Case of the Week

Background:

• Trigeminal neuropathy is a debilitating condition characterized by symptoms such as facial numbness and neuralgia.

Clinical Presentation:

• Although rare, absence of the Meckel cave has been reported in the literature as a potential cause of patients with trigeminal numbness and/or neuralgia, with an unknown etiology. Possible explanations include acquired constriction or failure in the development of the subarachnoid space within the cave. In some reported cases, the onset of symptoms occurring later in life may be attributed to changes in dural compliance and thickness, resulting in progressive compression of the Gasserian ganglion with aging. Interestingly, facial numbness can precede trigeminal neuralgia in patients with the absence of Meckel cave, as seen in our case.

Key Diagnostic Features:

• MRI is the best imaging tool for investigating trigeminal neuropathy in such cases, as it can reveal Meckel cave asymmetry resulting from the lack of CSF content on the affected side, which can be observed on T2-weighted images, including high-resolution, heavily T2-weighted sequences. Furthermore, MRI can demonstrate asymmetry of the trigeminal nerves with size reduction on the affected side, especially on its cisternal segment.

Differential Diagnoses:

Other main causes of trigeminal neuropathy, which can all be assessed with MRI, include:

• Neurovascular conflict, characterized by the compression of the cisternal segment of the trigeminal nerve at the transition zone caused by a loop of an artery or vein
• Neoplasms and other expansile lesions, such as primary trigeminal nerve neoplasms (eg, schwannomas), or compression resulting from lesions in the cerebellopontine angle (eg, vestibular schwannomas or arachnoid cysts)
• Perineural spread of neoplasms through the trigeminal branches; T1-weighted images with contrast may reveal thickening and abnormal enhancement of the nerve sheath
• Multiple sclerosis with demyelinating lesions along the trigeminal pathway in the brainstem; These lesions typically appear as foci with high signal intensity on T2-weighted images, and contrast enhancement may be observed if there is associated inflammatory process.

Treatment:

• Treatment options for trigeminal neuropathy related to absence of the Meckel cave include balloon compression (the inflation of the balloon may restore the CSF space of the Meckel cave) and stereotactic radiosurgery. Medication can also be used to provide symptomatic relief if neuralgia is present. Many of these cases are, however, resistant to medical treatment.