How should I explain isolated prolonged peak latency of the medial antebrachial cutaneous nerve to a patient?

Explaining Isolated Prolonged Peak Latency of the Medial Antebrachial Cutaneous Nerve

An isolated prolonged peak latency of the medial antebrachial cutaneous nerve (MACN) indicates a lesion affecting the lower brachial plexus (C8-T1 roots, inferior trunk, or medial cord) and should prompt evaluation for neurogenic thoracic outlet syndrome, traumatic plexopathy, or other focal nerve pathology. 1, 2, 3

Understanding the Clinical Significance

The MACN is a purely sensory nerve arising from the C8-T1 nerve roots through the medial cord of the brachial plexus. 1 When you observe an isolated prolonged peak latency in this nerve, you are identifying a specific electrodiagnostic pattern that localizes pathology to the lower brachial plexus while sparing more distal structures. 3

Normal Values and Diagnostic Thresholds

• Normal peak latency for the MACN ranges from 2.0 to 2.2 ± 0.2 ms using a 10-cm antidromic technique, with an upper limit of normal (97th percentile) of 2.6 ms. 4, 5
• Prolonged latency is defined as a peak latency ≥2.5 ms, which has 99% specificity and 73% sensitivity for diagnosing lower plexus pathology. 2
• Borderline prolongation occurs between 2.2-2.4 ms and requires additional diagnostic criteria for confirmation. 2

Key Diagnostic Pattern

The hallmark electrodiagnostic pattern includes:

• Prolonged MACN peak latency (≥2.5 ms) 2
• Normal median and ulnar nerve conduction studies (both motor and sensory), which distinguishes this from peripheral nerve pathology 3
• Normal or only slightly reduced interference pattern in C8-T1 innervated muscles on needle EMG 3
• Interside latency difference ≥0.3 ms (89% sensitivity, 63% specificity) 2

Common Etiologies to Consider

Neurogenic Thoracic Outlet Syndrome (NTOS)

• NTOS is the most common cause of isolated MACN abnormalities in patients presenting with "carpal tunnel syndrome-like" symptoms but normal median nerve studies. 3
• MACN testing has proven to be a reliable objective test for confirming NTOS, with 40 of 41 surgically confirmed cases showing at least one abnormal MACN parameter. 2
• Patients typically present with atypical upper limb pain and paresthesias without motor deficit or atrophy. 3

Traumatic Plexopathy

• Traumatic avulsion of C8-T1 roots can produce isolated MACN abnormalities. 1
• Iatrogenic injury from medical procedures, implants, or surgeries in the flexor forearm can damage the MACN. 4
• Perioperative compression or traction with arm abduction ≥90° is a recognized mechanism. 1

Neoplastic and Other Causes

• Tumors affecting the lower brachial plexus can present with isolated MACN prolongation before other findings emerge. 3
• In one series, 5 of 16 cases had obvious traumatic or neoplastic causes for mild lower plexus lesions. 3

Diagnostic Workup Algorithm

Step 1: Confirm the Electrodiagnostic Pattern

• Verify that median and ulnar nerve conduction studies (motor and sensory) are completely normal bilaterally. 3
• Document the specific MACN peak latency value and compare to the contralateral side. 2
• Calculate the interside amplitude ratio (abnormal if ≥2.0, with mean normal + 3 SD). 3

Step 2: Apply Multiple Diagnostic Criteria

Use the four-criteria approach for NTOS diagnosis:

• Peak latency >2.4 ms (specificity 99%, sensitivity 73%) 2
• Interside latency difference ≥0.3 ms (specificity 63%, sensitivity 89%) 2
• Amplitude <10 μV (specificity 97%, sensitivity 68%) 2
• Interside amplitude ratio ≥2.0 (specificity 100%, sensitivity 61%) 2

Having 3-4 positive criteria strongly confirms the diagnosis (56% of confirmed cases), while 2 positive criteria is seen in 29% of cases. 2

Step 3: Perform Needle EMG

• Examine C8-T1 innervated muscles for denervation potentials or reduced recruitment. 3
• Normal or only slightly reduced interference patterns support a mild lower plexus lesion. 3
• Significant denervation suggests more severe or chronic pathology. 3

Step 4: Consider C8 Nerve Root Stimulation

• C8 nerve root stimulation responses below 56 m/sec were abnormal in 54% of NTOS cases. 2
• This adjunctive test helps differentiate root-level from more distal plexus pathology. 2

Step 5: Imaging When Indicated

• MRI of the brachial plexus is the gold standard, with 84% sensitivity and 91% specificity for traumatic root avulsions. 1
• MR neurography with T2-weighted sequences is the reference technique for peripheral nerve pathology. 1
• High-resolution ultrasound provides 77-79% sensitivity and 94-98% specificity as a bedside alternative. 1

Technical Considerations and Pitfalls

Optimize Recording Technique

• Use proximal stimulation (upper arm) rather than distal (elbow) stimulation to achieve larger amplitude responses (mean difference 4.4-5.2 μV), improving technical reliability. 6
• Record during slight voluntary contraction to reduce muscle artifact, which is a common technical challenge. 6
• Both antidromic and orthodromic techniques are valid, with similar latency values (mean 2.11 vs 2.10 ms). 5

Avoid Misdiagnosis

• Do not diagnose carpal tunnel syndrome based on symptoms alone when MACN is abnormal and median nerve studies are normal—12 of 16 patients in one series were initially misdiagnosed with CTS. 3
• Remember that MACN amplitude is normally smaller than lateral antebrachial cutaneous nerve amplitude (p<0.01), so do not over-interpret low amplitudes in isolation. 5
• Side-to-side amplitude differences up to 67% (onset-to-peak) or 78% (peak-to-peak) can be within normal range. 4

Clinical Implications for Management

When NTOS is Confirmed

• Surgical decompression is indicated when objective MACN abnormalities confirm the clinical diagnosis. 2
• Two cases in one series had NTOS confirmed by surgical findings after MACN testing established the diagnosis. 3

When Cause is Unclear

• Nine of 16 cases in one series had no identifiable cause and were considered mild NTOS based solely on MACN findings. 3
• These patients benefit from conservative management initially, with surgical referral if symptoms progress. 3

Prevention of Iatrogenic Injury

• Limit arm abduction to ≤90° in supine patients during procedures. 1
• Position the arm in abduction on an arm-rest with forearm in supination or neutral position. 1
• Perform periodic positioning checks during lengthy procedures. 1