Inverted Supinator Reflex: Cervical Cord Lesion at C5-C6
When tapping the distal radius produces finger flexion instead of the normal elbow flexion/supination, this is an "inverted supinator reflex" that localizes to a cervical spinal cord lesion at the C5-C6 level.
What This Sign Indicates
The inverted supinator reflex is a pathological sign indicating:
- Loss of the normal brachioradialis reflex (which is mediated by C5-C6 nerve roots)
- Exaggerated finger flexion response from muscles innervated by segments below the lesion (C7-C8)
Anatomical Localization
The lesion is at the C5-C6 spinal cord level 1. This creates a specific pattern:
- At the level of the lesion (C5-C6): Destruction of the reflex arc causes loss of the normal brachioradialis reflex
- Below the level of the lesion (C7-C8 and lower): Upper motor neuron signs develop, including hyperreflexia of finger flexors
Underlying Mechanism
The finger flexion occurs due to increased alpha motor neuron excitability below the level of the lesion 1. When you tap the radius:
- The mechanical stimulus spreads through bone and soft tissue
- Finger flexor muscles (innervated by C7-C8) respond excessively due to loss of upper motor neuron inhibition
- The normal supinator response is absent because the C5-C6 reflex arc is damaged
Clinical Significance
This is a localizing sign for cervical myelopathy, most commonly seen with:
- Cervical spondylotic myelopathy
- Intrinsic cord lesions at C5-C6
- Compressive lesions affecting the cervical cord at this level
Important Caveat
A similar phenomenon exists in the lower extremity: absent quadriceps reflex with toe flexion when tapping the patella, which localizes to L3-L4 levels 2. Both signs follow the same principle of reflex loss at the lesion level with hyperreflexia below.