Unusual delayed and prolonged arm weakness and atrophy following botulinum toxin injection for musician's cramp: case report.

Botulinum toxin is considered first-line treatment for focal hand dystonia in musicians. Mild, temporary weakness is a common accompaniment of effective injection. We present a unique case of delayed-onset, severe, prolonged weakness and atrophy in a patient with musician's dystonia, successfully treated with botulinum toxin for over 10 years, following injection of his usual muscles at his well-established dose. This pianist received botulinum toxin treatment for more than 10 years, with a stable response. Six weeks after an injection, he developed progressive severe weakness and atrophy of the affected forearm involving both injected and uninjected muscles. Weakness and atrophy took over one year without further injections to resolve. The clinical course and laboratory testing were not suggestive of brachial neuritis, plexopathy, or neuralgic amyotrophy. The literature contains rare case reports of severe weakness and atrophy after botulinum toxin injection, sometimes with delayed onset and sometimes affecting distant muscles. Frequently presenting with pain, such cases often have evidence of plexopathy or neuralgic amyotrophy which were absent in our patient. Clinicians should be aware of this rare potential severe adverse event associated with botulinum toxin.