Diagnostic and Screening Laboratory Tests in the Assessment of Patients With Small Fiber Neuropathy: An Evidence-Based Review-Report of the American Association of Neuromuscular and Electrodiagnostic Medicine Small Fiber Neuropathy Task Force.

Small fiber neuropathy (SFN) presents with neuropathic pain, dysesthesia, and autonomic symptoms in the context of normal nerve conduction studies, necessitating specialized diagnostic approaches. This evidence-based review by the AANEM SFN Task Force evaluates the diagnostic utility of ancillary tests and appropriate screening laboratory investigations in the assessment of SFN. A comprehensive literature review was conducted using OVID MEDLINE and EMBASE from 1966 to January 2023. Studies were selected based on prespecified inclusion criteria requiring clinical symptoms consistent with SFN and normal large fiber conduction studies. Articles were independently reviewed and graded for evidence quality by multiple raters. Thirteen diagnostic and two laboratory screening studies met criteria. Skin punch biopsy assessing intraepidermal nerve fiber density showed Class II evidence (sensitivity 74%-78%, specificity 65%-80%). Additional metrics included intraepidermal fiber length and inter-fiber distance. Corneal confocal microscopy (CCM) showed potential (Class III) but lacked validation in isolated SFN. Indirect tests (laser/contact heat evoked potentials, cutaneous silent period) had variable sensitivity and high specificity. Lab screening identified metabolic/immune etiologies in up to 64%, though most evidence was Class III. Skin biopsy is the most validated direct diagnostic tool for SFN, though CCM and indirect assessments can aid in diagnosis. No test offers enough sensitivity or specificity to serve as a stand-alone gold standard. Despite limited high-level evidence, screening for metabolic and immune conditions may help identify etiologies. Standardized methods and population studies are needed to improve accuracy.