Peripheral neuropathy, meaning damage to the peripheral nervous system, encompasses nerve damage in a wide variety of areas and so has a broad array of possible symptoms. Common symptoms of peripheral neuropathy include numbness or loss of sensation, extreme sensitivity, or tingling or painful sensations that can escalate or spread through the body over time. Sufferers may experience muscle problems such as weakness or spasms or problems with coordination and balance. Involuntary functions controlled by the autonomic nervous system, such as digestion, circulation, and sexual function, can also be affected.
The peripheral nervous system encompasses all nerves outside of the brain and spine. Its functions include voluntary bodily movement, sensory perception, and control of involuntary functions such as digestion, blood circulation, and salivation. Peripheral neuropathy can be further divided into polyneuropathy, mononeuropathy, and mononeuritis multiplex, which respectively refer to damage to multiple nerves throughout the body, damage to a single nerve, and damage to multiple isolated nerves. Nerve damage can be the result of a variety of different causes, and the symptoms of peripheral neuropathy can vary according to their origin, whether the damage is systemic or local and whether the damage is to the motor, sensory, or autonomic nerves.
Polyneuropathy is caused by systemic problems. These are most often caused by metabolic problems, such as diabetes mellitus or malnutrition. Motor and sensory nerves are frequently both affected, and the autonomic nerves sometimes are as well. Genetic nerve diseases such as Charcot–Marie–Tooth disease and Friedreich's ataxia, autoimmune disorders such as lupus, and infectious diseases such as hepatitis C, Lyme disease, and human immunodeficiency virus (HIV) are also potential causes. Polyneuropathy can also be caused by poisoning due to excessive alcohol consumption, exposure to toxic levels of heavy metals, or adverse reactions to some drugs, including the chemotherapy drug vincristine and antibiotics such as nitrofurantoin and the fluoroquinolones.
The damage to the sufferer's sensory nerves causes feelings of numbness, prickling, or pain that usually start in the fingers and toes and then spread up the limbs as the damage progresses. These are usually the first symptoms of peripheral neuropathy to appear. The sufferer may become hypersensitive to pain or feel pain from what are normally harmless stimuli, such as a light touch. Alternately, the sufferers may become so numb that they do not notice when they have been injured. Sensory nerve damage can also impair the sufferer's sense of balance.
Damage to motor nerves can make movement difficult or clumsy and weaken the muscles. It may also cause painful cramping or visible involuntary muscular twitches called fasciculations. Over time, polyneuropathy can result in muscle atrophy and skeletal degeneration. Both sensory and motor neuropathy can also cause gait abnormalities.
If the autonomic nervous system is damaged, involuntary functions will become impaired. This can interfere with digestion, causing symptoms such as abdominal pain, nausea, or diarrhea, as well as interfering with the absorption of nutrients. Damage to nerves connected to the bowels and bladder can cause constipation, urinary retention, or fecal or urinary incontinence. The cardiovascular system can also be affected, resulting in abnormally slow or rapid heartbeat.
Mononeuropathy is caused by localized damage. The most common form is compression neuropathy, in which the nerve is physically compressed due to posture, body position, or the effects of injuries such as falls, blunt impacts, and repetitive stress damage. Compression can also be the result of weight gain, pregnancy, or the growth of a tumor or other lesion. Besides compression, mononeuropathy can also be caused by injuries that damage the nerve directly, inflammation from a localized infection, or inadequate blood supply. Some conditions that can cause polyneuropathy, such as diabetes, also increase the risk of mononeuropathies.
The effects are similar to the sensory and motor symptoms of peripheral neuropathy caused by systemic problems, such as muscle weakness and numbness, pain or tingling sensations, and problems with motor control. Mononeuropathy, however, only affects the specific part of the body associated with the damaged or compressed nerve and does not interfere with involuntary functions the way polyneuropathy sometimes does. Common examples include pain or tingling in the legs, buttocks, and lower back due to compression of one of the sciatic nerves, a condition called sciatica, and carpal tunnel syndrome, which causes similar symptoms in the hands and arms as a result of the compression of the median nerve in the wrist.
Mononeuritis multiplex begins with similar symptoms in separate locations, with more areas becoming affected as the condition progresses and more individual nerves are afflicted. Pain is often severe and most commonly focused on the legs, hips, and lower back. Like polyneuropathy, it is caused by systemic issues such as disease, poisoning, or metabolic problems, and as mononeuritis multiplex progresses and affects more areas of the body, the two can become difficult to distinguish.
