Ross Hauser, MD

Patients who come to Caring Medical with widespread pain, burning or tingling sensations, dizziness, abnormal sweating, digestive symptoms, or exercise intolerance may already carry several diagnoses. Fibromyalgia, hypermobile Ehlers-Danlos syndrome (hEDS), hypermobility spectrum disorder (HSD), postural orthostatic tachycardia syndrome (POTS), and small fiber neuropathy (SFN) can overlap clinically. The symptoms are real, but identifying which disorder—or combination of disorders—is responsible is often difficult.

Small fiber neuropathy is a disease of the body’s smallest sensory and autonomic nerve fibers. Research increasingly shows objective small-fiber abnormalities in subsets of people with fibromyalgia and hEDS/HSD. However, an abnormal small-fiber test in a person with fibromyalgia does not automatically prove classic small fiber neuropathy, and it does not identify the cause by itself.

At Caring Medical, we are also interested in a narrower clinical question: could cervical ligament injury, abnormal cervical motion, or cervical dysstructure contribute to autonomic and neurologic-like symptoms in a carefully selected subgroup of patients? This is a proposed mechanism—not an established cause of small fiber neuropathy. We evaluate it in context, after considering conventional neurologic, metabolic, autoimmune, infectious, toxic, nutritional, and genetic explanations.

What is small fiber neuropathy?

Small nerve fibers include thinly myelinated A-delta fibers and unmyelinated C fibers. These fibers transmit pain and temperature signals and participate in autonomic functions that the body regulates without conscious effort, including sweating, vascular tone, heart rate, blood pressure, gastrointestinal activity, and bladder function.

Small fiber neuropathy commonly produces burning, stabbing, electric, prickling, or pins-and-needles pain. Symptoms often begin in the feet and move upward in a length-dependent pattern, although non-length-dependent or patchy presentations also occur. A person may experience pain from normally nonpainful touch (allodynia), exaggerated pain from painful stimuli (hyperalgesia), altered heat or cold sensation, itching, or numbness.

When autonomic fibers are involved, symptoms may include:

• Lightheadedness or rapid heartbeat when standing
• Abnormal sweating or heat intolerance
• Dry eyes or dry mouth
• Changes in skin color or temperature
• Constipation, diarrhea, bloating, or early fullness
• Urinary frequency or other bladder symptoms
• Sexual dysfunction

A 2024 clinical review emphasized that SFN has many possible associations, including metabolic disease, immune-mediated disease, infection, toxin or medication exposure, and inherited sodium-channel disorders. The same review noted that diagnosis remains complex and generally requires more than symptoms alone. (1)

Why routine nerve testing can be normal

Standard electromyography and nerve-conduction studies primarily evaluate large nerve fibers. They may therefore be normal in isolated SFN. These studies are still useful when weakness, loss of reflexes, impaired vibration sense, or another large-fiber disorder is suspected.

Skin biopsy and other small-fiber tests

A small skin punch biopsy can measure intraepidermal nerve fiber density—the number of nerve endings within the epidermis. The sample is commonly taken from the lower leg, sometimes with a second sample from the thigh. Results must be interpreted against age- and sex-adjusted laboratory norms. There is no universal cutoff such as “fewer than seven fibers per millimeter” that applies to every patient, biopsy location, and laboratory.

Depending on the clinical picture, neurologists may combine biopsy results with:

• A detailed neurologic examination
• Quantitative sensory testing for temperature and pain thresholds
• Autonomic testing, such as QSART, heart-rate variability, or tilt-table testing
• Corneal confocal microscopy in specialized or research settings
• Blood tests directed at potentially treatable causes

No single test captures every sensory and autonomic function of the small-fiber system. A result should be interpreted together with the symptom pattern and examination rather than used as a stand-alone diagnosis.

Fibromyalgia and small-fiber abnormalities are related—but not identical

Fibromyalgia is a chronic pain disorder characterized by widespread pain, fatigue, unrefreshing sleep, cognitive difficulty, and increased sensitivity to sensory input. Current models emphasize altered pain processing in the central nervous system, but peripheral nervous system changes may also be relevant in some patients.

A 2022 meta-analysis of 20 studies involving 903 patients estimated that 49% had evidence of somatic small-fiber impairment and 45% had evidence of autonomic small-fiber impairment. The authors also reported substantial variability among the included studies, limiting how confidently these pooled figures can be applied to an individual patient. (2)

In a 2024 study, small-fiber pathology was found in 50% of a small fibromyalgia cohort, but pain intensity did not differ between those with and without the structural abnormality. Some participants reported small-fiber-type symptoms despite having no structural small-fiber loss. (3) This is important: burning pain or autonomic complaints do not, by themselves, establish SFN.

