When is a nerve biopsy necessary?

A nerve biopsy is required in rare, exceptional cases to diagnose ALS. In a nerve biopsy (which is often performed in combination with a muscle biopsy), branches of a cutaneous nerve on the outer side of the foot (the sural nerve) are removed during a minor surgical procedure and then analyzed neuropathologically. Exposing branches of the sural nerve requires a skin incision of one to two centimeters (followed by suturing).

A nerve biopsy is necessary only if it is not possible to distinguish between ALS and motor neuropathies (diseases of the nerve roots and nerve tracts) using other methods (EMG, electroneurography, neurofilament testing). Rare differential diagnoses that may require a nerve biopsy in individual cases include immune-mediated nerve disorders (immune neuropathies) or inflammatory conditions affecting blood vessels that cause damage to the nervous system (vasculitic neuropathy). Even these rare differential diagnoses can usually be distinguished from ALS based on symptoms (sensory symptoms or pain) versus ALS (typically no sensory symptoms and no neuropathic pain). Therefore, a nerve biopsy is required only in exceptional cases to diagnose ALS.