16.3 Polyneuropathies, Mononeuropathy Multiplex, and Muscle Disorders

Why polyneuropathy and muscle sit on the same outline cluster

Outline VI.B.6 (polyneuropathies), VI.B.5.e (mononeuropathy multiplex), and VI.B.7 (muscle disorders) ask you to recognize maps, not to issue a neuromuscular-clinic diagnosis from one nerve. Independent OpenExamPrep teaching for R.NCS.T. candidates treats Charcot–Marie–Tooth (CMT), acute inflammatory demyelinating polyradiculoneuropathy (AIDP) / Guillain–Barré syndrome (GBS), chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), toxic / metabolic / infectious neuropathies, vasculitic multiplex, and myopathy as different answers to three worksheet questions: Are SNAPs involved? Is slowing uniform or multifocal? Is the process length-dependent, asymmetric-named-nerve, or muscle? This is independent study of published outline headings. It is not an official AAET text and does not invent unpublished official cut scores.

Needle EMG of myopathic motor-unit potentials is physician interpretation. The technologist still must produce NCS that can exclude demyelinating neuropathy, exclude length-dependent sensory loss, or show the multiplex map.

CMT1: uniform demyelinating hereditary neuropathy

Charcot–Marie–Tooth disease type 1 (CMT1) is the classroom demyelinating hereditary polyneuropathy (often PMP22 duplication in CMT1A, though NCS does not name the gene). Clinical frame: long-standing distal weakness, pes cavus (high arches), hammer toes, family history, little pain compared with inflammatory neuropathies.

NCS teaching:

• Conduction velocity uniformly slow, often very slow. Median motor CV in demyelinating CMT teaching is commonly below about 38 m/s, and many CMT1 nerves sit in a 15–30 m/s classroom range. Those numbers are typical electrodiagnostic teaching, not unpublished official AAET cut scores.
• Slowing is symmetric and diffuse, not a patch of block at the elbow plus a normal opposite arm.
• Conduction block and marked temporal dispersion are not the CMT1 signature. Uniform myelin pathology slows the whole cable; acquired inflammatory demyelination desynchronizes and blocks patchily. A little dispersion at common entrapment sites can occur in any chronically slow nerve; it is not CIDP by itself.
• Distal latencies prolong; F waves prolong in proportion to the uniformly slow path.
• SNAPs are often small or absent because chronically demyelinated and secondarily axonal sensory fibers do not give large volleys. Do not call CMT1 a “pure motor” disease because the SNAP is hard to record.

If every nerve is 22 m/s and the patient has high arches since adolescence, you are in uniform hereditary demyelination, not AIDP of last Tuesday.

CMT2: axonal hereditary neuropathy

CMT2 is the axonal CMT cousin. NCS:

• Amplitudes low (SNAPs often first and most, CMAPs in wasted distal muscles).
• Velocities relatively preserved or only mildly slow, in proportion to loss of large axons — not the 20 m/s uniform map of CMT1.
• Still length-dependent and symmetric over years, still the high-arch / family-history frame more often than acute pain.

Mixing CMT1 and CMT2 is an outline error. One is a velocity disease with secondary axonal loss; the other is an amplitude disease from the start.

Hereditary neuropathy with liability to pressure palsies (HNPP) (often PMP22 deletion) is the related trap: recurrent focal block or slowing at common entrapment sites, sometimes with mildly slow background velocities. It is not vasculitis multiplex and not CMT1’s uniform 20 m/s cable, but it belongs in the hereditary conversation so you do not call every tomaculous entrapment “CIDP.”

AIDP / GBS: acute acquired demyelination

AIDP is the demyelinating form of GBS: acute, often post-infectious, areflexic weakness that nadirs within about four weeks. NCS is time-dependent. Typical electrodiagnostic teaching:

• Early (days): the most sensitive conduction clues are often F-wave absence or prolongation and H-reflex loss, because roots and proximal segments are inflamed. Distal CMAPs may still look deceptively normal. A “normal” distal-only worksheet in week 1 has not excluded AIDP.
• Later (often 1–3 weeks): conduction block, temporal dispersion, prolonged distal latencies, and slowed velocities appear as the demyelinating physiology becomes easier to record.
• Sural-sparing pattern: a taught AIDP sensory clue is an abnormal median or ulnar SNAP with a relatively preserved sural SNAP. That is the opposite of length-dependent axonal polyneuropathy, which attacks the sural first. Sural sparing is not required in every GBS case and is not an official AAET checkbox; it is a pattern you should recognize when it is present.
• Blink-reflex R1 delay can support proximal demyelination when facial weakness is part of the GBS picture (Chapter 8).

