Small Fiber Neuropathy is a disabling form of peripheral neuropathy that is increasing in prevalence among young non-diabetic patients. The peripheral nerves, which includes all nerves outside of the central nervous system (the brain and spinal cord), support motor, autonomic, and sensory functions for the limbs and organs of the body. Peripheral neuropathies common among aging and diabetic patients, arise most frequently as a result of damage from uncontrolled blood sugars and vitamin deficiencies. Research has been able to demonstrate that neuropathy symptoms are associated with loss of the smallest nerves present at the surface of the skin and organs. Most peripheral neuropathies are of an unknown origin though most research is pointing toward chronic inflammation as a root cause. Diabetes is no longer the leading cause of peripheral neuropathies.
The symptoms of peripheral and small fiber neuropathies are very diverse. To understand why that is, it may help to appreciate the types of nerves that are affected. The peripheral nerves are divided into 3 major functions or types.
- The motor nerves send impulses from the brain and spinal cord to all of the muscles in the body.
- Motor nerve loss or damage results in muscle weakness, muscle cramps and spasms, and difficulty with ambulation.
- The sensory nerves send messages from the periphery back to the spinal cord and brain. Sensory nerves are located superficially in the skin and deeply throughout the body and evaluate pressure, temperature, and motion.
- Sensory nerve loss or damage results in tingling, numbness, pain, and extreme sensitivity to touch.
- The autonomic nerves control the involuntary functions of the body such as heart rate, blood pressure, digestion, and sweating.
- Autonomic nerve loss or damage results in tachycardia, bradycardia, hyperhidrosis, anhidrosis, dysphagia, nausea, vomiting, diarrhea, constipation, difficulty with urination, abnormal pupil size, and sexual dysfunction.
Peripheral neuropathies result from loss of both sensory and autonomic nerves. The nerves most affected are the smallest and most vulnerable nerves located distally in the skin. Amongst the sensory nerve fibers are included the lightly myelinated A-delta nerve fibers, and the unmyelinated Group C nerve fibers. And among the autonomic nerve fibers are the unmyelinated Group C nerve fibers.
Unmyelinated Group C Nerve Fibers
- The unmyelinated Group C nerve fibers are the leading peripheral sensory nerve fiber type. These include postganglionic fibers in the Autonomic Nervous System, and nerve fibers at the dorsal roots.
- The purpose of these polymodal (multiple stimuli) nerves are to react to changes, such as thermal (hot or cold), mechanical(pressure and touch), or chemical in nature(hypoxia, osmolarity, hypoglycemia, toxicity).
- It is the loss of these unmyelinated Group C fibers that causes the burning, warmth, itching, and cramping pain.
Aδ-(Delta) Nerve Fibers
- The other nerve fiber leading to various forms of peripheral neuropathy includes the myelinated A-Delta fibers.
- Because they are myelinated the conduction velocity is faster than the C fibers.
- These A-delta axons respond to sharp and stinging pain.
Small Fiber Neuropathy
Small fiber neuropathy is a type of peripheral neuropathy resulting from damage or loss of the lightly myelinated A-delta and the unmyelinated Group C nerve fibers.
The final result of nerve loss includes both sensory and autonomic fibers and thus causes both sensory and autonomic symptoms. Review both sensory and autonomic symptom lists below. If you have symptoms on both sides of the chart you may have small fiber neuropathy.