Assessment of Amplitude Is Critical In Determining The Severity Of Cubital Tunnel Syndrome
Hollie A. Power, MD1; Ketan Sharma, MD, MPH1; Madi El Haj, MD MSc1; Amy M. Moore, MD1; J. Megan M. Patterson, MD2; Susan E. Mackinnon, MD1
1Division of Plastic and Reconstructive Surgery, Washington University School of Medicine, St. Louis, MO, 2Department of Orthopaedics, University of North Carolina, Chapel Hill, NC
Background
Cubital tunnel syndrome has a spectrum of presentations ranging from mild paresthesias to debilitating numbness and intrinsic atrophy. Commonly, classification of severity relies on clinical symptoms and slowing of conduction velocity (CV) across the elbow. However, changes in compound muscle action potential (CMAP) amplitude more accurately reflect axonal loss. We hypothesized that CMAP amplitude would better predict functional impairment and disease severity than CV alone.
Methods
A retrospective cohort of patients presenting to a single surgeon's practice with cubital tunnel syndrome between 2013-2017 were included in the study. All patients had nerve conduction study and electromyography performed at our institution. Clinical and electrodiagnostic variables were recorded. The primary outcome was preoperative functional impairment, defined by grip and key pinch strength ratio. Multivariate regression identified which clinical and electrodiagnostic variables predicted preoperative functional impairment.
Results
A total of 83 patients were included in the study with average age of 57 years (75% male). The majority of patients (87%) had abnormal electrodiagnostic studies: 54% had reduced CMAP amplitude and 74% had slowing of CV across the elbow. On univariate analysis, older age and longer symptom duration were both significantly associated with reduced CMAP amplitude and slowing of CV across the elbow, whereas BMI, laterality, primary vs. revision surgery, DASH and VAS scores were not. Multivariate regression analysis demonstrated that reduced first dorsal interosseous (FDI) CMAP amplitude independently predicted loss of preoperative grip and key pinch strength, while slowed CV across the elbow did not.
Conclusions
Reduced FDI CMAP amplitude predicted preoperative weakness in grip and key pinch strength, while isolated slowing of CV across the elbow did not. CMAP amplitude is a sensitive indicator of axonal loss and an important marker of the severity of cubital tunnel syndrome. It should be considered when counseling patients regarding their prognosis and determining the necessity and timing of operative intervention.
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