Axonal GBS variants exist. Acute motor axonal neuropathy (AMAN) drops CMAPs with relatively spared SNAPs; acute motor-sensory axonal neuropathy (AMSAN) drops both. Anti-ganglioside associations (for example GM1 in some AMAN cases) are clinical-lab correlations. Do not insist that every GBS worksheet show CIDP-style block and dispersion. Do teach that classic AIDP is the demyelinating, late-response-first story.

CIDP: chronic acquired, multifocal demyelination

CIDP is the chronic acquired demyelinating polyneuropathy. Symptom evolution is measured in months (typical teaching threshold more than about eight weeks, versus GBS’s four-week nadir). NCS:

• Multifocal demyelination: conduction block, temporal dispersion, segment-to-segment velocity differences, asymmetric or patchy distal-latency prolongation.
• Not uniformly slow like CMT1. A nerve at 28 m/s next to a nerve at 48 m/s with a block in the forearm is acquired, not CMT1A.
• Sensorimotor. SNAPs are commonly abnormal. This is a major contrast with MMN, which spares SNAPs.
• Proximal and distal weakness, areflexia, and a treatable inflammatory frame belong to the physician; your job is the non-uniform demyelinating map.

CIDP versus CMT1 versus AIDP in one sentence: CMT1 is old, uniform, very slow; AIDP is acute, late-response-first, then block/dispersion, sometimes sural-sparing; CIDP is chronic, patchy block and dispersion, not a lifelong 20 m/s cable.

Other toxic, metabolic, and infectious polyneuropathies

Most of this bucket is length-dependent axonal disease until a named exception says otherwise.

Diabetes mellitus (distal symmetric polyneuropathy, the everyday DPN): toes and feet first — sural and plantar SNAPs fall, distal CMAPs may fall later, velocities mildly slow only in proportion to axonal loss, not CMT1-slow. F waves can prolong mildly. Superimposed median neuropathy at the wrist is common; that is entrapment plus polyneuropathy, not CIDP. Diabetes can also produce radiculoplexus neuropathy (asymmetric, painful, proximal) and, less often, a multiplex picture — those are not the stocking-glove axonal default, and they need a different map (asymmetric CMAPs, not just a low sural).

Chemotherapy: vinca alkaloids and taxanes are typical length-dependent axonal sensory or sensorimotor neuropathies. Platinum agents more often behave like a sensory neuronopathy (section 16.1): SNAPs crash, including in the hands, with relatively spared motor studies. Do not file every “chemo neuropathy” as a stocking-glove axonal tale.

Alcohol, vitamin B12 deficiency, copper deficiency, uremia, and many idiopathic late-life neuropathies: again, distal axonal, SNAPs first. B12 and copper can add a myeloneuropathy clinically; NCS still shows the axonal neuropathy component when present.

Hypothyroidism can slow conduction and enlarge distal latencies; treat it as a metabolic contributor and still look for superimposed entrapments (carpal tunnel is common).

Toxic metals: lead classically emphasizes motor, sometimes upper-limb (wrist drop) more than a neat stocking sensory map. Arsenic is a painful axonal sensorimotor neuropathy, sometimes with skin clues. These are named exceptions to “all toxins look like diabetes.”

Infectious and inflammatory-infectious:

• Leprosy (Hansen disease): predilection for cool, superficial nerves (ulnar at the elbow, fibular at the fibular head, greater auricular, facial). Can look like sequential mononeuropathies or focal demyelinating/axonal lesions rather than a tidy dying-back polyneuropathy.
• Lyme: facial palsy, painful radiculoneuritis; NCS follows the localization (facial CMAP, limb roots) rather than a mandatory sural-first axonal pattern.
• HIV: distal axonal polyneuropathy is common; inflammatory demyelinating neuropathies also occur. Cytomegalovirus polyradiculopathy in advanced immunocompromise is a root pattern (section 16.1), not a toe-tip dying-back map.
• Diphtheria: demyelinating neuropathy is the historical teaching exception among infections.
• Hepatitis C / cryoglobulinemia and some vasculitic infections blur into mononeuropathy multiplex.

Unusual non-length-dependent or motor-predominant metabolic patterns (acute intermittent porphyria, some amyloid neuropathies that may be painful small-fiber early with later axonal NCS, Refsum and other rare demyelinating metabolic diseases) exist so you do not claim every metabolic neuropathy is diabetic DPN. If the map is patchy, proximal, purely motor, or neuronopathic, say so instead of forcing a stocking gradient.

