lunes, 26 de marzo de 2012

Metabolic & Nutritional Neuropathies

Diabetes Mellitus
Peripheral nerve involvement in diabetes is common and may be characterized by polyneuropathy, which is of mixed (sensory, motor, and autonomic) character in approximately 70% of cases and predominantly sensory in about 30%; mononeuropathy multiplex; or mononeuropathy simplex (Table 6–5). Such clinical manifestations can occur in isolation or in any combination. The incidence of peripheral nerve involvement may be influenced by the adequacy of diabetes control, which should, in any event, be optimal.

Table 6–5. Neuropathies Associated with Diabetes.









Type
Distribution
Polyneuropathy
  Mixed sensory, motor, and autonomic
Symmetric, distal, lower > upper limbs
  Primarily sensory

Mononeuropathy multiplex
Variable
Polyradiculopathy/plexopathy (Diabetic amyotrophy)
Asymmetric, proximal (pelvic girdle and thighs)
Thoracoabdominal radiculopathy
Chest, abdomen
Mononeuropathy simplex
  Peripheral
Ulnar, median, radial, lateral femoral cutaneous, sciatic, peroneal, other nerves
  Cranial
Oculomotor (III) > abducens (VI) > trochlear (IV)

Facial nerve
 
Clinical Features
The most common manifestation is a distal sensory or mixed polyneuropathy, which is sometimes diagnosed, before it becomes symptomatic, from the presence of depressed tendon reflexes and impaired appreciation of vibration in the legs. Symptoms are generally more common in the legs than in the arms and consist of numbness, pain, or paresthesias. In severe cases, there is distal sensory loss in all limbs and some accompanying motor disturbance. Diabetic dysautonomia leads to many symptoms, including postural hypotension, disturbances of cardiac rhythm, impaired thermoregulatory sweating, and disturbances of bladder, bowel, gastric, and sexual function. Diabetic mononeuropathy multiplex is usually characterized by pain and weakness and often has a vascular basis. The clinical deficit will depend on the nerves that are affected. Diabetic amyotrophy is due to radiculoplexopathy, polyradiculopathy, or polyradiculoneuropathy. Pain, weakness, and atrophy of pelvic girdle and thigh muscles are typical, with absent quadriceps reflexes and little sensory loss. Diabetic mononeuropathy simplex is typically abrupt in onset and often painful. CSF protein is typically increased in diabetic polyneuropathy and mononeuropathy multiplex.
Treatment and Prognosis
No specific treatment exists for the peripheral nerve complications of diabetes except when the patient has an entrapment neuropathy and may benefit from a decompressive procedure. The role of growth factors in treatment is currently under study. Pain is troublesome in some patients and responds to the measures outlined earlier (Idiopathic Inflammatory Neuropathies).
Postural hypotension may respond to treatment with salt supplementation; sleeping in an upright position; wearing waist-high elastic hosiery; fludrocortisone, 0.1–1 mg/d; and midodrine (an -agonist), 10 mg three times daily. Treatment is otherwise symptomatic. Diabetic amyotrophy and mononeuropathy simplex usually improve or resolve spontaneously.
Other Endocrinopathies
Hypothyroidism is a rare cause of polyneuropathy. More commonly, hypothyroidism is associated with entrapment neuropathy, especially carpal tunnel syndrome. Polyneuropathy may be mistakenly diagnosed in patients with proximal limb weakness caused by hypothyroid myopathy or in patients with delayed relaxation of tendon reflexes, a classic manifestation of hypothyroidism that is independent of neuropathy. Other neurologic manifestations of hypothyroidism such as acute confusional state , dementia , and cerebellar degeneration  are discussed elsewhere.
Acromegaly also frequently produces carpal tunnel syndrome and, less often, polyneuropathy. Because many acromegalic patients are also diabetic, it may be difficult to determine which disorder is primarily responsible for polyneuropathy in a given patient.
Uremia
A symmetric sensorimotor polyneuropathy, predominantly axonal in type, may occur in uremia. It tends to affect the legs more than the arms and is more marked distally than proximally. Restless legs, muscle cramps, and burning feet have been associated with it. The extent of any disturbance in peripheral nerve function appears to relate to the severity of impaired renal function. The neuropathy itself may improve markedly with renal transplantation. Carpal tunnel syndrome (see later) has also been described in patients with renal disease and may develop distal to the arteriovenous fistulas placed in the forearm for access during hemodialysis. In patients on chronic hemodialysis, it often relates to amyloidosis and the accumulation of 2-microglobulin.
Liver Disease
Primary biliary cirrhosis may lead to a sensory neuropathy that is probably of the axonal type. A predominantly demyelinative polyneuropathy can occur in patients with chronic liver disease. There does not appear to be any correlation between the neurologic findings and the severity of the hepatic dysfunction.
Vitamin B12 Deficiency
Vitamin B12 deficiency is associated with many features that are characteristic of polyneuropathy, including symmetric distal sensory and mild motor impairment and loss of tendon reflexes. Because controversy exists about the relative importance of polyneuropathy and myelopathy in producing this syndrome, vitamin B12 deficiency is considered in more detail below in the section on myelopathies. It should be noted, however, that nolyradiculoneuropathy, polyneuropathy, and myelopathy may follow bariatric surgery and relate to nutritional deficiencies, including but not limited to vitamin B12.