Cubital Tunnel Syndrome Affects Both Sensory and Motor Nerves on EMG
Cubital tunnel syndrome demonstrates both sensory and motor nerve abnormalities on electrodiagnostic testing, with motor conduction velocity slowing across the elbow being the most consistent finding, though sensory nerve involvement is also characteristic and can be severe. 1, 2
Primary EMG Findings in Cubital Tunnel Syndrome
Motor Nerve Abnormalities
- Motor conduction velocity (MCV) slowing across the elbow segment is the hallmark finding, with studies showing MCV ranging from 15.9-47.5 m/s (mean 32.7 m/s) in affected patients, with abnormalities detected in 95% of cases (21 of 22 limbs). 1
- Reduced motor conduction velocity in the forearm segment occurs in approximately 59% of cases (13 of 22 limbs), with MCV ranging from 15.7-59.6 m/s (mean 40.4 m/s). 1
- Motor weakness and muscle atrophy represent advanced disease, indicating severe and long-lasting nerve impairment. 3
Sensory Nerve Abnormalities
- Absent or abnormal sensory nerve action potentials (SNAPs) following stimulation of the little finger occur in approximately 64% of cases (14 of 22 limbs). 1
- Sensory action potential dispersion above the sulcus is significantly prolonged in 84.6% of patients, even in those with EMG scores of 0/10 who present with only sensory symptoms or mild sensory deficits. 2
- In severe cases, complete absence of sensory nerve conduction represents advanced disease with poorer prognosis for recovery. 4
Clinical Correlation with EMG Severity
Disease Staging Based on Electrodiagnostic Findings
- An EMG scoring system (0-10 scale) correlates with clinical severity: Grade 1 (least severe) shows median EMG score of 0.9/10, while Grade 4 (most severe) shows 7.1/10. 2
- The scoring incorporates terminal motor and sensory latencies, motor and sensory conduction velocities, sensory action potential amplitudes and dispersions, and needle EMG findings. 2
Prognostic Implications
- Patients with absent sensory nerve conduction experience significantly less improvement of sensory symptoms after surgery (53% improvement rate) compared to the general cubital tunnel population. 4
- Motor strength improvement in severe cases with absent sensory conduction is limited, with only 13% showing improvement postoperatively. 4
- Symptom duration directly impacts recovery potential, with shorter duration (<7 months) associated with better atrophy improvement compared to longer duration (>26 months). 3
Important Clinical Pitfalls
Early disease may show normal electrophysiology, particularly when testing is performed within the first week or in patients with mild, slowly progressive, or primarily proximal symptoms—repeat testing 2-3 weeks later can be diagnostic in these cases. 5
Asymptomatic ulnar entrapment is common, occurring in 14.7% of controls overall and reaching 30% in individuals aged 60 years or older, emphasizing the need for clinical correlation. 2