In our experience and others, a cause for neuropathy will not be found in as many as 50% of cases despite an extensive work-up.1–11 The chronic idiopathic polyneuropathies are likely a heterogeneous group of neuropathies. Most individuals have only sensory symptoms, but some may have mild weakness (e.g., toe extension) or slight abnormalities on motor conduction studies. The neuropathy may affect large- and/or small-diameter nerve fibers. As the etiology is unknown, only symptomatic management of the neuropathic pain is available.
CHRONIC, IDIOPATHIC, LENGTH-DEPENDENT SENSORY OR SENSORIMOTOR POLYNEUROPATHY
Most individuals present with numbness, tingling, or pain (e.g., sharp stabbing paresthesias, burning, or deep aching sensation) in the feet between the ages of 45 and 70 years.1–11 This is a common problem occurring in approximately 3% of adults as they age. In a large series of 93 patients with idiopathic sensory polyneuropathy, 63% presented with numbness and paresthesia along with pain, 24% with numbness or paresthesia without pain, and 10% with pain alone.9 Eventually, 65–80% of affected individuals develop neuropathic pain.6,9–11 Sensory symptoms are first noted in the toes and slowly progress up the legs and later into the arms. The average time to involvement of the hands is approximately 5 years.6,9
Neurological examination reveals the typical length-dependent pattern of sensory loss.6,7,9,11 Vibratory perception is reduced in 80–100%, proprioception is impaired in 20–30%, pinprick sensation is diminished in 75–85%, and light touch is decreased in 54–92% of those with the neuropathy. Strength is usually normal, although mild distal weakness and atrophy involving toe muscles may be appreciated in 40–75% of cases, and rarely of ankle dorsiflexors and plantar flexors.6,9,11 However, upper limb strength, including the hand intrinsics, should be normal. Muscle stretch reflexes are usually absent at the ankle and diminished at the knees and arms. Generalized areflexia though is less common and would point to a hereditary or acquired demyelinating neuropathy.
Within the category of idiopathic sensory or sensorimotor polyneuropathies are people who have only a small fiber sensory neuropathy.2,3,7,9 By definition, these individuals should have normal nerve conduction studies (NCS), and nerve biopsies, if performed, demonstrate a relatively normal density of large myelinated nerve fibers. Most people with small fiber neuropathy (approximately 80%) complain of burning pain in the feet, while 40–60% describe sharp, lancinating pain; paresthesias; or just numbness. Symptoms may involve the distal upper extremities. Rarely, the neuropathy is restricted to the arms and face or involves the autonomic nervous system.2,3 Examination reveals reduced pinprick or temperature sensation in almost all patients, while vibratory perception is impaired in half. Muscle strength is preserved. Likewise, muscle stretch reflexes are also usually normal, ...