Small fiber peripheral neuropathy


Small fiber peripheral neuropathy

Sensory neuropathy is a condition that affects the peripheral nerves responsible for transmitting sensory information from the body to the brain. 

It is characterized by damage to the nerve fibers that can result in symptoms such as pain, tingling, numbness or a loss of sensation, particularly in the hands and feet.

Sensory neuropathy can be caused by a variety of factors such as diabetes, autoimmune diseases, vitamin deficiencies, infections, and exposure to certain drugs or toxins. 

In some cases, the cause of sensory neuropathy is unknown.

The treatment of sensory neuropathy depends on the underlying cause. 

In cases where diabetes is the underlying cause of the condition, better blood sugar management may improve symptoms. 

Medications such as topical creams, oral medications, and nerve blocks may be used to manage symptoms.

Small fiber peripheral neuropathy is a type of peripheral neuropathy that occurs from damage to the small unmyelinated and myelinated peripheral nerve fibers. 

These fibers, categorized as C fibers and small Aδ fibers, are present in skin, peripheral nerves, and organs.

The role of these nerves is to innervate the skin and help control autonomic function.

It is estimated that 15–20 million people in the United States have some form of peripheral neuropathy.

SFN is a condition characterized by severe pain. 

Symptoms typically begin in the feet or hands but can start in other parts of the body. 

Some people initially experience a more generalized, whole-body pain, often described as stabbing or burning, or abnormal skin sensations such as tingling or itchiness. 

In some pain is more severe during times of rest or at night. 

With small fiber neuropathy often one cannot feel pain that is concentrated in a very small area, such as the prick of a pin, but have hyperalgesia and experience pain from stimulation that typically does not cause pain (allodynia). 

Individuals with SFN may also have a reduced ability to differentiate between hot and cold.

Sudomotor dysfunction is one of the most common and earliest neurophysiological manifestations of small fiber neuropathies.

Small fibers of the autonomic nervous system can be affected, leading to urinary or bowel problems, heart palpitations, dry eyes or mouth, or abnormal sweating. 

SFN may experience orthostatic hypotension, which can cause dizziness, blurred vision, or fainting.

SFN is considered a form of peripheral neuropathy, affecting the peripheral nervous system, which connects the brain and spinal cord to muscles and to cells that detect sensations such as touch, smell, and pain.. 

Like many polyneuropathies, the symptoms are typically length-dependent, starting in the longer nerves and progressively attacking shorter nerves. 

Symptoms often start in the hands and feet before progressing upwards.

Symptoms are usually more severe in the extremities. 

Mutations in the SCN9A or SCN10A gene can cause small fiber neuropathy, providing instructions for making the alpha subunits of sodium channels. 

Sodium channels transport positively charged sodium ions into cells and play a key role in a cell’s ability to generate and transmit electrical signals. 

The NaV1.7 and NaV1.8 sodium channels are found in nerve cell nociceptors that transmit pain signals to the spinal cord and brain.

The SCN9A gene mutations that cause small fiber neuropathy result in NaV1.7 sodium channels that do not close completely when the channel is turned off. 

The altered channels allow sodium ions to flow abnormally into nociceptors. 

This increase in sodium ions enhances transmission of pain signals, causing individuals to be more sensitive to stimulation that might otherwise not cause pain. 

The small fibers that extend from the nociceptors through which pain signals are transmitted degenerate over time. 

This degeneration accounts for signs and symptoms such as the loss of temperature differentiation and pinprick sensation. 

SCN9A gene mutations have been found in approximately 30 percent of individuals with small fiber neuropathy; SCN10A gene mutations are responsible for about 5 percent of cases. 

Other health conditions cause small fiber neuropathy.

Diabetes mellitus and impaired glucose tolerance are the most common diseases that lead to this disorder, with 6 to 50 percent of diabetics or pre-diabetics developing small fiber neuropathy. 

Other causes of this condition include Fabry disease, immune disorders such as celiac disease or Sjogren syndrome, sarcoidosis, and human immunodeficiency virus (HIV) infection.

Studies have suggested an association between autonomic small fiber neuropathy and postural orthostatic tachycardia syndrome, erythromelalgia, fibromyalgia, Ehlers–Danlos Syndrome and long covid.

The diagnosis of small fiber neuropathy often requires: Quantitative sensory testing (QST) to assess small fiber function by measuring temperature and vibratory sensation.

 Nerve conduction studies and electromyography are commonly used to evaluate large myelinated sensory and motor nerve fibers, but are ineffective in diagnosing small fiber neuropathies.

Abnormal QST results can be attributed to dysfunction in the central nervous system, and is limited by a patient’s subjective experience of pain sensation.

Electrochemical skin conductance and quantitative sudomotor axon reflex testing can measures sweating responses at local body sites and can evaluate the small nerve fibers that innervate sweat glands.

Electrochemical skin conductance  can evaluate for early diagnosis of small fiber neuropathy and follow-up of treatment efficacy.

A skin biopsy can measure epidermal nerve fiber density for the diagnosis of small fiber peripheral neuropathy: intraepidermal nerve fiber density (IENFD).

A value measured below the 0.05 Quantile IENFD values per age span, is considered a reliable positive diagnosis for small fiber peripheral neuropathy.

Treatment is based on the presence of an underlying cause, if any. 

For cases without known cause there is only symptomatic treatment.

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