Mononeuropathy multiplex (VI.B.5.e)

Mononeuropathy multiplex (mononeuritis multiplex) is asymmetric, stepwise involvement of named nerves (for example left fibular, then right ulnar, then left median) rather than a symmetric stocking. Classic teaching association: vasculitis (polyarteritis nodosa, rheumatoid vasculitis, ANCA-associated vasculitis, diabetic radiculoplexus overlapping this map). NCS:

• Axonal loss in individual nerve territories: SNAP and CMAP fall in that nerve, not in every nerve of that limb equally.
• Side-to-side differences are the point. A “symmetric polyneuropathy protocol” that only reports the worse sural will miss multiplex.
• Demyelinating block is not the vasculitis signature; ischemia infarcts axons. If you see MMN-style motor block with spared SNAPs, you are in MMN, not typical vasculitic multiplex.
• Leprosy, sarcoidosis, infiltrative disease, and HNPP can mimic a multiplex distribution for different reasons. The outline still wants the asymmetric named-nerve electrical map.

Worked contrast: a patient with years of numb toes, low sural SNAPs bilaterally, and mildly low distal CMAPs is length-dependent DPN. A patient who drops a foot on Monday and an ulnar hand on Friday, with a normal contralateral sural SNAP, is multiplex until proven otherwise. Record both sides and more than one nerve per limb.

Muscle disorders (VI.B.7)

Myopathy (inflammatory myositis, muscular dystrophy, toxic myopathy, endocrine myopathy) is a muscle-fiber localization. NCS role is mostly negative space:

• SNAPs typically normal. If SNAPs are gone, you do not have an isolated myopathy explanation for the sensory loss.
• CMAPs may be low if the recorded muscle is atrophic or severely weak (the generator is small). Distal latencies and velocities are not demyelinating-slow. There is no MMN-style conduction block as the myopathy mechanism.
• Motor NCS can look “axonal” in amplitude only. The physician’s needle EMG (short-duration, low-amplitude motor-unit potentials, early recruitment) separates myopathy from motor-neuron or neuropathic recruitment. That needle reading is not an R.NCS.T. task, but you should know why a low CMAP plus a normal SNAP plus normal velocity is compatible with myopathy or motor-neuron disease — and why sensory NCS and a search for block still matter to exclude neuropathy.

Distal myopathies can shrink foot or hand CMAPs and mimic motor neuropathy until SNAPs and needle EMG are considered. Periodic paralysis and some myotonic disorders may have normal routine NCS between attacks; they are not diagnosed by a CIDP protocol. Neuromuscular-junction disease (Chapter 10) can also lower CMAP amplitude, especially after repetitive stimulation; do not file every small CMAP as myopathy.

Independent OpenExamPrep one-liner: myopathy does not demyelinate peripheral nerve. If velocities are CMT1-slow or there is acquired block and dispersion, you are not finished with a muscle explanation.

Comparison table: CMT versus AIDP versus CIDP versus axonal DPN versus myopathy

Typical electrodiagnostic teaching. Classroom timing and velocity figures are not unpublished official AAET cut scores.

Worked laboratory menus (what to actually record)

Suspected CMT: bilateral motor and sensory studies in arms and legs, F waves, note uniformity. One slow entrapment does not make CMT1.

Suspected AIDP in week 1: do not stop at distal CMAPs. Add F waves in multiple nerves, H-reflexes, at least one blink reflex if the face is weak, and upper-limb and sural SNAPs to look for sural sparing. Repeat later in the admission if early studies are nondiagnostic.

Suspected CIDP: proximal and distal stimulation on several motor nerves looking for block and dispersion, sensory studies, comparison with the uniform-CMT expectation.

Suspected DPN: sural, plantar or distal motor, a second limb, and a median study because coincidental carpal tunnel is common. Do not call mild median slowing at the wrist “CIDP.”

Suspected multiplex: side-to-side named nerves that are clinically involved, plus at least one clinically spared nerve as an internal control.

Suspected myopathy: SNAPs, motor NCS to the weak muscle and a distal nerve, no demyelinating map. The physician adds needle EMG. If you find block, dispersion, or SNAP loss, you have found a neuropathy that myopathy does not explain.

Cover every named outline entity in one closing list: CMT1 (uniform demyelinating hereditary), CMT2 (axonal hereditary), AIDP/GBS (acute, late responses then CB/dispersion, sural-sparing teaching), CIDP (chronic acquired multifocal demyelination), other toxic/metabolic/infectious (mostly length-dependent axonal — diabetes, many chemotherapies, alcohol, B12, uremia — with named exceptions such as platinum neuronopathy, lead motor, leprosy, Lyme, diphtheria, HIV), mononeuropathy multiplex (asymmetric vasculitic named-nerve axonal loss), muscle disorders (normal SNAPs, possible low CMAP from atrophy, no demyelinating slowing). Independent OpenExamPrep study of VI.B.5.e and VI.B.6–7 is that list as maps on a worksheet, not as a claim of official partnership with AAET.