Small Fibre Neuropathy
Small fiber neuropathy (SFN) can affect any of the Aδ and C sensory fibres, the γ efferent fibres, and the preganglionic and postganglionic sympathetic C fibres. This leads to sensory, autonomic, or combined symptoms.
Small fibre neuropathy (SFN) and large fibre neuropathy belong to a group of diseases known as peripheral neuropathies. Polyneuropathy refers to cases where either the large fibres are affected, or both the large and small fibres are affected. SFN refers to isolated small fibre involvement. About 40-50% of patient with fibromyalgia meet the diagnostic criteria for SFN.
- See also: Basic Neurophysiology
Small fibre neuropathy results from damage to the nociceptive system. The majority of peripheral sensory nerves are unmyelinated C fibres and thinly myelinated Aδ fibres. Small fibre nerves also include the γ efferent fibres, and the preganglionic and postganglionic sympathetic C fibres. The large fibres in contrast are the Aβ afferents from the skin, muscle, and internal organs; and the α motor neurons.
- Characteristics of sensory nerve fibres
|Nerve fibre||Myelinated axons||Diameter (µm)||Conduction velocity (m/s)||Sensory information||Usefulness of electroneuromyography||Usefulness of QST|
|Aα||Yes||13-20||80-120||Proprioception, muscle spindle primary endings (Ia), golgi tendon organs (Ib) (and alpha motor neurons)||Yes (H reflex)||No|
|Aβ||Yes||6-12||33-75||Discriminative mechanoreception (touch, vibration), proprioception, pain modulation (block nociceptive information, allodynia in sensitisation)||Yes (sensory nerve conduction)||Yes|
|Aγ||Yes||4-8||15-40||Touch, pressure (and gamma motor neurons)||No|
|Aδ||Thin||1-5||3-30||"rapid" pain, crude touch, pressure, temperature. AMH type I for rapid mechanical pain (high heat threshold >53C), AMH type II for rapid heat pain (lower heat threshold 43-47C).||No||Yes|
|C||No||0.3-1.5||0.5-2.0||"slow" pain, touch, pressure temperature (and postganglionic autonomic). Polymodal.||No||Yes|
|Receptor Type||Fibre Group||Modality|
|Cutaneous and subcutaneous mechanoreceptors||Touch|
|Hair down||Aδ||Light stroking|
|Cold receptors||Aδ||Skin cooling (25°C)|
|Warm receptors||C||Skin warming (41°C)|
|Heat nociceptors||Aδ||Hot temperatures (>45°C)|
|Cold nociceptor||C||Cold temperatures (<5°C)|
|Mechanical||Aδ||Sharp, pricking pain|
|Polymodal||C||Slow, burning pain|
|Polymodal Muscle and Skeletal Mechanoreceptors||Limb proprioception|
|Stretch-sensitive free endings||Aδ||Excess stretch or force|
Disorders known to contribute to SFN are listed below. SFN is a generalised sensory nerve disease process with abnormalities in the structure and function of affected nerve fibres. It is histopathologically characterized by degeneration of small nerve fibre endings. The small unmyelinated nerves are affected.
- Diabetes mellitus and glucose intolerance
- Vitamin B12 deficiency
- Copper deficiency
- Hypervitaminosis B6
- Chronic kidney disease
- Dysimmunity/inflammatory diseases
- Sjögren’s syndrome
- Systemic lupus erythematosus
- Coeliac disease
- Ehlers-Danlos Syndrome
- Hepatitis C
- Ebstein-Barr virus
- Lyme disease
- Toxic agents and medications
- Alcohol (this can eventually cause large fibre neuropathy)
- Antibiotics (metronidazole, nitrofurantoin, linezolid, isoniazid)
- Anticancer agents (bortezomib, platin)
- Antiretroviral drugs
- Genetic diseases
- Fabry’s disease
- Familial amyloid polyneuropathy (transthyretin)
- Wilsons disease
- Sodium channelopathy
The pathophysiology is unknown. A demyelinating process is thought to be unlikely as the condition only affects small nerve fibres. Distal axonal loss or extraordinarily neuronal degeneration are potential mechanisms.
There are four stages of neuropathy in unmyelinated nerve fibres
- Mild proliferation: Increase in number of isolated small Schwann cell projections which are irregular in shape.
- Fibre loss: decreased fibre numbers along with increased empty Schwann cells.
- Regeneration: Signs of regeneration of unmyelinated fibres along with fibre loss. There is an increase in the total number of unmyelinated fibres and small fibres with a diameter of below 0.8μm
- Advanced regeneration: Empty Schwann cells return to normal. There is only an increase of small nerve fibres with a diameter of below 0.8μm, and small isolated projections of Schwann cells.
