Table 3.

Diagnostic steps in clinical trials and practice

Diagnostic method	Parameters of interest	Strengths	Weaknesses	Validation, use, and references
Patient history	Positive sensory symptoms: dysesthesia, paresthesia, neuropathic pain Negative sensory symptoms: numbness, thermal hypoesthesia, insensitivity to pain Distal or proximal muscle weakness Impaired fine motor skills Gait disturbances: stumbling, steppage, afferent ataxia, walking aids, falls Autonomic disturbances: orthostatic dysfunction, disturbed sweating, diarrhea and/or constipation, early satiety, unintended weight loss, incontinence, erectile dysfunction Cardiac symptoms: ankle edema, effort-dependent dyspnea, palpitations, dizziness Other organ involvement: carpal tunnel syndrome, vitreous opacities, or nephrotic syndrome Treatment side effects including specific (e.g. infusion reactions) and unspecific (e.g. headaches, urinary tract infections) reactions Family history	Easy, fast, non-invasive, and cheap to assess and to repeat Applicable in all disease stages Depicts a broad spectrum of disease aspects	Subjectivity	Recommendations on clinical practice [1–3]
Neurological examination	Sensory status: light touch, vibration, position, temperature, and pinprick sensation Gait patterns: afferent ataxia, steppage Isolated muscle strength (Medical Research Council) Deep tendon reflexes Inspection of the undressed extremities: wounds, ulcera, atrophy, edema	Easy, fast, non-invasive, and cheap to assess and to repeat Broad overview on different disease aspects Reliability and objectivity can be increased when performed by an experienced/the same examiner Applicable in all disease stages	Examiner dependency	NIS [4, 5] NIS-LL [6] mNIS+7 [5, 7, 8]
Nerve conduction studies	Neuropathy verification, analysis of patterns (length-dependent, sensorimotor) and qualities (axonal damage represented by reduced compound motor and sensory nerve action potentials) Pathological spontaneous activity in distal muscle electromyography as a sign of axonal damage Nerve conduction velocities may be delayed and F-waves abolished due to the loss of fast conduction axons, which can mislead to the diagnosis of CIDP [9]	Objective, quantifiable and repeatable tool to measure large fiber function Very sensitive in stage 1 and 2 Equipment is widely available	Uncomfortable/painful procedure Less sensitive in the very early stage 1 and from late stage 2 on	mNIS+7 [5, 7, 8]
Neuromuscular ultrasound	Cross-sectional areas are typically not or mildly enlarged only (other than in CIDP or demyelinating CMT) Increased muscle echogenicity in advanced disease stages	Objective, quantifiable tool to measure nerve structure Suitable to distinguish from CIDP Non-invasive, not painful	Requires specific equipment and training	Ultrasound pattern sum score (UPSS) [10]
Quantitative sensory testing	Early signs of small fiber involvement represented by cold detection threshold, mechanical pain threshold, paradoxical heat sensations (indicating dysfunction of Aδ fibers each), warm detection and heat pain threshold (indicating C fiber involvement), cold pain and pressure pain threshold (C and Aδ fibers) Distinction from (early) large fiber involvement (represented by mechanical detection and vibration threshold) and central sensitization (represented by dynamic mechanical allodynia and mechanical pain sensitivity)	Sensitive in very early stage 1 (small fiber involvement) Non-invasive, easy to learn	Less sensitive from late stage 1 on Highly dependent on the patient’s collaboration Time consuming Requires standardized equipment and training	DFNS protocol [11–14] mNIS+7 [5, 7, 8]
Autonomic testing	Orthostatic dysregulation (Winkler scale, Schellong test, tilt table) Disturbed sweating [SUDOSCAN, quantitative sudomotor axon reflexe testing (QSART)] Gastrointestinal disturbances (modified body mass index)	Sensitive already in the early disease stages	Relies partly on subjective patient descriptions Susceptible to interference, requires patient preparation (e.g. no caffeine, no skin care products) Tilt table: can cause uncomfortable effects like dizziness, fainting; not possible in cases of cardiac arrhythmia or pacemaker dependence	COMPASS-31 [7, 15] Winkler scale CASS score [16]
Questionnaires	Pain: neuropathic character, severity, course (PainDETECT, PainPREDICT, Neuropathic Pain Symptom Inventory (NPSI)) Disability: daily-life activities, impairment quantification (Rasch-built Overall Disability Scale (R-ODS)) Quality of life (Norfolk QoL)	Easy to assess, non-invasive, repeatable	Subjectivity	PainDETECT [17] NPSI [18] PainPREDICT [19] R-ODS [7, 8, 20, 21] Norfolk QoL [7, 8, 22, 23]
Histology	Congo red amyloid staining: amyloid depiction, most frequently used; not obligatory for treatment, helpful in undecisive cases Thioflavine S fluorescence microscopy: amyloid depiction ATTR immunohistochemistry: TTR specification	Salivary glands [24] and fat tissue aspirates [25] are easy accessible and show a high sensitivity Skin biopsies provide additional information on small fiber degeneration Sural nerve (and muscle) biopsies are helpful to distinguish from other neuropathies Myocardial biopsies might be obtained in the standard diagnostic procedures of cardiomyopathy workup Other potentially assessable tissues: deep rectal mucosa, carpal ligament material	Risk of false negative test results Should be evaluated in specified centers only	Amyloidosis guidelines and phenotyping studies [26, 27]

Overview on the different diagnostic work steps, including parameters of interest, validation, strengths, and weaknesses. A detailed description of all diagnostic procedures and a list of the cited references are provided in the supplementary material