(Kiosk 3) Complex Motor Unit Action Potential Analysis Secondary to Concurrent Myopathic and Neuropathic Recruitment Patterns in a Patient with Chronic Polyradiculopathy and Acute Necrotizing Myositis

Case Diagnosis: 70-year-old male with a past medical history of type 2 diabetes mellitus along with severe cervical and lumbar stenosis diagnosed with an acute necrotizing myopathy.

Case Description or Program Description: The patient presented with an insidious six-month history of progressive bilateral lower extremity weakness. His symptoms were associated with low back pain, lower extremity paresthesias, and more recently bilateral, proximal upper extremity weakness. He stated that he was having difficulty with mobility and abducting his shoulders above parallel.

Setting: Tertiary care hospital

Assessment/Results: Physical examination revealed significant weakness in shoulder abduction and hip flexion bilaterally. MRIs of the cervical and lumbar spine revealed severe multilevel stenosis. Laboratory studies revealed elevated creatine kinase at 2005. Electrodiagnostic testing was consistent with chronic polyradiculopathies in the cervical and lumbar regions. Additionally, myopathic recruitment was appreciated in the vastus medialis oblique along with myotonic discharges were seen in proximal muscle groups. Muscle biopsy of the right thigh revealed an active necrotizing myopathic process and HMG CoA-reductase autoantibodies were positive suggestive of a statin induced necrotizing myopathy.

Discussion (relevance): Overall, the electrodiagnostic study was difficult to interpret given complexity of having a concomitant myopathic and neuropathic process. The short-duration and small-amplitude MUAPs were only appreciated by instructing the patient to give a small amount of force with contractions. If the patient progressed to a large muscle force too quickly then the myopathic recruitment would be missed and replaced with long-duration large-amplitude MUAPs and an increased recruitment ratio.

Conclusions: This case brings awareness to the complexities of MUAP analysis for physiatrists who perform electrodiagnostic testing. It is imperative to be cognizant not only of the sounds and waveforms that are produced but to understand how those relate to the amount of force produced with muscle contractions. Physiatrists who are attentive to this will arrive at appropriate diagnoses and allow for optimized outcomes for their patients.