SFN associated with diabetes may have a different underlying cause. This may involve axon swelling, and there can be progression to proximal large fibre or polyneuropathy.
The majority of peripheral sensory nerves are unmyelinated C fibres and thinly myelinated Aδ fibres. There is no clear way of diagnosing pathology in these fibres.
Symptoms are typically length dependent, i.e. most notable in the distal extremities. However it can also present in a non-length dependent manner. Non-length dependent features may be more likely to have an immune mediated disease association. i
Classical symptoms include distal burnings, pain, numbness, paraesthesia, and autonomic symptoms. Autonomic symptoms can include sweating alterations, temperature dysregulation, dry mouth and eyes, and erectile dysfunction.
Patients with SFN may have severe symptoms but a normal physical and neurological examination. Proprioception, light touch, and vibration sense may also be normal. Some patients may have decreased pinprick, decreased thermal sensation, hyperalgesia in the affected region, and slightly decreased vibratory sense.
Most affected patients have a combination of positive signs (e.g. hyperalgesia, and allodynia), and negative signs (e.g. diminished pin prick and temperature sense).
40-50% of patients with fibromyalgia meet the diagnostic criteria for SFN. In one small study, fibromyalgia patients with SFN were more likely to report dysautonomia and paraesthesias.
There is no gold standard for diagnosis. One suggestion is making the diagnosis based on 2/3 abnormal findings of clinical features, quantitative sensory testing, and skin biopsy; or quantitative sudomotor axon-reflex test (QSART) as an alternative to skin biopsy.
The following classifications can be use.
- Possible SFN: symptoms or clinical signs of small fibre damage
- Probable SFN: symptoms or clinical signs of small fibre damage and normal sural nerve conduction studies
- Definite SFN: symptoms or clinical signs of SFN-damage, normal sural nerve conduction studies and decreased intra-epidermal nerve fibre density (IENFD) and/or abnormal quantitative sensory testing (QST) thermal thresholds
Screen for Systemic Causes
Blood tests may include HbA1c, serum electrophoresis, vitamin B12, full blood count, hepatitis B and C serology, celiac serology, copper, and ANA.
These tools are obviously limited by their subjective nature. Neuropathic pain can be diagnosed using the Douleur Neuropathique 4 (DN4), and other similar instruments. There are multiple specific questionnaires for small fibre neuropathy, with validation done in different populations.
Nerve Conduction Studies
Standard electrophysiologic testing is typically normal in SFN as the pathology lies in the small unmyelinated nerve fibres. However nerve conduction studies can be used to exclude large fibre involvement. This is recommended over the other QST procedures for testing large fibres.
Testing for Sensory Symptoms
Quantitative Sensory Testing
Quantitative sensory testing (QST) is an extension of the physical examination, and can be used to diagnose peripheral nervous system disorders. Testing involves thermal, pressure, vibration, and electrical stimulation, with the full battery of tests involving 13 parameters. The results are compared to normative values.
Due to the time consuming nature of full QST, specific thermal threshold testing (TTT) can be used to test small fibre function. In TTT an electrode is used that has a baseline temperature of 32C and this can increase up to 50C or down to 0C. There are two testing methods
- Method of limits (reaction time dependent): start at the baseline temperature and increase or decrease the temperature. The test button is pressure twice, first when the temperature change is felt, and second when it becomes painful.
- Method of levels (reaction time independent): there are two buttons for yes or no. For each stimulus the patient is asked whether the thermode becomes colder or not. There is no pain threshold testing in method of levels, only thermal detection.
Normative values for TTT: normal temperature detection thresholds lay above 41C (C fibres) and below 25C (Aδ fibres), and temperature pain thresholds lay above 45C (Aδ fibres) and below 5C (C fibres).
There are some important limitations to QST in general. There is great inter-observer variability and a lack of world-wide standardisation with iportant differences in methods between normative values.. Either central or peripheral nervous system abnormalities can cause the same deficit. QST requires good cognitive function and good conscious patient reactions. There are numerous other factors that can affect the results such as environmental conditions, gender, instructions, habituation, and motor performance.
The Quantitative Sudomotor Axon Reflex Test (QSART) is used to evaluate autonomic function, in particular the peripheral sympathetic cholinergic nervous system. It measures the response of the autonomic sudomotor nerves. Iontophoresis is used to introduce acetylcholine into the skin, which stimulates the sweat glands. The volume of sweat produced is measured. Some patients with SFN have increased sweat production. 
