Spinal myoclonus in the periscapular muscles after mastectomy assessed by FDG-PET

We describe a 64-year-old woman who developed spinal myoclonus around the left scapula after long thoracic nerve injury by mastectomy. Involuntary muscle twitching was semi-rhythmic, and ultrasonography identified contraction of the serratus anterior, teres major, and rhomboid muscles. FDG-PET imaging revealed markedly increased glucose uptake only in the serratus anterior. Lidocaine injection into this muscle resulted in complete cessation of the involuntary movement, and then she was successfully treated with botulinum toxin type A. These findings raise the possibility that the myoclonus was primarily caused by ectopic firing of the injured long thoracic nerve, then spreading to adjacent muscles possibly via a central mechanism mediated by group Ia afferents. The new imaging tools, such as FDG-PET and ultrasonography, were useful to determine the therapeutic target muscle.