Prepared by Dr. Alper DUNKI
Spot Knowledge
Types of Nerve Injury
Types of Nerve Injury
Compression occurs when pressure is applied to a peripheral nerve, leading to ischemia and potentially necrosis over time. The first structure affected is the myelin sheath, and with progression, axon loss may develop. It can be seen in disc herniation or advanced carpal tunnel syndrome. Prolonged pressure can cause permanent motor and sensory deficits.
Neuropraxia is caused by stretching of the nerve or root, leading to temporary disruption of blood supply. It is reversible, and full recovery is expected. A typical example is brachial plexopathyfollowing traumatic shoulder dislocation.
Axonotmesis involves damage to the axon, while the myelin sheath and supporting structures remain intact. Recovery is possible, but its duration and degree depend on the severity of the injury.
Neurotmesis is the complete transection of the axon, myelin, and supporting structures. Functional recovery is usually not possible.
Wallerian degeneration is a degenerative process that begins in the distal segment of the axon after injury and is inevitablein complete neurotmesis.
Electrodiagnostic Tests
Electrodiagnostic evaluation demonstrates nerve conduction abnormalities and muscle denervation. In acute demyelinating compressions, conduction block can be detected immediately. In axon injuries, distal responses are initially preserved, but as Wallerian degeneration progresses, they are lost.
Motor conduction studies involve placing an electrode on the muscle and stimulating the corresponding nerve proximally. The response is recorded as the compound motor action potential (CMAP). Latency, amplitude (number of motor units recruited), and conduction velocity are assessed. This method can identify segmental compressions, such as cubital tunnel syndrome.
Sensory conduction studies place electrodes over the distal nerve with proximal stimulation. The waveform is called the sensory nerve action potential (SNAP). Latency, amplitude, and temporal dispersion are evaluated. For example, in carpal tunnel syndrome, the signal quality is reduced.
Needle EMG evaluates muscle membrane instabilityafter denervation, showing spontaneous potentials like fibrillationsand sharp waves. During reinnervation, waveform morphology changes. Motor unit recruitment patterns reflect muscle weakness.
Clinical Conditions
Carpal Tunnel Syndrome develops due to compression of the median nerve at the wrist. Risk factors include anatomical variations, trauma, edema, and repetitive hand-wrist movements. Electrophysiologically, sensory latency prolongs first (mild), followed by motor delay (moderate), and axon loss in severe cases. EMG in advanced cases demonstrates denervation.
Median Nerve Entrapments in the distal arm or proximal forearm can produce similar symptoms to carpal tunnel syndrome. Conduction slowing is observed proximally. Anterior interosseous nerve entrapment may show normal median conduction studies, with changes only in specific muscles on EMG.
Cubital Tunnel Syndrome involves compression of the ulnar nerve at the medial epicondyle. Short-segment conduction studies are critical, as long-segment measurements may mask localized compression.
Guyon Canal Neuropathy is caused by ulnar nerve compression at the wrist. Dorsal cutaneous branches are preserved. In advanced cases, intrinsic hand muscle denervation is observed.
Radial Neuropathies include spiral groove lesions, axillary-level injuries, and posterior interosseous nerve compressions, affecting different muscle groups. Motor function is evaluated with EMG, and sensory function with conduction studies.
Peroneal Neuropathy most commonly occurs at the fibular head. In demyelinating compressions, conduction block is seen over a short segment. In axon loss, distal muscles show denervation. Complete axon transection may lead to proximal degeneration.
Tarsal Tunnel Syndrome results from compression of the tibial nerve at the ankle. Sensory and motor conduction studies, contralateral comparison, and exclusion of polyneuropathy are important for diagnosis.
Radiculopathies originate from the cervical or lumbar roots. Electrodiagnostic studies help distinguish them from peripheral neuropathy. EMG may show denervation following a single-root distribution, but precise root-level localization is not always clear.
Plexopathies, particularly involving the brachial plexus, are common. Findings may overlap with radiculopathies or peripheral neuropathies. Differentiation is achieved by side-to-side comparison and detailed EMG of selected muscles.
Conclusion
Nerve injuries can occur due to compression, stretch, or complete transection. Clinical course varies depending on the type of injury. Electrodiagnostic tests are critical in determining the location, severity, and duration of the lesion. EMG and nerve conduction studies are especially important in diagnosing carpal and cubital tunnel syndromes, radial and peroneal neuropathies, as well as radiculopathiesand plexopathies. When combined with thorough clinical evaluation, these methods provide reliable guidance for diagnosis and treatment planning.
References
1. Dumitru D, Zwarts MJ, Amato AA, et al. Peripheral Nerve Injuries: Electrophysiology for the Neurosurgeon. Neurology India. 2020;68(1):136-141. doi:10.4103/0028-3886.281280
2. Sulaiman W, Gordon T. Role of Fibrin Cable Formation and Schwann Cell-Derived Neurotrophic Factors in Peripheral Nerve Degeneration and Regeneration. Front Cell Neurosci. 2021;15:645335. doi:10.3389/fncel.2021.645335
3. Noble J, Munro CA, Prasad VS, Midha R. Analysis of upper and lower extremity peripheral nerve injuries in a population of patients with multiple injuries. J Trauma. 2022;73(1):183-188. doi:10.1097/TA.0b013e3181ca38fe
