Feeling sunburnt in winter? It could be small fiber neuropathy.

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Nerves are composed of bundles of individual fibers (axons)

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Nerve fibers (axons) come in a variety of shapes and sizes.  Some are wrapped in insulation (myelinated) others are bare (unmyelinated).

Human nerve

You can see from the figure (above) that small unmyelinated fibers make up the majority of human sensory nerves.  These small unmyelinated fibers convey pain and temperature sensitivity.

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Small fiber neuropathy

Some diseases, particularly diabetes, preferentially affect these small unmyelinated fibers, leaving the other fibers relatively unaffected, resulting in small fiber neuropathy.

Symptoms of small fiber neuropathy are usually a mixture of numbness (sensory loss) and neuropathic pain.

The pain can be superficial and burning, deep aching, pins-and-needles, electrical shocks, or knife-like stabbing.  Innocuous contact (such as with clothing or bedclothes) can become painful like a sunburn.

Small fiber symptoms often worsen at night (when there are fewer distractions) and in the cold.

The symptoms usually begin in the feet, often first affecting the toes and/or soles.  As the condition worsens, the symptoms usually spread proximally up on to the legs and ultimately on to the hands, leading to a “glove and stocking” pattern.

Peripheral Neuropathy, Length Dependent

In most neuropathies, the ends of longest nerves are affected first (left), leading to a glove and stocking distribution of pain and numbness (right).

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Autonomic dysfunction from small fiber neuropathy can cause burning redness in the feet (“erythromelagia”):

erythromelagia

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Also, loss of innervation to the sweat glands can cause decreased sweating peripherally (where the neuropathy is worse), and lead to increased sweating on the head and trunk:

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Sweat test showing decreased sweating in t extremities (yellow) and increased sweating on the head and trunk (purple).

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A Diagnostic Challenge!

It is the large myelinated fibers which sub-serve strength and deep tendon reflexes.  Furthermore, it it these same large myelinated fibers which are tested during a conventional nerve conduction study.

So the physical signs and electrophysiologic findings we typically rely on to diagnose neuropathy may be absent in small fiber neuropathy.

The most widely available diagnostic test is the punch skin biopsy to quantify epidermal innervation.

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Skin biopsies, showing normal epidermal innervation (left) and epidermal denervation in small fiber neuropathy (right).

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Managing Small Fiber Neuropathy

So, you know you have sunburn from small fiber neuropathy, now what?

The most important first step is to look for an underlying (treatable) cause, particularly occult diabetes, with blood work that includes a glucose tolerance test.  In the case of diabetes, monitoring and controlling the blood glucose, is the most important next step.

Otherwise, treatment is usually limited to symptomatic measures, using drugs like gabapentin, pregabalin and/or duloxetine.

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2012-13 Neurology Student Research Presented at Drexel University College of Medicine

Monmouth neurology students presenting their posters during medical student research day at Drexel University College of Medicine in Philadelphia on March 20, 2013:

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1: Addressing blood glucose control in diabetic peripheral neuropathy:  A missed opportunity for neurologists?

Darsi Pitchon and Seun Ku Kim

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Key points:

Most neurology visits for diabetic neuropathy did not include counseling about blood glucose levels, unless they were with a neuromuscular fellowship trained sub specialist.   Because neuropathy can be the presenting and/or predominant problem in diabetes, some of these patients may be primarily followed by their neurologist, so this is a missed treatment opportunity.

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2.  Acute myelopathy with normal CSF and imaging:

Denis Chang

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Chang

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Key Points:

Spinal cord infarction from fibrocartilaginous embolism can present with acute quadriparesis in young patients with normal CSF and MRI scans.  If this diagnosis is suspected, a follow-up MRI 2-3 days can be quite helpful.  This is not an inflammatory process, and will not improve with steroids or other immunosupressive medications, which can hurt more than they help.

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3. The benefits of an on-line neurology clinical course for 4th year clerkship students

Ilya Grinberg

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Key points:

The on-line video course used by the Monmouth neurology clerkship led to improved test scores on an on-line clinical test compared to students who did their clerkship at other sites.  The on-line clinical test is a low cost but effective method of evaluating students’ clinical skills in neurology over multiple clerkship sites.

Diabetic Neuropathy, You’d better be checking your blood sugars!

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Diabetes causes high blood sugar levels, either from lack of insulin (type I diabetes) or insulin resistance (type II diabetes).

Many diabetics develop numbness, tingling, neuropathic burning pain, or weakness that starts in the feet and progresses up the legs (“glove and stocking” pattern) from diabetic polyneuropathy.

Neuropathic pain in feet

Neuropathic pain in feet

Loss of protective sensibility from neuropathy can cause painless injuries that result in ulceration, infection and bony deformity “Charcot joints”.

Diabetic ulcer

Diabetic ulcer

Diabetic foot deformity, Charcot joint

Diabetic foot deformity, Charcot joint

There are many medications that can reduce the symptoms of neuropathic pain, such as gabapentin, pregabalin and duloxetine.

However, the only treatment that has been shown to be effective for diabetic neuropathy is improved control of blood sugar.

While many patients and their doctors rely on the glycosylated hemoglobin to measure how well their diabetes is controlled, it is important to recognize that this is an average measure of blood glucose levels, and patients with mild diabetes can have peaks (which are neurotoxic) and troughs (which can also be harmful), which average each other out, leading to a “normal” glycosylated hemoglobin level , and a false sense of security that everything is OK.

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Blood sugars – you can have a normal “average” level, but still get nerve damage from high peaks after you eat

If you have diabetic neuropathy, we suggest that you check your finger stick glucose regularly and write the numbers down in a book or on a computer spreadsheet, so that you can review them with your doctor.

We usually suggest the following schedule to our patients:

Pick the same 2 days each week (no matter what you have eaten those days) to do the test.  On those days, check a “fasting” sugar when you first wake up (should be <130, ideally <110), a second sugar test before dinner (should be <130, ideally <110), and a third test 2 hours after dinner (should be <180, ideally <140).

Remember, although improved blood sugar control will prevent progression of your neuropathy, it won’t make it better, so start today before things get too bad!