Musculocutaneous Neuropathy and Lateral Antebrachial Cutaneous SNAP
Yes, musculocutaneous neuropathy will cause absence of the lateral antebrachial cutaneous (LABC) sensory nerve action potential (SNAP) on nerve conduction studies, as the LABC nerve is the terminal sensory branch of the musculocutaneous nerve. 1, 2
Anatomical Basis
The musculocutaneous nerve arises from the lateral cord of the brachial plexus (C5-C7 nerve roots) and provides motor innervation to the biceps brachii, brachialis, and coracobrachialis muscles before terminating as the lateral antebrachial cutaneous nerve, which supplies sensation to the lateral forearm. 2
When the musculocutaneous nerve is injured, the LABC SNAP will be absent or reduced because the sensory fibers traveling through the musculocutaneous nerve cannot reach the recording electrodes. 1
The LABC nerve is purely sensory in its terminal distribution, making SNAP testing a direct assessment of musculocutaneous nerve sensory fiber integrity. 3
Electrodiagnostic Findings in Musculocutaneous Neuropathy
Nerve conduction studies alone are insufficient to confirm musculocutaneous neuropathy—needle EMG examination of the biceps, brachialis, and coracobrachialis muscles must be performed as a routine part of the diagnostic evaluation. 1
Absent or reduced LABC SNAP indicates sensory fiber involvement but does not localize the lesion along the nerve course. 1
Motor nerve conduction studies of the musculocutaneous nerve are technically difficult and not routinely performed, making needle EMG critical for detecting denervation in the biceps and brachialis muscles. 1
Bilateral absence of LABC sensory responses suggests an inflammatory cause (such as neuralgic amyotrophy/Parsonage-Turner syndrome) rather than traumatic or compressive injury. 1
Clinical Presentation Patterns
Pain and sensory disturbance are more common presenting symptoms than weakness in musculocutaneous neuropathy. 1
Weakness may not be clinically apparent in some patients because other muscles (brachioradialis, pronator teres) can provide adequate elbow flexion and supination, potentially masking motor deficits. 1
Numbness, paresthesias, and dysesthesias over the lateral forearm are the predominant sensory complaints. 2, 3
Common Etiologies
The most common cause of musculocutaneous neuropathy is acute trauma or iatrogenic surgical injury (65% of cases). 1
Postsurgical etiology is the leading cause of isolated LABC neuropathy, likely from patient positioning during orthopedic procedures causing stretch or compression. 3
Antecubital fossa needle placement (phlebotomy, IV placement) is the second most common cause of LABC neuropathy. 3
Idiopathic and inflammatory causes (such as Parsonage-Turner syndrome) each account for approximately 14% of cases. 1
Other causes include strenuous activity, sports participation, cast placement, and trauma. 2
Important Diagnostic Pitfalls
Early in the disease course, all electrodiagnostic studies may be normal—repeat testing in 3-4 weeks is necessary if clinical suspicion remains high. 4
Musculocutaneous neuropathy symptoms may mimic C5-C6 cervical radiculopathy or brachial plexus lesions, requiring MRI and electrodiagnostic studies to differentiate. 2
Anatomical variations exist where the musculocutaneous nerve does not perforate the coracobrachialis muscle or shares a common sheath with the median nerve, which could cause unexpected combined paralysis patterns. 5, 6
Bilateral LABC nerve conduction studies should be performed to help establish the diagnosis and identify inflammatory causes. 3