Male, 37yo, only symptom is lhermitte's, brain mri is clear. is ms still most likely cause or something else?

Lhermitte phenomenon: Most commonly seen in ms, is rarely seen in other cervical cord lesions, including subacute combined degeneration(b12 deficiency; nitrous oxide), spondylosis, prolapsed/herniated disc, spinal tumors, syringomyelia, post-radiation myelopathy/radiation myelitis & following trauma. In a young person w/ no history of trauma or radiation & normal cervical mri, ms is still likely. Need cervical mri.

Significance: Lhermitte's is sign of problem involving posterior columns of spinal cord, and could be due to vertebral or disc compression, B-12 deficiency, and even MS. Did you have spinal MRI? If you possess but one localized area of complaint, this is not an initial sign of MS until other possibilities are excluded.