Small studies and comparative research have generally shown that reduced intra-epidermal nerve fiber density (IENFD) correlates with clinical and functional measures of small‑fiber neuropathy (SFN) and may be more sensitive than nerve conduction studies for detecting SFN (Teoh et al. 2008; Scherens et al. 2009; Lauria et al. 2005).
Skin biopsy with quantification of IENFD (3‑mm punch biopsy at distal leg, immunohistochemistry with anti‑PGP9.5, analysis of multiple sections) is supported by guideline-level recommendations as a validated technique for diagnosis of distal small‑fiber sensory polyneuropathy (EFNS/PNS, Lauria et al. 2005; England et al. 2009).
Skin biopsy techniques and reporting vary across studies; standardized methods (specific fixatives, sectioning, staining, and stereologic or validated morphometric analysis) improve reproducibility and diagnostic performance (Lauria et al. 2005; Gibbons et al. 2009, 2010; EFNS/PNS 2010).
Noninvasive functional tests (quantitative sensory testing [QST], stimulated skin wrinkling, autonomic testing) have variable sensitivity and specificity relative to IENFD; some modalities (e.g., stimulated skin wrinkling) may be nearly as sensitive as IENFD in certain settings and are useful when biopsy is unavailable (Teoh et al. 2008; Scherens et al. 2009).
Commercial (standard) biopsy processing that lacks stereologic quantitative analysis may miss subtle or localized denervation and may not correlate well with functional testing in some conditions (Kharkar et al. 2012).
Stereologic and automated morphometric approaches to measuring sweat gland (sudomotor) innervation show promise and may be more sensitive than descriptive semi‑quantitative methods; however, methods are not standardized and descriptive/semi‑quantitative approaches have poor reliability (Gibbons et al. 2009, 2010; EFNS/PNS 2010; Provitera et al. 2014; Liu et al. 2015).
Evidence is heterogeneous across disease states: studies report reduced IENFD or altered sudomotor innervation in conditions including fibromyalgia, POTS, REM sleep behavior disorder, hereditary and iatrogenic transthyretin (TTR) amyloidosis, Fabry disease, Ehlers‑Danlos syndromes, multiple system atrophy, CRPS‑I, and idiopathic small‑fiber neuropathy, but clinical implications vary and diagnostic utility is not established for many indications (multiple studies cited: Caro & Winter 2014; Kosmidis et al. 2014; Haensch et al. 2014; Schrempf et al. 2016; Masuda et al. 2017; Chao et al. 2014; Provitera et al. 2014; Kharkar et al. 2012; Cazzato et al. 2016; Haroutounian et al. 2021; Vecchio et al. 2020).
Use as a research endpoint and potential biomarker: IENFD and stereologic sweat gland measures have been used as outcomes in small clinical studies and may detect change over time or with intervention in research settings, but routine clinical use for monitoring disease progression or treatment response is not established (Jacobs & Cheng 2011; Liu et al. 2015; Masuda et al. 2017).
Limitations and diagnostic performance: sensitivity and specificity vary by condition and study; for example, in uncertain Fabry disease cohorts, IENFD and QST could not reliably distinguish affected from unaffected individuals (van der Tol et al. 2016). In endometrial assessment, PGP9.5‑positive nerve fibers in eutopic endometrium did not reliably distinguish endometriosis (Liutkeviciene et al. 2019).
Systematic reviews and guideline authors note heterogeneity in diagnostic criteria for idiopathic SFN and call for standardized, evidence‑based diagnostic criteria and further studies to validate methods and clinical utility (Haroutounian et al. 2021; EFNS/PNS 2010).