Common Entrapment Neuropathies and Chronic Inflammatory Demyelinating Neuropathy
Abstract & Commentary
By Michael Rubin, MD, Professor of Clinical Neurology, Weill Cornell Medical College. Dr. Rubin reports no financial relationships relevant to this field of study.
Synopsis: Entrapment neuropathies are not more common in the setting of chronic inflammatory demyelinating neuropathy and suggest a surgically treatable lesion.
Source: Rajabally YA, Narasimhan M. Electrophysiologic entrapment syndromes in chronic inflammatory demyelinating polyneuropathy. Muscle Nerve DOI: 10.1002/mus.22146.
Is chronic inflammatory demyelinating polyneuropathy (CIDP) associated with an increased incidence of multiple entrapment neuropathies at common sites of compression, and, ipso facto, does the presence of multiple entrapment neuropathies at common sites of compression support a diagnosis of CIDP, or assist in differentiating it from other demyelinating neuropathies? To address this question, a prospective study of 31 patients with CIDP was performed to determine if common sites of compression were more likely to be involved than their immediately adjacent segments. All patients fulfilled European Federation of Neurological Societies/Peripheral Nerve Society guidelines for the diagnosis of CIDP.1 None of the patients gave a history suggestive of pre-existing entrapment neuropathy. Using standard percutaneous recording procedures, bilateral electrodiagnostic studies of the median, ulnar, and peroneal nerves at the wrist, elbow, and fibular head, respectively, were performed, recording at the abductor pollicis brevis, abductor digiti minimi, and extensor digitorum brevis, respectively, and compared to the forearm segments for the median and ulnar nerves, and the below-knee segment for the peroneal nerve. Conduction block was defined as a drop in amplitude of 30% or more on proximal, compared to distal stimulation, and demyelination of a nerve segment was felt to be present if a 25% or more difference in conduction velocity was found between the common entrapment site and the non-entrapment site. Statistical analysis was provided using Fisher's exact test, and t-tests were used to calculate differences of the mean.
No difference in demyelination was found in the median nerve, comparing the common entrapment site at the wrist to its proximal adjacent segment. Interestingly, in both the ulnar and peroneal nerves, there was significantly more frequent demyelination at non-entrapment sites, as measured by conduction velocity, compared to sites more prone to compression neuropathy. With respect to conduction block, no segmental differences were seen in the ulnar nerve, and conduction block was more frequent below the knee than across the fibular head in the peroneal nerve. In CIDP, there is no predilection for demyelination at common entrapment sites, and evidence of such demyelination should suggest a potentially surgically treatable lesion rather than favor a diagnosis of CIDP.
Commentary
Voltage-gated Na+ channels (Nav) and ankyrin G, a cytoplasmic protein, are concentrated at nodes of Ranvier. Paranodal proteins include paranodin, also known as Caspr, and contactin. Electron microscopic study of superficial peroneal nerve biopsies obtained from 12 patients with CIDP and 10 with chronic inflammatory axonal polyneuropathy revealed that, compared to controls, CIDP patients demonstrated immunofluorescence of paranodin/Caspr that was more widespread, extending into the internodes, whereas Nav, and KCNQ2, a potassium voltage-gated channel subunit also found in the nodal region, were less altered. Labeling of paranodin was irregular and/or decreased in chronic inflammatory axonal polyneuropathy internodes, whereas control nerves demonstrated Nav restricted to the nodes of Ranvier, flanked by paranodin. These abnormalities are the first such described at the nodes of Ranvier in chronic inflammatory demyelinating or axonal polyneuropathy and may have future value in clinically distinguishing between these disorders.2
References
1. Hughes RA, et al. European Federation of Neurological Societies/Peripheral Nerve Society guideline on management of chronic inflammatory demyelinating polyradiculoneuropathy: Report of a joint task force of the European Federation of Neurological Societies and the Peripheral Nerve Society. Eur J Neurol 2006;13:326-332.
2. Cifuentes-Diaz C, et al. Nodes of ranvier and paranodes in chronic acquired neuropathies. PLoS ONE 2011;6:e14533. doi:10.1371/journal.pone.0014533.
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