What is the associated dermatome and where should I test for sensation in a patient with radial nerve injury after a motor vehicle collision (MVC) with right wrist pain, reduced strength in wrist extension, and diminished right brachioradialis reflex?

Radial Nerve Injury Assessment Following Motor Vehicle Collision

The associated dermatome for radial nerve injury is C6-C7, and sensation should be tested on the dorsal aspect of the first web space between the thumb and index finger (the "anatomical snuffbox") and the dorsolateral aspect of the forearm.

Clinical Presentation and Anatomical Considerations

The patient's presentation with right wrist pain after an acceleration-deceleration injury, markedly reduced strength in wrist extension, and diminished right brachioradialis reflex is highly suggestive of radial nerve injury. This clinical picture represents a classic pattern of radial nerve dysfunction:

• Reduced wrist extension strength indicates impairment of the extensor carpi radialis longus and brevis muscles
• Diminished brachioradialis reflex indicates involvement of the C6 nerve root or the radial nerve itself
• The acceleration-deceleration mechanism in an MVC is a common cause of radial nerve injury, particularly at the spiral groove of the humerus 1

Dermatome and Sensory Testing

Primary Sensory Testing Areas:

1. First dorsal web space (between thumb and index finger) - This is the most reliable and sensitive area to test for radial nerve sensory function 1, 2
2. Dorsolateral aspect of the forearm - This area corresponds to the lateral antebrachial cutaneous nerve territory, which can overlap with radial nerve distribution 3

Anatomical Basis:

The radial nerve provides sensory innervation primarily through its superficial branch to the dorsal-radial aspect of the hand and the dorsolateral forearm. The C6 and C7 dermatomes overlap in this distribution 4.

According to the American College of Radiology guidelines, the brachial plexus is formed from the C5 to T1 ventral rami, with the radial nerve being one of its terminal branches 1, 2. The radial nerve specifically receives contributions primarily from C6-C7 nerve roots.

Assessment Algorithm

1. Test sensory function in the following areas:

   • First dorsal web space (between thumb and index finger)
   • Dorsal aspect of the thumb
   • Dorsolateral aspect of the forearm

2. Compare with the unaffected side to detect subtle differences in sensation

3. Document the type of sensory deficit:

   • Light touch (using cotton wisp)
   • Pinprick sensation
   • Two-point discrimination (if available)

Clinical Pearls and Pitfalls

• Important caveat: There is significant overlap between the lateral antebrachial cutaneous nerve and the superficial branch of the radial nerve in approximately 41.2% of individuals 3. This means sensory testing alone may not definitively distinguish between a radial nerve injury and a more proximal brachial plexus injury.

• Avoid the common pitfall of relying solely on dermatome maps, as there is considerable variation in the exact distribution of C6 and C7 dermatomes between individuals 4.

• Remember: While the patient presents with motor symptoms (reduced wrist extension strength), sensory testing is crucial for localizing the level of injury and determining prognosis.

• Clinical pearl: The dorsal aspect of the distal radial forearm has twice the frequency of sensory impairment in C6 radiculopathy compared to C7 radiculopathy 4, which may help distinguish between a nerve root injury and a more distal radial nerve injury.

Timely and accurate assessment is critical, as outcomes for radial nerve injuries are significantly better when surgical intervention (if needed) occurs within 5 months of injury 5.