As we know, our nervous system consist of the central nervous system (the brain and spinal cord), and peripheral nervous system (the nerves exiting the brain and spinal cord and all their branches). So, we have 43 pairs of peripheral nerves (12 cranial nerve pairs, and 31 pairs that exit the spinal cord: cervical thoracic, lumbar, sacral and coccygeal).
Peripheral nerve injuries result from:
Systemic diseases (diabetes, autoimmune disease)
Localised damage (trauma, compression, tumours)
They manifest as neurological deficits to the level of the lesion and manifest with neurological deficit distal to the level of the lesion.
Depending on the underlying cause, these injuries may be accompanied by changes in the muscular, soft tissue or vascular components of the affected region.
Anatomy
On the outside of each peripheral nerve, there is collagenous tissue: epineurium. Surrounding every fascicle within the nerve is the perineurium. Individual nerve fibers within the fascicles are embedded in endoneurium, which fills the space bound by the perineurium.
The peripheral nerve consists of myelinated and unmyelinated nerve fibers. Unmyelinated axons are surrounded by the plasma membrane of the Schwann cell. Myelinated axons are surrounded by the myelin sheath of the Schwann cell, that wraps around the axon and insulates it with layers of cell membrane.
Gaps in myelin occur at regular intervals, called the nodes of Ranvier. The segments of axon covered by myelin between the gaps are called the internodal segments. The myelin sheath has a low capacitance and a high resistance to electrical current, so that current flow is directed longitudinally along the axon. Ion channels within the axon membrane are distributed at the node of Ranvier and under the myelin sheath.
The expression of ion channels and the insulation by the myelin sheath result in rapid mode of nerve transmission called saltatory conduction.
Seddon classification
The Seddon classification divides peripheral nerve injuries into neurapraxia, axonotmesis, or neurotmesis based upon the severity and extent of injury to the structural components of the peripheral nerve, including Schwann cells, axons, and surrounding connective tissue.
- Neurapraxia
- Burning
- Numbness
- Stinging
- Weakness
- Proprioception issues
- Bone fractures
- Dislocation
- Tears/injuries to the ligaments and tendons
- Post-operation
- Post-partum
- Tooth extractions
- Axonotmesis
- Immediate weakness
- Pain
- Muscle atrophy (due to NMJ failure) - follows after a few weeks
- Complete loss of sensory, motor and SNS functions
- Crush injuries
- Nerve stretch injuries (car accidents, falls, etc)
- Percussion injuries (gunshot wounds)
- Neurotmesis
- No pain
- Muscle atrophy
- Complete loss of sensory, motor and SNS functions
- Sharp injuries
- Traction injuries
- Percussion injuries
- Exposure to neurotoxic substances
This is the mildest form of nerve injury that results in focal demyelination of the axon. The distal part of the axon is unaffected, so there is nerve transmission and continuity to the target (but slower, due to the temporary disruption of nerve conduction).
A full recovery is expected, over the span of a few hours - months (if severe).
Symptoms are felt near the injury site:
Causes include:
Here, the axon is irreversibly damaged and the myelin sheath is similarly involved, but the endoneurium, perineurium and epineurium remain intact. The distal part of the axon undergoes 'Wallerian degeneration' in which it completely disintegrates and gets digested. The proximal end also degenerates up to a few cm's, but there is potential for axonal regrowth as the Schwann cell from the proximal stump converts to a regenerative phenotype and releases growth factors.
Partial recovery is expected, but will take a lot longer than neurapraxia.
Symptoms:
Causes:
This is the most severe form. The axon, myelin sheath, and surrounding stroma are all irreversibly damaged. The external continuity of the injured nerve is usually disrupted. No significant regeneration occurs with such a lesion, because the nerve tries to regenerate in a very disorganised manner, leading to the formation of traumatic neuroma (painful nodular thickening of the nerve). Regeneration can only happen if surgical re-anastomosis is performed.
Symptoms:
Causes include severe:
Sunderland classification
This classification involves 5 grades of nerve injury:
Seddon | Sunderland | Damage |
Neurapraxia | Grade 1 | Myelin damage |
Axonotmesis | Grade II | Axonal damage |
Neurotmesis | Grade III | Axon + endoneurium damage |
Grade IV | Axon + endoneurium + perineurium | |
Grade V | Axon + endoneurium + perineurium + epineurium |
Upper extremity nerve injuries
Generally speaking:
Distal nerve injuries cause claw hands (ulnar or median claw), because distal lesions cause a loss of lumbrical function (flexion of MCP and extension of IP joints), while extrinsic flexors remain intact.
Proximal nerve injuries will result in hand distortions (hand of benediction) which are visible when the person is trying to make a fist.
Cervicothoracic nerve injuries
Lower extremity nerve injuries
A few general notes:
The difference between tibial and peroneal nerve injuries:
TIPPED
- Tibial → impaired foot Inversion and Plantarflexion
- Peroneal → impaired foot Eversion and Dorsiflexion
Investigations
The diagnosis of peripheral nerve injuries depends on a thorough clinical history, a neurological examination, and sometimes diagnostic tests.
Imaging:
- X-ray - detects compression or transection due to dislocated or fractured bones
- CT/MRI - evaluate the causes of injury, such as nerve tumours, soft tissue pathologies, etc
Electrodiagnostic studies: detect and grade nerve injury.
- Electroneurography (nerve conduction study) - analysis of the msucle's electrical activity in response to stimulation of its supplying nerve
- Needle electromyography (EMG)
Management
Conservative management:
- Watchful waiting (expectant management) - for injuries with high chances of spontaneous recovery such as neurapraxia
- Activity modification
- Splinting - to reduce stiffness and contracture of joints
- Physiotherapy
- Electrical stimulation - supports regeneration of the proximal axons and reinnervation of the denervated muscles after surgical repair
- Analgesia
- NSAID's
- Local anaesthetics
- Drug therapy
- Chronic neuropathic pain → gabapentin
- Used in combination with surgery to enhance remyelination
Surgical management:
- Nerve repairs - to reconstruct nerve continuity
- Nerve transfers - redirecting a health nerve towards a denervated nerve in order to restore innervation to the target
- Tendon transfers - a tendon from an adequately powerful muscle is redirected/ transferred towards another tendon in order to restore its function
