The Two-Dimensional Framework of Nerve Injury

Classifying nerve injury severity is essential for communication, prognosis, and treatment decisions. Traditional classification systems (Seddon, Sunderland) cannot account for the full spectrum of nerve pathology, from familiar phenomena such as local anesthetics to recently discovered autoimmune nodopathies. A two-dimensional framework is proposed that separates two aspects of severity conflated in previous classifications and applies to focal and diffuse nerve injuries. The first dimension, quantitative degree, is the proportion of axons involved; the second, pathological type, is the underlying pathology of the involved axons. Pathological type comprises conduction block (conduction failure in live axons) and axon loss (conduction failure of dead axons). Conduction block has two anatomic origins: axonal block, due to primary axonal dysfunction, and demyelinating block, due to primary myelin dysfunction. Axonal block can be organized into three mechanistic clusters: ionic, displacement, and nodopathic block, with distinct pathophysiology, histology, and recovery times. Both axonal and demyelinating conduction block produce electrodiagnostic features historically attributed to demyelination, although subtle differences can sometimes be identified. Severity and recovery time increase from ionic block to axon loss. A single mechanism can cause multiple pathological types, sometimes simultaneously, with the type determined by injury intensity and duration. The two dimensions integrate into assessment: quantitative degree is estimated clinically, and conduction block or axon loss is determined electrodiagnostically. This framework highlights that conduction block can result from processes at the axon or myelin. This includes autoimmune nodopathies, which are often mistaken for demyelinating disorders, but accurate identification can change treatment and may be lifesaving.