A rare genetic peripheral neuropathy disorder due to gain-of-function mutations in voltage-gated sodium channels present in the small peripheral nerve fibres characterised by neuropathic pain of varying intensity (often beginning in the distal extremities and with a burning quality) associated with autonomic dysfunction (for example orthostatic dizziness, palpitations, dry eyes and mouth), abnormal quantitative sensory testing and reduction in intraepidermal nerve fibre density. Large fibre functions (such as normal strength, tendon reflexes and vibration sense) and nerve conduction studies are typically normal. [from SNOMEDCT_US]