A peripheral neuropathy is a disorder that affects the nerves that control sensation in the body and communicate this to the central nervous system (brain and spinal cord). Most typically, these are “length dependent,” meaning that the regions furthest away from the spinal cord and brain are affected first. This often results in a “stocking-glove” distribution, where the legs are first affected in the feet and ankles that would be covered by a stocking and the arms are first affected in the hand that would be covered by a glove. If an underlying cause goes untreated, the symptoms can spread more proximally or closer to the central nervous system.
The most common symptoms from peripheral neuropathy relate to the ability to perceive pain (nociception) and temperature (thermal perception). When these nerves malfunction, it may cause the person to feel cold-like pain, over-sensitivity (allodynia or hyperesthesia), or a “pins-and-needles” sensation. It can ultimately lead to numbness.
Larger peripheral nerves with myelination can be affected, which results in characteristic findings on a type of nerve-muscle testing called Electromyography (EMG).
However, small fiber neuropathy can result in an EMG being “normal,” because the EMG only tests the large myelinated fibers. In this case, some people are told that their peripheral nerves are “normal” and go undiagnosed and may not be given appropriate medications directed at nerve pain.
Smaller myelinated or unmyelinated fibers can be affected in small fiber neuropathy, which tends to be even more painful than large fiber neuropathies. The over-sensitivity can be so great that people have difficulty sleeping due to pain caused by their sheets touching the feet during the night. Occasionally, small fiber neuropathies can also cause lightheadedness, abnormal sweating, dry mouth, dry eyes, constipation, difficulty urinating, or erectile dysfunction.
To aid with the diagnosis, EMG is performed to exclude a large fiber neuropathy. When this is normal, the Neurologist will take care to pursue a skin biopsy, which will have characteristic findings of a small fiber neuropathy. If this can be confirmed, testing is performed to find the underlying cause, such as diabetes or pre-diabetes, thyroid disease, sarcoidosis, vitamin deficiencies, celiac disease, or other causes.
Small fiber neuropathy can be quite painful, but fortunately the Neurologist is able to prescribe various medications that are effective for this pain. Classes of medications include antiepileptics (such as gabapentin or pregabalin), tricyclic antidepressants (such as nortriptyline or amitriptyline), SNRIs (such as venlafaxine), topical creams containing various anesthetics, or occasionally tramadol. The specific choice of the medication will be tailored by a discussion with the Neurologist, taking into account the person’s age, consideration for side effects, and potential medication interactions.
When a person has unexplained significant pain in the feet and/or hands, a Neurologist can partner for a thorough evaluation to find and treat the cause, such as small fiber neuropathy.