General disorders in peripheral nerve lesions
Damage to nerve trunks is always accompanied by a complex of changes in their innervation zone. Sensory disturbances may manifest as hyperesthesia or complete anesthesia. In partial conduction block or during certain phases of regeneration (especially with causalgia), qualitative changes in sensation—hyperpathia—occur. Notably, the actual zones of sensory loss often turn out to be significantly smaller than the anatomical boundaries of innervation of a specific nerve. This is explained by the overlap phenomenon: individual areas of skin are not innervated strictly autonomously by a single trunk, but jointly with branches of neighboring nerves.
Vasomotor, secretory, and trophic disorders are most common in injuries of the median (n. medianus) and tibial (n. tibialis) nerves, whereas they are less typical for the radial (n. radialis) and peroneal (n. peroneus) nerves. These include:
- Vascular reactions: as a rule, cyanosis and changes in skin temperature are observed. Hyperemia is less common and indicates nerve irritation.
- Secretory dysfunctions: manifest as anhidrosis (absence of sweat) or hyperhidrosis.
- Trophic changes: atrophy of paralyzed muscles and skin is typical. Hyperkeratosis and hypertrichosis develop. Nails lose their gloss, become dry, brittle, and acquire pronounced striation. In severe cases, bone osteoporosis occurs and trophic ulcers open up (especially if blood vessels are simultaneously damaged).
Ulnar nerve injury (n. ulnaris)
The clinical picture in ulnar nerve damage is strictly specific. The main motor impairment is the inability to flex the IV and V fingers of the hand, while they are constantly in a state of abduction. A typical deformity occurs in which the hand takes on the appearance of a "claw hand". The patient is unable to perform a simple action—scratch the table with the nail of the little finger. Sensation disappears on the ulnar surface of the lower third of the forearm, the hand, and in the region of the IV and V fingers.
Specific tests are used to confirm the diagnosis:
- Froment's sign: the patient is asked to pull a piece of paper apart forcefully with both hands while holding it between the I and II fingers. Due to weakness of the adductor pollicis muscle (m. adductor pollicis) in ulnar nerve palsy, the thumb of the affected hand compensates by flexing at the interphalangeal joint (due to the long flexor innervated by the median nerve).
- Pitres' test: the hand lies palm down on the table, and the patient is physically unable to adduct the IV and V fingers to the midline.
- Fist-clenching test: when attempting to make a fist, the IV and V fingers remain extended.
Radial nerve injury (n. radialis)
The radial nerve is most frequently injured at the border of the middle and lower thirds of the arm—at the site where it spirals around the humerus. Motor impairments consist of the complete inability to extend at the elbow and wrist joints.
The primary visual marker is "wrist drop": with the arm extended forward, the patient's hand hangs limply downward. Neurological examination reveals loss of the triceps reflex. Anesthesia covers the posterior surface of the arm, the posterior surface of the forearm, and the region of the I finger.
Median nerve injury (n. medianus)
Trauma to the median nerve deprives the patient of the ability to pronate the hand and flex the first three fingers. Due to adduction of the thumb and general flattening of the hand, an "ape hand" deformity forms. Anesthesia extends to the lateral surface of the palm, the I, II, III fingers, and the lateral half of the IV finger.
Specific diagnostic signs:
- Mill sign: a patient with crossed fingers is unable to deftly rotate one thumb around the other.
- Decourt's test: when attempting to pull a sheet of paper between the II and flexed I fingers, the thumb on the affected side does not flex, but merely adducts toward the second.
- Scratch test: inability to scratch the table with the nail of the index finger.
- When attempting to make a fist, the I and II fingers do not flex.
For quick memorization of hand deformities: RADIAL nerve — "dropped" hand (like a ray falling downward); MEDIAN — "ape" hand (flattened in the middle); ULNAR — "claw" hand (extended fingers resemble claws).
Class and exam questions
Why is the zone of anesthesia often smaller than its anatomical innervation zone in nerve trauma?
This is due to the overlap of innervation zones. Individual skin areas on the human body are not innervated autonomously by a single nerve, but jointly with neighboring nerve trunks.
What is the essence of Froment's test?
The patient pulls a sheet of paper with the first two fingers of both hands. In ulnar nerve injury, the thumb of the affected hand flexes due to the compensatory action of the long flexor (innervated by the median nerve), since the intrinsic adductor muscle is paralyzed.
What trophic nail changes occur in nerve lesions?
Nails lose their natural gloss, become dry, brittle, and acquire a characteristic pronounced striation.
What else to study on this topic
- Motor impairments and reflex loss in femoral nerve injuries
- Symptomatology of sciatic nerve lesions and complete foot drop
- Tibial nerve: inability to plantarflex and anesthesia of the foot
- Common peroneal nerve: foot drop, internal rotation, and loss of sensation
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