Lacosamide-Responsive Paroxysmal Tonic Spasms Associated With a New Lateral Cerebral Peduncle Lesion in Multiple Sclerosis: A Case Report.

Paroxysmal tonic spasms (PTS) are abrupt, brief, involuntary, and often painful muscle contractions commonly seen in demyelinating disorders such as multiple sclerosis (MS) and neuromyelitis optica spectrum disorder (NMOSD). These movements are frequently associated with new demyelinating lesions in the corticospinal tract. First-line treatment is usually carbamazepine, a sodium channel blocker, but intolerance and side effects may limit its use. We report a case of a 31-year-old woman with clinically stable relapsing-remitting MS who developed sudden, painful spasms of the left upper limb. MRI of the brain showed a new lesion in the lateral part of the right cerebral peduncle, consistent with her symptoms. She was first started on carbamazepine, which was discontinued due to gastrointestinal side effects. Lacosamide, another antiepileptic with a similar mechanism of action, was introduced at a dose of 50 mg twice daily, resulting in rapid and complete resolution of her symptoms. During follow-up, the patient remained symptom-free and clinically stable. This case highlights the occurrence of PTS secondary to a lateral cerebral peduncle lesion. Lacosamide was highly effective in controlling symptoms, suggesting it is a good alternative for patients who cannot tolerate carbamazepine. Awareness of PTS and timely imaging can guide adequate therapy and improve patients' quality of life. Lacosamide may represent an effective and well-tolerated alternative when first-line therapy cannot be used.