Compression of peripheral nerves result in symptoms relating to damage to the motor and sensory axons that are affected.
The level of nerve damage depends on whether conduction is blocked, axons are transected or whole nerves are transected (known as neuropraxis, axonotmesis and neurotmesis respectively).
Damage to Motoneurones results in Weakness or Paralysis, Loss of Reflex Activity and Atrophy in the denervated or partially denervated Muscles.
Sensory symptoms include paraesthesia and pain.
Clinical Neurophysiological Investigations involved Nerve Conduction Tests and Electromyography, which allow diagnosis and the abily to follow the course of recovery. These may be supplemented by MRI visualisation of sites of nerve compression.
The diagram shows the different levels of damage that can occur in a nerve trunk.
Trauma and Compression
Axons are susceptible to damage by trauma which either transects the nerve or compresses it. Compression affects blood flow to the nerve and the transmission of impulses along the axonal membrane; pressure block of impulse transmission in myelinated fibres may be reversible (see Neuropraxia). This can give rise to sensory and motor symptoms.
Nerve compression also interferes with axoplasmic transport, so that the nutrition of nerve endings is affected in the longer term. Both orthograde and retrograde transport are affected by nerve compression.
Motor symptoms include weakness or paralysis of muscles, and sensory symptoms include tinglings and numbness (paraesthesiae) and pain. All of these occur in the field of distribution of the nerve distal to the compression.
Axonotmesis occurs when the pressure causes the axons to be divided, but can regenerate following decompression, when they regrow down their tubes.
Image source: GPonline
Wasting of the thenar muscles in a patient with carpal tunnel syndrome
Carpal Tunnel Syndrome
Carpal Tunnel Syndrome is common: the median nerve becomes compressed as it passes into the hand between bones and ligaments.
Median nerve compression gives rise to muscle weakness and atrophy, resulting in an inability to undo bottle tops and grasp objects.
Nerve conduction tests show slowing of conduction in the terminal branches of alpha motoneurones; needle electromyography of the small muscles innervated by the median nerve shows the presence of fibrillation potentials. During recovery the fibrillation potentials disappear and there may be evidence of Large Motor Units.
In addition sensory symptoms such as tinglings and numbness are present in the distribution of the median nerve as a result in compression of sensory axons.
The Sensory Nerve Action Potential is slowed as the nerve passes through the carpal tunnel. Vibration and Temperature thresholds may also change in sensory neuropathies.
Image source: Living Handbooks
Severe Ulnar Nerve Lesion at the elbow. Note the marked clawing of the hand characteristic of an ulnar nerve lesion. Also wasting of the adductor pollicis muscle normlly innervated by the deep branch of the ulnar nerve.
Ulnar Nerve compression
The ulnar nerve is vunerable to damage as it passes behind medial epicondyle the humerus where it may be damaged by a sudden blow.
Hitting the 'funny bone' results in symptoms due to transient compression of the ulnar nerve at the elbow.
Fractures of the bone at this site can cause more serious nerve injury that affects many small muscles of the hand including the adductor pollicis which adducts the thumb.
Sensory symptoms occur in the skin of the little finger and the medial side of the fourth finger.
Nerve conduction is slowed over the elbow segment of the nerve.
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Path of the common peroneal nerve as it winds round the neck of the fibula and branches into deep and superficial branches. The superficial branches innervate the extensor digitorum brevis muscle which dorsiflexes the foot. Damage to this nerve causes footdrop, and commonly results from fractures in the region of the neck of the fibula. The deep branches innervate peroneal muscles that evert the foot.
Damage to the Peroneal Nerves
Foot Drop is a sign that the peroneal axons are affected somewhere along their course, which may be at the fibular neck, or at more proximal sites such as the intervertebral foraminae.
The common peroneal nerve winds round the neck of the fibula where is is vulnerable to damage by pressure and by fractures of the adjacent bone.
Its branches are both superficial and deep, innervating the doral muscles of the foot and the peroneal muscles; it also provides sensation to skin in the leg and dorsum of the foot.
Image source: accessphysiotherapy.mhmedical.com
The diagram shows the cutaneous distribution of the superficial and deep branches of the common peroneal nerve.
Nerve conduction tests allow changes in conduction velocity to be measured and EMG of extensor digitorum brevis and the peroneal muscles may identifiy signs of denervation such as fibrillation potentials or enlarged motor unit size.
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Typical Sign's of Bell's Palsy
Bell's Palsy is a lower motoneurone lesion of the facial nerve, which passes through the skull to innervate the muscles involved in facial expression.
The nerve appears to become inflammed in a narrow bony canal, which compresses the nerve; bony fractures or regional surgery also cause this lesion.
One side of the face droops, there is no control of the muscles of the forehead or eyelid.
EMGs of facial muscles may show fibrillation potentials.