Quantitative sudomotor axon reflex testing
QSART is an autonomic study that measures sweat output in response to acetylcholine, which reflects the function of postganglionic sympathetic unmyelinated sudomotor nerve fibers. Electrodes are placed on the arms and legs to record the volume of sweat produced by acetylcholine iontophoresis, in which a mild electrical stimulation on the skin allows acetylcholine to stimulate the sweat glands. The output is compared with normative values.
One prospective study showed that 67 (72.8%) of 92 patients with painful feet had abnormal results on QSART, ie, low sweat output.28 A retrospective study found that 77 (62%) of 125 patients with clinical features of distal small fiber neuropathy had a length-dependent pattern of QSART abnormalities.22 QSART abnormalities were detected in some patients without autonomic symptoms.
If these tests are not available
Skin biopsy and QSART are objective, reproducible, sensitive, and complementary in diagnosing small fiber neuropathy. One or both can be ordered, depending on whether the patient has somatic symptoms, autonomic symptoms, or both. However, these two tests are not widely available. Only a few laboratories in the country can process skin biopsy specimens to evaluate intraepidermal nerve fiber density. Nevertheless, it is easy to learn the skin punch biopsy procedure, and primary care physicians and neurologists can perform it after appropriate training. (A concern is avoiding damage to the epidermis.) They can then send specimens to one of the cutaneous nerve laboratories (but not to a routine reference laboratory).
A special technique, including unique fixative and cryoprotectant, is used to fix and process the biopsy specimens, because routine techniques for processing dermatologic punch biopsy specimens often result in lower intraepidermal nerve fiber densities. Therefore, it is very important to contact the laboratory regarding fixative and processing before performing a biopsy.
QSART requires specialized equipment and must be performed on site. In addition, the test is very sensitive to drugs that can affect sweating, such as antihistamines and antidepressants, and such drugs must be discontinued 48 hours before the study.
Basic laboratory tests to find the cause
Once the diagnosis of small fiber neuropathy is established, the next important step is to order a battery of laboratory tests to search for an underlying cause. The tests should include the following:
- Complete blood cell count
- Comprehensive metabolic panel
- Lipid panel
- Erythrocyte sedimentation rate
- Thyroid-stimulating hormone level
- Free thyroxine (T4) level
- Antinuclear antibody
- Extractable nuclear antigens
- Angiotensin-converting enzyme (ACE) level
- Serum and urine immunofixation tests
- Vitamin B12 level
- 2-hour oral glucose tolerance test.
Oral glucose tolerance testing is much more sensitive than measuring the hemoglobin A1c and fasting glucose levels in detecting diabetes and prediabetes. These two conditions were detected by oral glucose tolerance testing in more than 50% of patients with otherwise idiopathic sensory-predominant peripheral neuropathy and normal hemoglobin A1c and fasting glucose levels.13,14 Therefore, every patient with small fiber neuropathy without a known history of diabetes or prediabetes should have an oral glucose tolerance test.
Special laboratory tests in special cases
- If there is a history of gastrointestinal symptoms or herpetiform-like rash, then testing for gliadin antibody and tissue transglutaminase antibodies as well as small-bowel biopsy may be pursued to evaluate for celiac sprue.
- Serologic tests for HIV or hepatitis C should be ordered if the patient has risk factors.
- If there is a significant family history, further genetic testing should be considered.
- Lip biopsy or bone marrow biopsy should be considered if clinical suspicion is high for Sjögren disease, seronegative sicca syndrome, or amyloidosis.
- The serum ACE level has a low sensitivity and specificity; therefore, if sarcoid is suspected clinically, additional confirmatory testing, such as computed tomography of the chest, should be ordered despite a normal ACE value.
HOW DO YOU TREAT SMALL FIBER NEUROPATHY?
Treatment of small fiber neuropathy should target the underlying cause and neuropathic pain. Cause-specific treatment is a key in preventing small fiber neuropathy or slowing its progression.
Glucose control, weight control, and regular exercise
As glucose dysmetabolism is the condition most often associated with small fiber neuropathy (and since individual components of the metabolic syndrome are potential risk factors for it), tight glycemic control and lifestyle modification with diet control, weight control, and regular exercise are of paramount importance in patients with these conditions.
The Diabetic Prevention Program,29 a study in 3,234 people with prediabetes, found that diet and exercise were more effective than metformin (Glucophage) in preventing full-blown diabetes. At an average of 2.8 years of follow-up, the incidence of diabetes was 11.0 cases per 100 patient-years in a group assigned to receive placebo, compared with 7.8 in those assigned to receive metformin (31% lower), and 4.8 (58% lower) in those who were assigned to undergo a lifestyle intervention that included at least 150 minutes of physical activity per week with a weight-loss goal of 7%. Put another way, to prevent one case of diabetes over 3 years, 6.9 patients would have to undergo the lifestyle intervention program, or 13.9 would have to receive metformin. Since impaired glucose tolerance neuropathy may represent the earliest stage of diabetic neuropathy, the neuropathy at this stage may be reversible with lifestyle intervention and improvement of impaired glucose tolerance.
This concept is supported by a 3-year study in 31 people, which showed that lifestyle intervention significantly improved impaired glucose tolerance, reduced the body mass index, and lowered total serum cholesterol levels.30 Changes in these metabolic variables were accompanied by significant improvement of neuropathy as evidenced by significantly increased intraepidermal nerve fiber density, increased foot sweat volume, and decreased neuropathic pain.30