Beyond Diagnostic Accuracy: Ultrasound as a Structural, Mechanical, and Procedural Phenotyping Tool in Carpal Tunnel Syndrome

Median nerve cross-sectional area (CSA) is the best-validated sonographic metric for carpal tunnel syndrome, but an isolated cutoff cannot represent the full structural and clinical heterogeneity of entrapment neuropathy. This Perspective proposes a descriptive phenotyping framework in which the interpretation of sonographic findings depends on the clinical question and stage of care; the framework is intended to organize reporting and generate testable hypotheses, not to serve as a validated classification or treatment-allocation rule. At diagnosis, internal-reference ratios and dynamic nerve mobility complement inlet CSA by characterizing focal swelling and impaired gliding. Before intervention, ultrasound identifies anatomical variants and secondary lesions, maps transverse and longitudinal safe corridors, and supports real-time device tracking and technique-specific completion checks. After release, median nerve CSA generally decreases over weeks to months, but structural change correlates inconsistently with symptoms, and persistent enlargement alone does not establish treatment failure. Postoperative ultrasound is therefore most useful for symptom-directed problem-solving when incomplete release, perineural fibrosis, or recurrent compression is suspected; the available evidence does not support routine serial imaging during satisfactory recovery. Ultrasound should complement, rather than replace, clinical evaluation and electrodiagnostic testing by linking morphology, biomechanics, procedural anatomy, and symptom-directed postoperative assessment.