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SFN info

Small Fiber Neuropathy: A Concise Overview

Small fiber neuropathy (SFN) is a condition characterized by damage to the small sensory nerve fibers in the body. The nerve damage in SFN often initially manifests as a burning sensation or pain, commonly starting in the feet and, less frequently, in the hands. As the condition progresses, these symptoms can extend to other body parts, leading to a diverse range of pain types and intensities.

These damaged fibers are components of the peripheral nervous system, which includes all neural pathways not part of the brain or spinal cord. They are present both in the skin—referred to as somatic fibers—and in various organs, known as autonomic fibers.

  • Somatic Fibers in the Skin:
    These fibers are essential for sensing pain, temperature, and tactile sensations. They extend into the skin and are responsible for regulating our responses to temperature changes and pain stimuli. Unlike larger nerve fibers that primarily handle muscle movements and remain generally unaffected.

  • Autonomic Fibers in Various Organs:
    Part of the autonomic nervous system, these fibers play a crucial role in regulating automatic or involuntary bodily functions. They oversee essential processes like heart rate, blood pressure, digestion, and respiration, ensuring these functions operate seamlessly. In small fiber neuropathy, damage to these fibers can disrupt these critical autonomic processes.

Technically, small fiber neuropathy arises from the selective deterioration of small myelinated nerve fibers, known as A-delta fibers, as well as unmyelinated C fibers1. Myelin, a fatty substance, insulates nerve cells, facilitating rapid signal transmission.

Despite being relatively rare, with an estimated incidence of 53 cases per 100,000 individuals2, SFN is believed to be underdiagnosed. Many cases may go unnoticed or are misdiagnosed3.

Symptoms

Symptoms of small fiber neuropathy commonly begin in the feet, manifesting as either a burning pain or a vague discomfort. Initially, these symptoms may be mild. The reason symptoms often start in the feet or hands is that SFN is typically length-dependent 3. This means that the nerve fibers farthest from the central body, such as those in the feet, are affected first, followed by a gradual spread to other parts of the body.

In some individuals, the disease is not length-dependent, leading to a varied presentation where pain and other symptoms sporadically manifest across the body, including the face, upper arms, and back 3.

Length-dependent small fiber neuropathy is frequently linked to diabetes or nerve damage induced by toxic exposure (chemicals, drugs, alcohol, etc.). In contrast, non-length-dependent SFN is often observed when autoimmune diseases are the underlying cause 4.

Somatic Symptoms

  • Burning pain
  • Loss of sensation
  • Tingling
  • Pinpoint pain
  • Numbness 5
  • Widespread muscle and skeletal pain
  • Fatigue
  • Cognitive impairments
  • Headaches
  • Temporomandibular disorders 3, 4
  • Restless legs syndrome 6

Autonomic Symptoms

  • Gastrointestinal issues
  • Urinary tract problems
  • Abnormal sweating
  • Dry eyes and mouth
  • Sexual dysfunctions
  • Heart rhythm abnormalities
  • Proneness to fainting 7

Diagnosis

The diagnosis of small fiber neuropathy can be challenging as conventional nerve conduction studies and electromyography (EMG) generally focus on large fibers and often appear normal in SFN patients. Instead, physicians may employ the following tests in combination with your symptoms.

  • Skin Biopsy: Quantitative assessment of small fiber density in skin samples.
  • Quantitative Sensory Testing (QST): Measures sensory perception thresholds.
  • Autonomic Testing: Assesses autonomic nerve function, particularly useful if autonomic symptoms are present.
  • Blood Tests: Rule out underlying systemic conditions that might contribute to SFN.

Underlying Causes

Small fiber neuropathy can either be idiopathic, meaning the cause is unknown, or secondary to various medical conditions and factors, including: 8

  • Diabetes: The most common cause.
  • Autoimmune Diseases: Such as Sjögren’s syndrome and lupus.
  • Infections: Like Lyme disease and HIV.
  • Vitamin/Micronutrient related: Example Vitamin B12 deficiency.
  • Toxic: Example Excessive alcohol use and.Heavy metal toxicity
  • Genetic Factors: Familial cases are increasingly recognized. Fabry’s disease AndMutation in sodium channels are two examples.

Footnotes

  1. Tavee, J. and Zhou, L., 2009. Small fiber neuropathy: a burning problem. Cleve Clin J Med, 76 (5), pp.298. ↩

  2. Peters, M.J., Bakkers, M., Merkies, I.S., Hoeijmakers, J.G., van Raak, E.P. and Faber, C.G., 2013. Incidence and prevalence of small-fiber neuropathy: a survey in the Netherlands. Neurology, 81 (15), pp.1356-1360. ↩

  3. Voortman, M., Fritz, D., Vogels, O.J., Eftimov, F., van de Beek, D., Brouwer, M.C. and Drent, M., 2017. Small fiber neuropathy: a disabling and underrecognized syndrome. Current Opinion in Pulmonary Medicine, 23 (5), pp.447-457. ↩ ↩2 ↩3 ↩4

  4. Themistocleous AC, Ramirez JD, Shillo PR, Lees JG, Selvarajah D. The Pain in Neuropathy Study (PiNS): A cross-sectional observational study determining the somatosensory phenotype of painful and painless diabetic neuropathy. 2016;157:1132-45. ↩ ↩2

  5. Djouhri L, Lawson SN. A-Fiber Nociceptive Primary Afferent Neurons: A Review of Incidence and Properties in Relation to Other Afferent A-Fiber Neurons in Mammals. Vol 46.; 2004. doi:10.1016/j.brainresrev.2004. 07.015 ↩

  6. Lauria G, Lombardi R. Small fiber neuropathy: is skin biopsy the holy grail? Curr Diab Rep 2012; 12:384–392. ↩

  7. Khan S, Zhou L. Characterization of nonlength-dependent small-fiber sensory neuropathy. Muscle Nerve 2012; 45:86–91. ↩

  8. Farhad K. Current Diagnosis and Treatment of Painful Small Fiber Neuropathy. Curr Neurol Neurosci Rep. 2019;19(12):103. Published 2019 Nov 26. doi:10.1007/s11910-019-1020-1. ↩