Small Fiber Morphology and Function in Inclusion Body Myositis-A Multimodal Assessment Including Confocal Corneal Microscopy.

Introduction: The extent of peripheral nerve involvement in inclusion body myositis (IBM) remains a topic of ongoing scientific discussion. In this study, we aimed to explore the involvement of small nerve fibers in IBM using a range of diagnostic methods, including, for the first time, corneal confocal microscopy (CCM).

Methods: Nineteen clinic-pathologically or clinically defined IBM patients underwent comprehensive clinical and electrophysiological examinations. A multimodal small fiber examination was performed, including a skin biopsy of the thigh and lower leg, quantitative sensory testing of the feet, and CCM. Pain levels and quality of life were also assessed using standardized questionnaires (Small Fiber Neuropathy Screening List, PainDETECT, Brief Pain Inventory, Short Form-36).

Results: The motor and sensory neurography identified large fiber neuropathy in 67% of cases. Regarding small fibers evaluation, skin biopsy revealed pathological findings in 72% of cases and CCM in 32% of cases. Quantitative sensory testing results predominantly indicated large fiber damage (61%), with small fiber pathology identified in only 12% of cases. Questionnaire responses suggested a slight pain-related impact on patients' quality of life.

Discussion: Small nerve fiber pathology is a frequent finding in skin biopsies of IBM patients. However, noninvasive methods like CCM appear less sensitive than skin biopsy for diagnosing small fiber pathology in IBM patients. Further studies are needed to refine diagnostic approaches and better understand the role of small fiber involvement in IBM.