A separate 2024 review concluded that approximately half of women with fibromyalgia have reduced skin innervation. It also cautioned that “small fiber pathology” in fibromyalgia should not automatically be equated with classic SFN because the clinical patterns and underlying biology may differ. (4)

Clinically, length-dependent burning pain, temperature loss, altered sweating, skin-color changes, and reduced distal hair or nail growth may point more strongly toward SFN. Widespread deep aching, prominent fatigue, sleep disturbance, headache, and temporomandibular symptoms may fit fibromyalgia more closely. The two can coexist, and neither diagnosis should be dismissed simply because the other is present. (5)

Small fiber neuropathy in Ehlers-Danlos syndrome and hypermobility disorders

Pain in EDS is often multifactorial. Joint instability, recurrent subluxation, tendon and muscle overload, central sensitization, nerve entrapment, and autonomic dysfunction may all contribute. Research also supports an association between EDS and objective small-fiber abnormalities.

In a foundational 2016 study, 20 adults with several EDS subtypes had reduced intraepidermal nerve fiber density. Because the study was small and did not include a broad population-based control group, it established an association rather than a universal feature of EDS. (6)

A 2022 hEDS/HSD cohort found abnormal structural testing in 58% and abnormal functional testing in 70%; 49% had abnormalities on both measures. The patients had been referred because SFN was suspected, so these percentages should not be interpreted as the prevalence in all people with hEDS/HSD. (7)

Another study reported generalized small-fiber loss in hEDS and suggested that both sensory and autonomic fibers may be involved. Again, this was an observational study and could not determine why the nerve changes developed. (8)

More recent evidence has strengthened the association. A 2025 study of hEDS patients reported mild but widespread autonomic abnormalities in 90%, while SFN was detected in 64% by structural criteria and 82% when structural and functional criteria were combined. Because this was a referral-based clinical cohort, selection bias remains an important limitation. (9)

A small 2025 family study involving five female participants also found A-delta fiber dysfunction in all participants, with C-fiber impairment in those who had dysautonomia. The sample is too small for broad conclusions, but it provides a useful direction for larger research. (10)

These studies support careful neurologic and autonomic evaluation in symptomatic hEDS/HSD patients. They do not prove that connective-tissue laxity directly damages small fibers, nor do they show that treating joint laxity reverses SFN.

Finding the underlying cause: what should be evaluated first?

Small fiber neuropathy is not one disease with one treatment. It is a pattern of nerve injury or dysfunction with many possible causes. Diabetes and impaired glucose regulation are common considerations, but the evaluation may also include vitamin B12 and other nutritional deficiencies, thyroid disease, kidney or liver disease, alcohol exposure, chemotherapy and other neurotoxic medications, monoclonal gammopathy, Sjögren syndrome, sarcoidosis, celiac disease, HIV, hepatitis C, and selected genetic disorders.

A 2026 structured laboratory review divides testing into first-line studies for common or treatable causes and second-line studies directed by the patient’s history and examination. (11) The exact workup should be individualized by a neurologist or another clinician experienced in peripheral neuropathy. Labeling unexplained neuropathy “idiopathic” before a reasonable investigation may miss a treatable condition.

When cervical instability should be explored

Some of our patients have neuropathic or autonomic symptoms together with substantial neck pain, a history of whiplash or other cervical trauma, generalized ligament laxity, symptoms provoked by head position, or objective findings of abnormal cervical motion. In this subgroup, we consider whether a cervical structural problem could be a contributing comorbidity.

Our proposed model is that ligament laxity and abnormal cervical alignment may alter mechanical forces around neural and vascular structures in the neck. This could potentially influence symptoms through several pathways, including cervical nerve irritation, altered proprioceptive input, venous outflow restriction, or vagus nerve stretch or compression. Our 2024 hypothesis paper describes this model and reports findings from a retrospective outpatient cohort; it does not establish that cervical instability causes SFN or fibromyalgia. (12)

In 2025, our team published the cervicovagopathy hypothesis, supported by a retrospective chart review of a highly selected symptomatic population. We proposed that cervical dysstructure may contribute to vagus nerve dysfunction in some patients. This is relevant to autonomic symptoms, but vagus nerve ultrasound measurements are not a validated diagnostic test for small fiber neuropathy, and prospective independent studies are needed. (13)

