Diabetic sensorimotor polyneuropathy
Diabetic sensorimotor polyneuropathy is characterized by symmetric numbness, paresthesias, or pain in the distal lower limbs (or a combination thereof). Examination may disclose stocking and glove sensory loss, impaired vibration and proprioception in the toes, reduced or absent Achilles tendon reflexes, and weakness or atrophy of the intrinsic muscles of the foot, which can result in foot abnormalities such as pes cavus and hammertoes. Signs and symptoms progress in a centripetal fashion and are not confined to a single nerve or dermatomal distribution.
In 2009, the Toronto consensus panel was convened to update the definition of diabetic sensorimotor polyneuropathy. The panel proposed the following definition: “a symmetrical, length-dependent sensorimotor polyneuropathy attributable to metabolic and microvessel alterations as a result of chronic hyperglycemia exposure and cardiovascular risk covariates.” It further defined painful diabetic polyneuropathy as “pain arising as a direct consequence of abnormalities in the peripheral somatosensory system in people with diabetes. The symptoms are distal, symmetrical, often associated with nocturnal exacerbations, and commonly described as prickling, deep aching, sharp, like an electric shock, and burning.
Small fiber neuropathy
Small fiber neuropathy is another phenotype of painful diabetic neuropathy. Small diameter myelinated and unmyelinated fibers are affected, resulting in pain such as that seen in “burning feet” syndrome. The Toronto consensus panel defined “possible” small fiber neuropathy as symptoms or signs of small fiber involvement. “Probable” small fiber neuropathy requires the addition of a normal sural nerve sensory response on nerve conduction studies. “Definite” small fiber neuropathy requires the same criteria for probable small fiber neuropathy as well as a confirmatory test such as skin biopsy or quantitative sensory testing. Of note, most patients with diabetes and small fiber neuropathy have concomitant large fiber involvement or develop abnormalities in large fibers, transitioning to typical diabetic sensorimotor polyneuropathy.
Other subtypes of painful diabetic neuropathy
Other subtypes of painful diabetic neuropathy include diabetic lumbosacral radiculoplexus neuropathy, mononeuropathy, treatment-induced neuropathy, and mononeuritis multiplex.
Diabetic lumbosacral radiculoplexus neuropathy presents with asymmetric pain and weakness in the proximal lower limb. Weight loss and improved glucose control, such as after starting insulin, can be associated features. Cervical and thoracic subtypes are also described. No treatment has been proved to be effective in diabetic lumbosacral radiculoplexus neuropathy or its cervical and thoracic subtypes.
Carpal tunnel syndrome is the most common mononeuropathy seen in patients with diabetes. Other mononeuropathies that cause pain and are common in patients with diabetes include ulnar mononeuropathies at the elbow and lateral femoral cutaneous neuropathies (meralgia paresthetica).
Treatment induced neuropathy is characterized by the acute onset of pain and autonomic dysfunction in the setting of improved glucose control. This condition can occur after insulin or oral hypoglycemic drugs are started and patients often improve with time, particularly those with type 1 diabetes.
Diabetic mononeuritis multiplex results in a stepwise progressive dysfunction of specific nerves and leads to pain, sensory loss, and weakness. Peroneal and ulnar nerves are particularly susceptible. Perivascular infiltrates can be seen on biopsy, supporting an immune mediated vasculopathy.