Classification, differential diagnosis, and staging of diabetic peripheral neuropathy.
The peripheral nerve disorders associated with diabetes are complex and probably involve a variety of causative mechanisms. This may give rise to difficulty in the classification of individual cases. A broad separation into rapidly reversible or more persistent phenomena is helpful. The former, which can be categorized as "hyperglycemic neuropathy," include minor sensory symptoms, reduced nerve conduction velocity, and resistance to ischemic conduction failure. From analogy with experimental studies in animals, nerve hypoxia is likely to play a significant role in their origin. Of the more persistent phenomena, a distal symmetric polyneuropathy that predominantly affects sensory and autonomic function is the most common manifestation. A distal axonopathy of dying-back type may represent the underlying pathogenetic basis. Other more persistent phenomena consist of focal and multifocal lesions giving rise to cranial, thoraco-abdominal, and limb neuropathies, including proximal lower limb motor neuropathy (diabetic amyotrophy). Some of these may have an ischemic basis. Multifocal proximal lesions can summate to produce an approximately symmetric diffuse distal neuropathy. Focal lesions at sites of entrapment or external compression may reflect an abnormal susceptibility of diabetic nerve to compressive damage. There is also evidence that focal inflammatory, including vasculitic, lesions may be involved in proximal lower limb neuropathies. Finally, superimposed chronic inflammatory demyelinating polyneuropathy may occur. For the evaluation of possible treatment regimens, it is essential that cases should be correctly classified as to type. Thus, the features falling into the category of hyperglycemic neuropathy should not contaminate the assessment of distal symmetric polyneuropathy. For this type, a widely accepted scheme for staging devised by P. J. Dyck is available. Other schemes are also available for the assessment of such cases, with differing degrees of complexity. Evaluation by serial nerve biopsies has also been proposed.