Sural nerve biopsy may be only minimally abnormal, or even normal. It is performed using a 3mm punch under sterile technique. It can be taken from any body part, but the standard biopsy is 10cm above the lateral malleolus in the region of sural nerve innervation to enable evaluation of the loss of the most distal sensory endings that are typical of length dependent axonal neuropathy. Another site is the upper thigh (20cm distal to the iliac spine). The guidelines on performing the skin biopsy were established by the European Federation of Neurological Societies in 2005. The biopsy should be 3mm thick to enable assessment of both the epidermis and dermis. Suturing is not required. Risks include infection, tenderness at biopsy site, delayed healing, bleeding, allergy.
The biopsy sample is stained immunohistochemically with antibodies against protein gene product 9.5 (PGP 9.5). This protein is a marker for peripheral nerve fibres and neuroendocrine cells. Most cutaneous nerve fibres are unmyelinated, but in the dermis of hair skin 10% are small diameter myelinated fibres (A-delta fibres). A fibre count is done, and single axons are counted that cross or originate at the epidermal-dermal junction. The result is the Epidermal Nerve Fibre Density (ENFD). Reduced ENFD has a 90% specificity and 82.6% sensitivity for small fibre neuropathy.
In New Zealand the PGP9.5 stain doesn't appear to be available, but it may be possible to request an epidermal nerve fibre density assessment through other stains. It is viewed as a controversial investigation by pathologists in New Zealand.
Testing for Autonomic Symptoms
There are a variety of tests for autonomic symptoms. The most applicable for clinical care are cardiovagal tests, water induced skin wrinkling, neuropad, sudoscan, pupillometry, and bladder function tests.
Water-induced and EMLA-induced Skin Wrinkling
The patient places both hands in a bath of water heated to 40 degrees. The wrinkling is graded for the second to fifth fingers. Grade 0: wrinkling absent; grade 1: slight wrinkling and the fingertip is not smooth; grade 2: two or less lines of shallow wrinkling on the fingertip; grade 3: three or more lines of clear cut lines of wrinkling on the fingertip; grade 4: wrinkling completely distorts the pulp of the fingertip. The grade is averaged across fingers.
EMLA can be used as an alternative to induce skin wrinkling. Apply 1g to the distal digit tips of the 2nd to 5th fingers, and leave it to soak into the skin for 30 minutes after covering with Tegaderm.
In both cases, with sympathetic nerve dysfunction, there is less wrinkling seen due to prevention of vasovonstriction. Studied in patients with sensory neuropathy, the diagnostic accuracy depends on the cut off of the comparison standard of reduced intraepithelial nerve fibre density. Overall both tests have good positive predictive values, but poor negative predictive values. There was no correlation between grade of wrinkling and nerve density. For wrinkling grading, a score of 12 or more for each hand was used as the cut off for normal, and a difference of 4 or more difference per hand was used as a cut-off for a different score.
One abnormal cardiovagal test is possible cardiovascular autonomic neuropathy, two positive tests is definite, and severe is defined as orthostatic hypotension in addition to definite.
Heart rate response to valsalva (parasympathetic) is a test where the RR interval is measured on ECG to determine heart rate variability. There are five phases: ; phase (0) deep inspiration, phase (I) onset of strain, phase (II) continued strain, phase (III) release, phase (IV) recovery. The ratio is calculated by taking the shortest RR interval in phase II and dividing it by the longest in phase IV. An abnormal result is a lack of bradycardia during phase IV and a decreased valsalva ratio.
Heart rate response to postural change (parasympathetic) is where the patient changes from supine to upright and the heart rate is measured. A normal result is where the heart rate increases initially and then reduces. The 30:15 ratio is calculated, taking the bradycardia at 30 seconds and dividing it by the tachycardia at 15 seconds. Normally the heart rate increases by 10 seconds. In autonomic dysfunction there is no bradycardia.
Heart rate response to deep inspiration (parasympathetic) is looking for variability of the amplitude of individual electrical complexes on ECG with deep inspiration.
Blood pressure response to valsalva (sympathetic): In autonomic dysfunction there is no overshoot in blood pressure and bradycardia reflex.
Blood pressure response to postural change (sympathetic): The blood pressure should increase by 10mmHg after 1-2 minutes.
There is no known cure for SFN. Standard neuropathic pain medications can be trialled such as gabapentinoids, topiramate, TCAs, and SNRIs. The combination of a TCA and gabapentinoid may be more effect than monotherapy. Opioids are not recommended, but weak opioids such as codeine or tramadol can be considered for short periods of use only. Topical lidocaine or capsaicin has been used.
- Maslinska et al. Small fibre neuropathy as a part of fibromyalgia or a separate diagnosis? Int. J. Clin. Rheumatol. (2018) 13(6), 353-359. Full Text
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