Pathophysiology inferred from electrodiagnostic nerve tests and classification of polyneuropathies. Suggested guidelines
Introduction
The distinction between axonal loss and demyelination is one of the main goals of the examination of a polyneuropathy (PNP) due to its important implications for diagnosis, treatment and prognosis. For instance, differentiation of axonal and demyelinating pathophysiologies is a major determinant for the effective choice of therapy in the case of acute inflammatory demyelinating polyradiculoneuropathy (AIDP), chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) or multifocal motor neuropathy (MMN). Nerve biopsy findings may be the most accurate means of classifying nerve abnormalities, however, in biopsies changes may be overlooked whenever the abnormalities are patchy or restricted to motor nerves. Besides, nerve biopsy is an invasive and inconvenient procedure that cannot be applied for follow-up studies. Nerve conduction studies (NCS) are therefore essential for determining the pathophysiology of peripheral nerves. In NCS primary demyelination is indicated by marked reduction in motor or sensory conduction velocity (CV), conduction block or increased temporal dispersion, whereas primary axonal loss may be indicated by a decrease in amplitude or area of the sensory nerve action potential (SNAP) or the compound muscle action potential (CMAP). However, electrophysiological differentiation between demyelination and axonal loss can be a challenging task as increased temporal dispersion or distal conduction block due to demyelination may result in amplitude reduction, and in axonal neuropathy loss of large fast conducting fibres may cause conduction slowing. Moreover, the pathophysiology is sometimes difficult to interpret with secondary demyelination in primary axonal neuropathy and vice versa (Johnsen and Fuglsang-Frederiksen, 2000). Additionally, other pathophysiologies should be kept in mind, e.g. channelopathies may lead to conduction slowing or failure via functional disturbances of ion-channels at the nodal areas (Gutmann and Gutmann, 1996, Kaji, 2003, Yokota et al., 1994).
For the past 12 years a European group of neurophysiologists have collected samples of their patient examinations for peer review medical audit through the Internet and at regular workshops in the multicentre project ESTEEM (European Standardized Telematic tool to Evaluate Electrodiagnostic Methods) (Fuglsang-Frederiksen et al., 1996, Vingtoft et al., 1994, Vingtoft et al., 1995). During the years, the group has established its own sets of electrophysiological criteria for pathophysiological interpretation of individual nerve tests and for PNP classification, based on existing criteria in the literature (Ad Hoc Subcommittee of the American Academy of Neurology AIDS Task Force, 1991, Albers, 1993, Albers and Kelly, 1989, Albers et al., 1985, Asbury and Cornblath, 1990, Barohn et al., 1989, Behse and Buchthal, 1977, Bouche et al., 1983, Bromberg, 1991, Buchthal and Behse, 1977, Gherardi et al., 1983, Gilliatt, 1966, Hadden et al., 1998, Harding and Thomas, 1980, Ho et al., 1997, Hughes et al., 2001, Italian Guillain-Barré Study Group, 1996, Meulstee et al.,1995, Nicolas et al., 2002, Saperstein et al., 2001, Van den Bergh and Pieret, 2004, Van der Meche et al., 2001), the experience with a patient material of 572 peer reviewed electrodiagnostic examinations, and productive discussions between the physicians at workshops. The criteria have proved useful in daily practice at the participating laboratories and the group believe that they should now be presented to the international neurophysiological community.
Section snippets
Data structure
Since 1992, seven physicians from six European countries have prospectively collected random samples of their patient examinations into a database, which now contains 1401 cases including 350 PNP cases. A common data structure implemented in a PC program (Johnsen et al., 1994b) is used to collect, exchange, and compare the heterogeneous data originating from the different laboratories in electronic form. The data structure is, in accordance with the electrophysiological examination, composed of
Sensory nerve conduction studies
The suggested limits are presented in Fig. 1. Normal studies have changes ≤±2 SD from mean of controls. The limits for probable demyelination are >4.5 SD decrease in CV, while definite demyelination requires >5.5 SD decrease in CV. Criteria for axonal loss are met when the decrease in SNAP amplitude is >2.5 SD and the CV is normal or slightly decreased (≤2.5 SD). Studies not fulfilling the above criteria are considered neuropathic.
CMAP amplitude versus distal motor latency
The suggested limits for motor conduction studies evaluated by
Discussion
The ESTEEM project has since 1992 been active as an international collaboration of peers. The physicians in the project are from different European countries and fundamental differences in training and tradition exist among the physicians. Therefore, some of the challenges in the project were to obtain agreement on a common terminology for electrophysiological definitions and to develop consistent interpretations of electrodiagnostic tests. This work has led to the present suggestions for
Conclusions
The criteria presented here result from seven neurophysiologists' several years of systematic review of patient cases from six different European electrodiagnostic laboratories. The physicians involved in the project use the criteria routinely and find them practical and useful and believe that they may also be helpful to other neurophysiologists.
Use of well-defined criteria assures consistent interpretations of electrophysiological tests and improves the quality of the neurophysiological
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