Small Fiber Neuropathies Flashcards
(37 cards)
What are Small Fiber Neuropathies (SFN)?
Heterogeneous disorders affecting unmyelinated C-fibers and thinly myelinated A-delta fibers, causing sensory/autonomic symptoms without motor deficits.
What is the impact of Small Fiber Neuropathies?
Debilitating pain (85% of patients), temperature loss, autonomic dysfunction (e.g., sweating, cardiovascular).
What are common causes of Small Fiber Neuropathies?
- Diabetes
- Metabolic syndrome
- Alcohol
- Amyloidosis (hereditary TTR)
- Chemotherapy
- HIV
- Autoimmune diseases (Sjögren’s, SLE)
What are genetic causes of Small Fiber Neuropathies?
- SCN9A/10A/11A variants
- Fabry’s disease
- Hereditary TTR amyloidosis
What percentage of Small Fiber Neuropathies cases are idiopathic?
~50% of cases.
What emerging associations are linked to Small Fiber Neuropathies?
- Fibromyalgia
- Parkinson’s
- Hypermobile Ehlers-Danlos
What are common sensory symptoms of Small Fiber Neuropathies?
- Pain: Burning, superficial, sharp, or cold sensations
- Temperature Loss: Distal > proximal or non-length-dependent
What autonomic symptoms are associated with Small Fiber Neuropathies?
- Sudomotor dysfunction
- Orthostatic hypotension
- GI dysmotility
What red flags should be noted in Small Fiber Neuropathies?
- Non-length-dependent patterns
- Rapid progression
- Systemic symptoms (e.g., weight loss)
What are the Besta Criteria for diagnosing Small Fiber Neuropathies?
Requires ≥2 of:
* Clinical signs (pinprick/thermal loss, allodynia)
* Abnormal QST thermal thresholds
* Reduced intraepidermal nerve fiber density (IENFD)
What are the Neurodiab Criteria for diagnosing Small Fiber Neuropathies?
Possible: Symptoms/signs of SFN.
Probable: Symptoms/signs + normal sural NCS.
Definite: Symptoms/signs + normal sural NCS + abnormal IENFD/QST.
What does Thermal Threshold Testing (QST) measure?
Warm/cold detection thresholds (C-fibers/A-delta).
What is the sensitivity and specificity of Thermal Threshold Testing (QST)?
Sensitivity: 50% (cold > warm). Specificity: Moderate.
What does the Sympathetic Skin Response (SSR) measure?
Sudomotor function (postganglionic C-fibers).
What is the sensitivity and specificity of the Sympathetic Skin Response (SSR)?
Sensitivity: 40%. Specificity: Low.
What does the Cutaneous Silent Period (CSP) measure?
A-delta fiber function.
What is the normative latency for the Cutaneous Silent Period (CSP)?
<80 ms (upper limbs), <120 ms (lower limbs).
What are the types of Pain-Related Evoked Potentials?
- Laser-evoked (gold standard)
- Electrical/contact heat
What is the sensitivity and specificity of Pain-Related Evoked Potentials?
Sensitivity: 78%. Specificity: 81%.
What is the gold standard investigation for Small Fiber Neuropathies?
Skin biopsy (IENFD) at distal leg (3 mm punch biopsy).
What is the sensitivity and specificity of Skin Biopsy?
Sensitivity: 70–88%. Specificity: High.
What does Autonomic Testing measure?
- Valsalva ratio (parasympathetic)
- Orthostatic BP/heart rate (sympathetic)
What is QSART and its sensitivity?
Quantitative Sudomotor Axon Reflex Test; Sensitivity: 80% (sudomotor).
What does Sudoscan/Body Scan measure?
Electrochemical skin conductance.
