3

Clinical Implications and Pitfalls

• The combination of 3rd and 6th nerve palsies with decreased vision requires urgent evaluation, even with pupil sparing, as this presentation is atypical for isolated microvascular disease 2, 4
• Do not rely solely on pupil sparing to rule out compressive lesions, especially when multiple cranial nerves are involved and vision is affected 1, 5
• Recent studies have shown that 16.5% of patients with acute ocular motor mononeuropathies have structural lesions on MRI, supporting the need for neuroimaging regardless of presumed etiology 4
• Plain CT without contrast is largely not useful in the workup of diplopia and should be avoided 3

Differential Diagnosis to Consider

• Cavernous sinus pathology: meningioma, schwannoma, metastatic lesions, inflammatory or infectious processes 1
• Orbital apex syndrome: tumors, inflammatory disease, infection 1
• Basilar subarachnoid space pathology: infectious meningitis (TB, fungal, Lyme disease) or non-infectious causes (sarcoid, neoplasm) 1
• Less likely given the pupil sparing: posterior communicating artery aneurysm 6