Our 2026 retrospective study evaluated 84 consecutive hEDS patients who sought care at an outpatient neck center. Cervical structural abnormalities, reduced internal jugular vein area at C1, and smaller vagus nerve cross-sectional measurements frequently co-occurred. The referral setting, lack of an internal control group, retrospective design, and inability to establish direction or causation are important limitations. The findings are hypothesis-generating and should not be generalized to every person with hEDS. (14)

Earlier work by our group described capsular ligament laxity as a possible source of chronic neck pain and cervical instability. That publication was a narrative review, not a clinical trial demonstrating treatment of neuropathy. (15)

Patients can read more about our clinical approach to cervical instability and chronic neck pain, vagus nerve dysfunction and the cervical spine, dysautonomia and joint hypermobility, and hEDS cervical and neurovascular findings.

Treating confirmed or suspected small fiber neuropathy

When a cause is identified, treating it is the priority—for example, improving glucose control, replacing a documented nutritional deficiency, removing a neurotoxic exposure when medically appropriate, or managing a confirmed autoimmune or infectious disorder. Neuropathic pain may be treated with medications such as selected antidepressants, gabapentinoids, sodium-channel agents, or topical therapies, chosen according to the patient’s health history and risk profile. Exercise, sleep treatment, physical rehabilitation, psychological support, and management of orthostatic or gastrointestinal symptoms may also be part of care.

Immunotherapy should not be used simply because SFN is unexplained. An updated expert consensus statement concluded that intravenous immunoglobulin is not recommended for idiopathic SFN based on Class I evidence. (16) Immunotherapy may still be considered by specialists in selected immune-mediated neuropathies, but diagnosis, expected benefit, adverse effects, and insurance implications require careful review.

The role and limits of Prolotherapy

At Caring Medical, Prolotherapy is considered when examination and imaging support painful ligament or joint instability. Dextrose Prolotherapy is an injection treatment intended to stimulate a local healing response in injured connective tissue. A systematic review and meta-analysis reported possible benefit for several chronic musculoskeletal pain conditions, while also calling for larger, more uniform, longer-term trials. The included evidence was not specific to SFN, fibromyalgia, or upper cervical instability. (17)

We therefore make an important distinction: Prolotherapy may be offered to address a diagnosed musculoskeletal problem such as ligament-related neck pain or instability. It has not been proven in controlled clinical trials to regenerate small nerve fibers, cure fibromyalgia, or reverse small fiber neuropathy. Any improvement in neurologic or autonomic symptoms that occurs after cervical treatment should be considered a clinical observation until confirmed through appropriately designed prospective studies with objective outcome measures and comparison groups.

Symptoms that require prompt or urgent evaluation

Burning or tingling symptoms are not always benign, and not every neurologic complaint is small fiber neuropathy. Urgent or emergency assessment is warranted for sudden one-sided weakness or numbness, facial droop, difficulty speaking, a new severe headache, loss of consciousness, new bowel or bladder retention or incontinence, saddle anesthesia, rapidly progressive weakness, inability to walk, severe shortness of breath, chest pain, or signs of a severe allergic reaction.

Prompt conventional medical evaluation is also important when symptoms progress rapidly; follow a new medication, infection, or toxic exposure; occur with unexplained weight loss, fever, rash, swollen joints, or cancer history; or include objective weakness, loss of reflexes, vision loss, or significant balance deterioration. Cervical manipulation or injection should not substitute for appropriate evaluation of stroke, spinal cord compression, inflammatory neuropathy, diabetes, autoimmune disease, nutritional deficiency, or another potentially treatable disorder.

Our clinical perspective

The emerging research helps explain why some patients diagnosed with fibromyalgia or hEDS/HSD have burning pain, temperature sensitivity, allodynia, or autonomic symptoms that cannot be explained by joint pain alone. Objective small-fiber testing can help identify a clinically meaningful subgroup, but it does not explain every symptom or determine the cause automatically.

At Caring Medical, we believe the cervical spine deserves consideration when a patient has compelling neck-related features—particularly significant neck pain, trauma or hypermobility, positional symptom changes, and objective structural findings. We also recognize the present limits of the evidence. The relationship among cervical instability, vagus nerve morphology, dysautonomia, fibromyalgia, and SFN remains under investigation.

The most responsible approach is not to force every symptom into one diagnosis. It is to identify established and treatable causes first, determine whether fibromyalgia and SFN are truly present, evaluate cervical pathology when the clinical history supports it, and explain clearly which conclusions are proven, observed, or still hypothetical.

For questions about whether a cervical evaluation may be appropriate, patients may contact Caring Medical.

References

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