An EMG measures the electrical activity in a muscle in response to stimulation. It is used in conjunction with a neurological exam to help physicians diagnose a wide range of problems, including:
- Nerve damage from disk compression in the neck or back
- Nerve compression from carpal tunnel syndrome
- Neuromuscular diseases such as amyotrophic lateral sclerosis (ALS), poliomyelitis, myasthenia gravis, and muscular dystrophy
- Peripheral neuropathy caused by conditions such as diabetes, pernicious anemia, and heavy metal toxicity
The test consists of two parts:
In the nerve conduction study, electrodes are placed on the skin and a safe level of electrical stimulation is sent through the nerve, causing the muscles in your arm or leg to contract. The sensation you will feel is similar to a static electric shock. Patients wearing pacemakers are not at risk, since this current is not strong enough or close enough to the pacemaker to interfere with its operation.
In the second part, the EMG, a tiny pin electrode is inserted under the surface of the skin into the muscle. Electrical stimulation will indicate if there is damage to a specific nerve, or if the problem involves the muscle itself.
This test is mildly uncomfortable, but is specially designed to not be painful. It will take between 30 to 60 minutes.
Preparing for your EMG at DENT
- Your arms and legs should be clean and free of lotions or creams.
- Meals and medication may be taken unless your physician advises otherwise.
Skin Biopsy for Evaluation of Small Fiber Neuropathy and Autonomic Neuropathy
Small fiber neuropathy (SFN) is caused by damage to the small, unmyelinated fibers in the peripheral nerves that innervate the skin and internal organs, including the cardiovascular system, gastrointestinal tract, and bladder, among others. These fibers convey pain and temperature sensations from the skin, and mediate autonomic functions. Dysfunction of the small nerve fibers can cause sensory or autonomic symptoms. The neurological examination and EMG and nerve conduction studies that measure the large nerve fibers are usually normal so that the diagnosis requires examination of the small nerve fibers in the skin. This can be done by taken skin samples by punch skin biopsy at standard sites, and the density of the small nerve fibers is determined by morphometry after immunostaining with an antibody to the axonal marker PGP- 9.5. Two standardized tests are available; the Epidermal Nerve Fiber Density (ENFD) test which measures the density of the small sensory nerve fibers in the skin, and the Sweat Gland Nerve Fiber density (SGNFD) test which measures the density of the small autonomic nerve fibers in the sweat glands. The SGNFD test may also show a decrease in nerve fiber density in some patients with small fiber neuropathy whose ENFD test is normal.
As mentioned above, the skin samples are taken by a 3mm punch skin biopsy at standard sites, distally at either the foot or distal leg (calf), and proximally at the thigh, after prepping the skin with alcohol and betadine solution followed by the application of local anesthetic, 2% lidocaine with epinephrin. No sutures are required and only small band-aids are applied at the skin biopsy sites. Samples will then be shipped to a standardized laboratory for processing and reading. Values are compared to those obtained in normal subjects.
Making the diagnosis of SFN provides an explanation for the symptoms, and helps direct further testing for potentially reversible causes, whose treatment can result in improvement of the neuropathy.