Laboratory Research Methods

In accordance with the recommendations of the American Academy of Neurology (AAN), for a patient with distal symmetric neuropathy the laboratory workup should include a complete blood count, vitamin B12 level, methylmalonic acid, serum glucose level, and serum protein electrophoresis with immunofixation.

Of course, the clinical situation itself may alter the strategy for using additional diagnostic methods, including laboratory tests. As laboratory research methods, when there is a corresponding clinical suspicion, one should consider screening for porphyria (qualitative analysis of a freshly collected urine sample for porphobilinogen content using Ehrlich's reagent), sarcoidosis (angiotensin-converting enzyme activity), screening for paraneoplastic syndrome (anti-onconeural antibodies, tumor markers, etc.), screening for heavy metals, and screening for celiac disease (antibodies to gliadin, tissue transglutaminase antibodies, endomysial antibodies, anti-reticulin antibodies). Cerebrospinal fluid examination is not considered a routine test in the presence of neuropathy with a symmetric, distal pattern of neurological deficit distribution; however, it is mandatory in polyradiculopathies and polyradiculoneuropathies.

The role of autoantibody testing in patients with peripheral neuropathies remains not entirely clear. Antibodies target membrane-associated and intracellular proteoglycans. Detection of autoantibodies at low titers in the general population increases the risk of false-positive results for this test. Therefore, it is recommended to use antibody panels that include the various antibodies detected in different neuropathies.

Antibodies to gangliosides. Gangliosides are specific glycolipids consisting of a hydrophobic ceramide fragment embedded in the axonal lipid membrane and a hydrophilic oligosaccharide fragment with a sialic acid residue located outside the cell, which can be recognized by specific antibodies. In the nomenclature, the first letter designates the ganglioside, the second corresponds to the number of sialic acid residues, the number represents the amount of the complete tetrasaccharide, and the last lowercase letter indicates the isomeric position of the sialic acid residue. Since different types of gangliosides may share similarities in oligosaccharide sequence, a single type of antibody may bind to several gangliosides. A factor limiting antibody access to gangliosides is the blood-nerve/blood-brain barrier. Antibodies to gangliosides can be detected using enzyme-linked immunosorbent assay (ELISA), immunodot assay, flow cytometry, and glycoarray. Different methods are not consistent with one another and may preferentially detect different types of antibodies due to differences in glycan orientation. Ganglioside complexes are defined as interacting partnerships between two structurally different gangliosides that create new antibody binding sites. Thus, antibodies against the GM1 and GD1a ganglioside complex are more frequently detected in acute motor and sensory axonal neuropathy (a variant of Guillain-Barré syndrome) than antibodies against individual gangliosides. Different inflammatory antibodies are associated with different variants of inflammatory neuropathies: the pharyngeal-cervical-brachial form of Guillain-Barré syndrome with antibodies to GT1a, Miller Fisher syndrome with antibodies to GQ1b, and acute sensory ataxic neuropathy with antibodies to GD1b. A strong association between anti-GQ1b and related ganglioside antibodies has led to the formation of the concept of "anti-GQ1b antibody syndrome," although false-positive reactions with them are extremely rare. Antibodies to GM1 can be detected in half of the cases of multifocal motor neuropathy, along with other examples that will be discussed in more detail in the relevant sections. The formation of a ganglioside complex enhances antibody detection by any of the available methods, possibly due to the formation of a heterodimer generating a new epitope.

Antibodies to Myelin-Associated Glycoprotein (Anti-MAG). Detected by enzyme-linked immunosorbent assay with high specificity. At high titers (more than 10,000 per µl), it shows cross-reactivity with GM1. There is no correlation between the clinical manifestations of neuropathy and the antibody titer level. Long-lived plasma cells may be the source of these antibodies.

Other proteins of the internodal and nodal region have also been identified as possible antibody targets in inflammatory neuropathies. According to a number of studies, antibodies to neurofascin 186 have been found in more than half of patients with MMN, while antibodies to neurofascin 186, contactin, and laminin have been found in 5% of patients with chronic inflammatory demyelinating neuropathy. Since these antibodies are uncommon, they cannot be used as a screening tool for inflammatory neuropathy; however, they are of great interest for explaining the pathophysiology of the cases discussed. Further research is needed to develop additional biomarkers and clarify their use in clinical practice.

Text author: Dmitry S. Druzhinin, MD, DSc