Are you wondering what is neuropathy? Neuropathy is damage to the fibers of a nerve or nerves by injury or disease. The damage might be to the axon, myelin sheath which covers the nerve fibers, or both. Neuritis is a more specific term when the lesion is one of inflammation. The most common pattern of nerve involvement is one in which several nerves are affected (polyneuropathy) involving both sides of the body equally (symmetrically). The disease can affect somatic nerves, cranial nerves, or autonomic nerves. Since all three types of nerves are outside of the brain and spinal cord they are a part of the peripheral nervous system. Thus, the terms peripheral neuropathy and neuropathy are synonyms.
The terms cranial neuropathy and autonomic neuropathy are more specific terms referring to involvement of cranial and autonomic nerves respectively. When afferent nerve fibers are involved it is termed sensory neuropathy. When efferent nerve fibers are involved it bears the name motor neuropathy. In most cases both types of fibers are involved but sensory symptoms usually precede motor ones.
Peripheral neuropathy symptoms and signs
Signs and symptoms vary depending upon the extent and relative degree of involvement of the two types of nerves as well as how long the disorder has been present.
Symptoms in general depend upon the nerve or nerves involved and the pathology. If the lesion is mainly inflammation, the most common symptoms are pain and dysesthesia. The classic pain is burning. But other common descriptors are electric shock-like, shooting, tingling, pins and needles, coldness, pricking, numbness and stabbing. The pain is in the part of the body with the damaged nerve(s) and travels along the course of the nerve(s).
The pain can sometimes resemble that associated with a pinched nerve. But it differs in that it does not radiate (spread out) relative to the spinal cord and spinal nerve roots. Additionally, the pain caused by a pinch nerve corresponds to a dermatome instead of involving the full circumference of a portion of the limb or the entire limb.
Patients frequently mistakenly think that pain from neuropathy is due to poor circulation. A reliable distinction between the two though is that the pain of peripheral arterial disease usually occurs while walking. Additionally, its descriptors tend to be aching, squeezing, or tightness in the calves, buttocks, and/or thighs.
It is common for pain from neuropathy in the lower limbs to go undiagnosed because of swelling and the assumption that it is the cause of the pain. Unless the swelling is severe or linked to inflammation though, it does not commonly cause pain. Also, the descriptors for the pain due to swelling alone tend to be aching or tightness, not those of neuropathic pain.
Dysesthesia is distorted sensation of stimulation, particularly of touch. Allodynia and hyperalgia are two of the more common forms. It might be a consequence of inflammation and/or structural damage to nerve fibers. Its basis is the alteration of the transmission of signals through afferent nerves.
With greater structural damage to nerve fibers sensation decreases or sometimes ceases and numbness ensues. Thus, in those situations a decrease or cessation of pain or dysesthesia does not mean the condition is improving. To the contrary, it is a sign of worsening.
Sensation tends to change over time the longer neuropathy is present. Changes can occur in any or all four of the main sensory modalities (types of sensation) that physicians use to perform a neurological examination. They are the following:
- Pin prick sensation – perception of sharpness produced by light pricking of the skin with a sharp object.
- Tactile sensation– perception of light touches with an object that is not sharp.
- Vibratory sensation – perception of vibration from the base of the handle of a tuning fork placed against a bone of the upper and lower extremities (most commonly of the feet and hands).
- Proprioceptive sensation – perception of upward and downward movement of a limb (usually the toes and fingers). Peripheral neuropathy causes
The earliest sensory change in most cases is that of vibratory sensation.
In addition to changes in the nature and degree of signs and symptoms over time, peripheral neuropathy symptoms and signs also change in space. They usually begin in the feet and subtly ascend up the legs. If symptoms develop in the hands they usually do so around the time the ones in the legs reach the knees. Hand symptoms tend to spread over time upward into the arms. Because of this pattern in which they spread typical sensory symptoms often end up being in a stocking-glove distribution.
Symptoms in just the feet and hands are a reason for concern about two separate problems. They are neuropathy causing the symptoms in the feet and carpal tunnel syndrome causing the symptoms in the hands.
Structural damage to efferent nerve fibers results in motor symptoms and signs. They usually don’t appear until well after the onset of pain or sensory disturbances. The main motor signs and symptoms are weakness and ataxia. Ataxia of the lower extremities is in the form of a problem with balance. When it involves the upper extremities there is clumsiness and a tendency to drop things. Another motor sign might be a loss of or decrease in deep tendon reflexes.
Dysautonomia – abnormal functioning of the autonomic nervous system – is the result of an autonomic neuropathy. Signs and symptoms vary depending on the nerve affected. Some of the more common ones are:
- orthostatic hypotension (with associated lightheadedness or fainting)
- an abnormal heart rate or rhythm
- trouble urinating (particularly beginning or finishing)
- abnormal sweating (either too much or too little)
- sexual dysfunction (erectile in man and vaginal dryness and women)
Peripheral neuropathy causes
There are a number of labeled causes of neuropathy. But a full understanding of the pathogenesis in most cases is lacking. A prime example is diabetes mellitus, which is the main cause in the modern world. Even though diabetic neuropathy is a very common complication of the disease, there are a number of different theories which attempt to explain the cellular events and reactions involved in its development. The incidence of diabetic neuropathy is proportional to how long someone has the disease. It tends to develop after diabetes has been present for several years. But ironically, there are rare instances in which signs and symptoms of it develop before diabetes is apparent.
Some other causes are the following.
- Vitamin B12 deficiency
- kidney failure
- connective tissue disorders
- blood vessel disease resulting in poor circulation to nerves
- some forms of cancer
- certain toxins
- some medications
- autoimmune disorders (abnormal immune system function)
- HIV infection
Many times though, the cause is unknown, in which case it is termed idiopathic neuropathy. Entrapment neuropathy is a special type of nerve damage caused by pressure from bodily tissue against a nerve. Carpal tunnel syndrome is one of the more common forms.
Peripheral neuropathy diagnosis
An EMG with nerve conduction velocity test often confirms a suspected diagnosis, but the test does have its shortcomings. First of all, it is costly and not always necessary if convincing signs and symptoms are present. Secondly, there is some discomfort associated with the test. Thirdly, it only assesses large nerve fibers, not small ones. Therefore, a normal study does not exclude a diagnosis of peripheral neuropathy involving small nerve fibers. Nevertheless, the presence of the latter is often apparent based on its most common signs and symptoms. They are pain, loss of sensation and those of dysautonomia. There are a number of tests to evaluate the function of small nerve fibers, but a discussion of them is beyond the scope of this article.
There are other factors which are helpful in recognizing and diagnosing neuropathy of certain types in terms of cause. The more common ones are:
- confirmed diabetes mellitus
- vitamin B12 deficiency
- a positive family history of an inherited neuropathy
There are other tests and factors which are sometimes helpful in making a diagnosis of neuropathy, but a discussion of them is beyond the scope of this article.