Median Nerve Injury at the Forearm Below the Elbow: Lost Functions
A median nerve injury at the forearm just below the elbow results in loss of thumb opposition, thumb abduction, flexion of the distal phalanx of the index and middle fingers, and sensory loss over the palmar aspect of the thumb, index, middle finger, and radial half of the ring finger. 1
Motor Deficits
The motor losses from a median nerve injury at this level include:
- Loss of thumb opposition - the ability to touch the thumb to the other fingertips is abolished, severely impairing pinch and grasp functions 1, 2
- Loss of thumb abduction - inability to move the thumb away from the palm in a perpendicular plane 1
- Loss of flexion of the distal phalanx of the index and middle fingers - this reflects involvement of the flexor digitorum profundus to these digits, which is innervated by the anterior interosseous nerve (AIN), a pure motor branch of the median nerve 1, 3
- Weakness of wrist flexion and pronation - the pronator teres and flexor carpi radialis are affected at this level 4
- Loss of flexion of the proximal interphalangeal joints of all fingers - due to paralysis of the flexor digitorum superficialis 4
Critical Clinical Point
The thenar muscles (abductor pollicis brevis, opponens pollicis, and superficial head of flexor pollicis brevis) are paralyzed, creating the characteristic "ape hand" deformity where the thumb lies in the same plane as the fingers. 2, 4 This devastatingly impairs the ability to pinch and grasp objects, which is the primary functional consequence of median nerve injury 2.
Sensory Deficits
The sensory losses are equally significant:
- Loss of light touch and two-point discrimination on the palmar aspect of the thumb, index finger, middle finger, and radial half of the ring finger 1
- Loss of sensation over the radial 3½ digits on the palmar surface 2
- Preserved sensation over the thenar eminence - this area is supplied by the palmar cutaneous branch of the median nerve, which branches proximal to the wrist and may be spared depending on the exact injury location 4
Assessment Technique
Compare sensory function with the unaffected contralateral side for reference, as this provides the most reliable baseline for detecting subtle deficits. 1 Test both light touch and two-point discrimination, as these modalities assess different fiber populations 1.
Distinguishing Complete Median Nerve Injury from Isolated AIN Syndrome
A critical pitfall is mistaking a high median nerve injury for an isolated anterior interosseous nerve (AIN) syndrome:
- Pure AIN lesions cause only motor deficits (loss of flexion of the distal phalanx of thumb and index finger, weakness of pronation) without any sensory loss 3
- However, in displaced supracondylar fractures near the elbow, the clinical syndrome often shows predominant but not exclusive affection of AIN-innervated muscles, with some sensory involvement 3
- The presence of any sensory deficit in the median nerve distribution indicates involvement beyond just the AIN 3
Diagnostic Confirmation
Nerve conduction studies (NCS) and electromyography (EMG) are the gold standard for confirming median nerve injury, measuring distal motor latency, conduction velocity, and amplitude. 1 These studies can detect denervation changes in median-innervated muscles and should be compared with the contralateral side and normative values 1.
MR neurography is the preferred imaging modality for evaluating median nerve injuries, as it provides excellent soft-tissue contrast and can identify nerve discontinuity, neuromas, and perineural edema. 1 Ultrasound serves as a useful adjunct for identifying compression, hematoma, or pseudoaneurysm, and allows real-time dynamic assessment of nerve movement 1.
Prognosis Considerations
Despite severe initial deficits and development of neuroma, the prognosis for median nerve lesions associated with forearm fractures can be excellent under conservative management with detailed neurological follow-up. 3 Motor recovery may require up to 11 months and differs between involved muscles 3. However, many patients do not achieve ideal outcomes and are left with permanent sequelae even with timely microsurgical repair 4.