Entrapment syndromes and hand nerve lesions
Peripheral nerves are most frequently damaged due to trauma, ischemia, or compression in anatomically narrow spaces.
- Median nerve (nervus medianus): compressed within the carpal tunnel beneath the flexor retinaculum. Patients experience pain and numbness in the hand, particularly at night (brachialgia parestetica nocturna). In advanced stages, the lateral thenar muscles undergo atrophy. Due to the high proportion of autonomic fibers in this nerve, complex regional pain syndrome (formerly known as Sudeck's atrophy) frequently develops. A specific sign of injury is the "hand of benediction".
- Ulnar nerve (nervus ulnaris): the second most common site of compression. The nerve is damaged in the ulnar canal during prolonged leaning on a hard surface. The result is hypoesthesia of the ulnar border of the hand, hypothenar atrophy, and the formation of a "claw hand".
- Radial nerve (nervus radialis): its damage leads to "wrist drop" syndrome. If both the median and ulnar nerves are damaged simultaneously, an "ape hand" deformity is formed.
Topical diagnostics and segmental innervation
To differentiate a spinal nerve root lesion from peripheral nerve pathology, physicians evaluate the functions of specific muscles.
- Cervical plexus (C1–C4): controls deep neck muscles. A crucial element is the phrenic nerves (C3–C5), which ensure inspiration.
- Brachial plexus (C5–Th1): is responsible for the upper extremities. The axillary nerve innervates the deltoid muscle (shoulder abduction), and the musculocutaneous nerve innervates the biceps brachii (elbow flexion). The radial nerve innervates the triceps muscle and finger extensors.
- Lumbar (Th12–L4) and sacral (L5–S1) plexuses: control the lower extremities. The femoral nerve flexes the hip, and the obturator nerve adducts it. The sciatic nerve is responsible for knee flexion, and the tibial nerves for plantar flexion of the foot. Pelvic sphincter function depends on the pudendal nerve (S2–S4).
Multiple peripheral nerve fiber damage is called polyneuropathy (or polyneuritis if the etiology is strictly infectious-inflammatory). They are classified into axonal, demyelinating, sensory, motor, and autonomic types.
Myasthenia gravis: neuromuscular transmission failure
The primary marker of synaptic pathology is pathological fatiguability of skeletal muscles. In myasthenia gravis (an autoimmune disease), the body produces antibodies against acetylcholine receptors. Due to receptor deficiency, nerve impulses fail to reach the muscles.
- Clinical presentation: weakness is strictly dependent on exertion. Extraocular muscles are usually affected first (resulting in ptosis and diplopia), followed by dysphagia and weakness in the proximal limb segments.
- Diagnostics: based on electromyography (EMG), which reveals a decrement (decrease) in the action potential during repetitive nerve stimulation. A pharmacological test with cholinesterase inhibitors (e.g., edrophonium chloride) is also performed—muscle strength temporarily returns following the injection.
- Treatment: immunosuppressants, long-acting cholinesterase inhibitors are used, and thymectomy is sometimes indicated for young patients.
Myopathies: primary muscle disease
Unlike myasthenia gravis, in myopathies muscle weakness progresses slowly and does not depend on physical activity.
- Features: muscle fiber atrophy is often masked by fatty tissue replacement (lipomatosis). A paradox arises: the muscle appears voluminous (pseudohypertrophy) but lacks strength.
- Differential diagnosis: myopathies present no sensory disturbances, autonomic dysfunctions, or fasciculations (the latter being typical for neurogenic paresis). Pain and cramps are more characteristic of metabolic forms than congenital ones.
- Evaluation: creatine kinase (CK) blood test, EMG, and molecular genetic testing play a key role. An accurate genetic diagnosis is critically important for prognosis and counseling the patient's family.
Briefly: answers to topical questions
Which muscles are innervated by the radial nerve according to segmental innervation?
- Triceps brachii, anconeus (C6–C8) — extension of the forearm at the elbow joint.
- Brachioradialis (C5–C6) — flexion of the forearm at the elbow joint.
- Extensor carpi radialis longus and brevis (C6–C8) — extension and lateral abduction of the hand.
- Extensor digitorum (C6–C8) — extension of digits II–V, finger abduction, extension of the hand at the wrist joint.
- Extensor digiti minimi (C6–C8) — extension of the little finger.
- Extensor carpi ulnaris (C6–C8) — extension and adduction of the hand.
- Supinator (C5–C7) — supination.
- Abductor pollicis longus (C6–C7) — abduction of the thumb, hand abduction.
- Extensor pollicis brevis (C7–C8) — extension of the proximal phalanx of the thumb.
- Extensor pollicis longus (C7–C8) — extension of the distal phalanx of the thumb.
- Extensor indicis (C6–C8) — extension of the index finger.
To memorize hand deformities in nerve lesions: Ulnar nerve produces a "claw hand", Radial nerve produces "wrist drop", Ape hand is a combined lesion, and the median nerve produces the "hand of benediction".
Class and exam questions
What is the difference between polyneuropathy and polyneuritis?
Polyneuropathy is a diffuse nerve lesion of any etiology, whereas polyneuritis has a strictly infectious or inflammatory origin.
How to distinguish myasthenia gravis from myopathy based on clinical presentation?
In myasthenia gravis, weakness increases with physical exertion and decreases at rest. In myopathy, weakness is constant, progresses slowly, and does not depend on activity.
What does the presence of muscle fasciculations indicate?
Fasciculations indicate a neurogenic nature of the lesion. They are absent in primary muscle pathologies (myopathies).
What else to study on this topic
- Segmental innervation of the pelvic girdle and lower limb muscles
- The role of electromyography (EMG) in the differential diagnosis of paresis
- Metabolic and myotonic dystrophies (Steinert-Batten-Curschmann disease)
- Cholinesterase inhibitor pharmacological tests in myasthenia gravis
- Complex regional pain syndrome in autonomic fiber lesions
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