Getting the Right Diagnosis: Testing and Accuracy
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CMT1A is diagnosed using Nerve Conduction Velocity (NCV) tests that show uniform nerve slowing below 38 m/s, followed by genetic testing. Confirming a PMP22 gene duplication is essential for an accurate diagnosis and avoids mistaking CMT1A for autoimmune conditions like CIDP.
Key Takeaways
- • A diagnosis of CMT1A relies on Nerve Conduction Velocity (NCV) tests showing uniform slowing of electrical nerve signals.
- • In adult CMT1A patients, nerve conduction speeds are typically less than 38 meters per second in the arms.
- • Confirming the diagnosis requires a genetic test showing a duplication of the PMP22 gene.
- • Distinguishing CMT1A from autoimmune conditions like CIDP is critical to avoid unnecessary and ineffective immune-suppressing treatments.
Getting an accurate diagnosis is the first step toward effectively managing Charcot-Marie-Tooth disease type 1A (CMT1A). Because CMT1A is a slowly progressive condition that shares symptoms with other nerve disorders, doctors rely on a combination of electrical “stress tests” for the nerves and definitive genetic mapping [1][2].
The Diagnostic Hallmark: Nerve Conduction Velocity (NCV)
A Nerve Conduction Velocity (NCV) test measures how fast electrical signals travel through your nerves. In CMT1A, the primary problem is the “insulation” (myelin) of the nerve, which causes signals to slow down uniformly [3][4].
- The Classic Threshold: For most adults with CMT1A, the “gold standard” finding is a conduction velocity of less than 38 meters per second (m/s) in the arms [1]. For context, normal speeds are typically above 50 m/s.
- The “Intermediate” Range: Some patients with very mild symptoms may have velocities in the “intermediate” range—between 38 and 45 m/s [1][5]. In these cases, genetic testing is absolutely essential to distinguish CMT1A from other types of CMT [1].
- Infants and Children: In children under one year old, NCV tests can sometimes appear normal or near-normal, as the electrical abnormalities may take time to develop [3].
Avoiding the “CIDP” Misdiagnosis
One of the most common diagnostic pitfalls is mistaking CMT1A for Chronic Inflammatory Demyelinating Polyneuropathy (CIDP) [6]. This is a serious error because CIDP is an autoimmune condition treated with expensive immune-suppressing therapies (like steroids or IVIG) that do not help CMT1A patients [6].
| Feature | CMT1A (Genetic) | CIDP (Autoimmune) |
|---|---|---|
| Progression | Very slow, steady over decades [7] | Can be rapid, fluctuating, or “relapsing” [7] |
| Symmetry | Usually very symmetrical (both sides equal) | Often asymmetrical (one side worse) [8] |
| NCV Pattern | Uniform slowing across all nerve segments [4] | Conduction block or “patchy” slowing [9] |
| Family History | Often present (but not always) | Usually absent |
CMT1A and Guillain-Barré Syndrome (GBS)
While CMT1A is a genetic condition, patients can still develop acquired nerve issues like Guillain-Barré Syndrome (GBS), an acute inflammatory attack on the nerves [8]. It is important to know that having CMT1A does not fundamentally increase your biological risk of contracting GBS. However, if a person with CMT1A experiences a sudden, rapid increase in weakness over a few days—especially after a viral illness—it is vital to seek emergency care. This “superimposed” GBS can look different in CMT1A patients, often appearing more severe on electrical tests than expected, and requires immediate medical attention to distinguish it from the slow progression of CMT1A [8][10].
Genetic Testing: The Final Word
While NCV tests suggest CMT1A, genetic testing is the only way to confirm it [1]. The test looks specifically for a duplication of the PMP22 gene on chromosome 17 [11].
In some cases, a patient may have a “point mutation” (a small typo in the gene) rather than a full duplication. This is known as CMT1E and can sometimes result in more severe symptoms than the typical CMT1A duplication [12].
Diagnosis Completeness Checklist
When reviewing your medical records, ensure your diagnostic report includes the following:
- [ ] Genetic Confirmation: Documentation of the PMP22 duplication [2].
- [ ] NCV Values: Specific speeds (in m/s) for the ulnar and median nerves [5].
- [ ] Symmetry Check: Note whether the nerve slowing is “uniform” or “diffuse” [3].
- [ ] Severity Score: A CMTNS (CMT Neuropathy Score) to serve as a baseline for future comparison [1].
- [ ] Metabolic Screening: Results for HbA1c (diabetes) or Vitamin B12, as these conditions can worsen CMT1A [13][14].
Frequently Asked Questions
What does a nerve conduction velocity (NCV) test show in CMT1A?
How is a CMT1A diagnosis officially confirmed?
Why is CMT1A sometimes misdiagnosed as CIDP?
Can a child with CMT1A have normal nerve test results?
What should I do if my weakness suddenly gets much worse?
Questions for Your Doctor
- • What were the exact conduction velocities measured in my or my child’s median and ulnar nerves?
- • Does the report show 'uniform slowing' or are there signs of 'conduction block' that might suggest an inflammatory condition like CIDP?
- • If my NCV was above 38 m/s, did we confirm the diagnosis with a PMP22 duplication genetic test?
- • Are there any signs of 'temporal dispersion' in the testing that might indicate a secondary issue like diabetes?
- • How does my current Charcot-Marie-Tooth Neuropathy Score (CMTNS) compare to previous years?
- • Should we be screening for other metabolic issues, like glucose intolerance, that could be mimicking a faster progression?
Questions for You
- • Have my symptoms progressed slowly over years, or was there a sudden, rapid worsening of weakness (which might suggest a superimposed issue like GBS)?
- • Has a doctor ever suggested an immune-based treatment (like IVIG or steroids) that didn't seem to help?
- • Do I have a copy of my genetic test results that specifically mentions the 'PMP22 duplication'?
- • Have I noticed any unusual fluctuations in my symptoms, such as getting better and then worse again?
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References
- 1
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Mechanisms and treatment strategies of demyelinating and dysmyelinating Charcot-Marie-Tooth disease.
Hertzog N, Jacob C
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PMID: 36926710 - 3
Young infants with PMP22 duplication can have minor nerve conduction study abnormalities.
Davion JB, Cassim F, Péréon Y, Nguyen The Tich S
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PMID: 36253232 - 4
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PMID: 27412484 - 5
Charcot-Marie-Tooth disease type 1C: Clinical and electrophysiological findings for the c.334G>a (p.Gly112Ser) Litaf/Simple mutation.
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PMID: 28164329 - 6
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Neurogenetics 2025; (26(1)):37 doi:10.1007/s10048-025-00808-9.
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PMID: 32765395 - 8
Guillain-Barré syndrome in patients with Charcot-Marie-Tooth type 1A disease, probably a non-random association.
Davion JB, Devaux J, Tard C, et al.
Neurophysiologie clinique = Clinical neurophysiology 2025; (55(4)):103071 doi:10.1016/j.neucli.2025.103071.
PMID: 40120365 - 9
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Uncini A, Cavallaro T, Fabrizi GM, et al.
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PMID: 38600691 - 10
Guillain-Barré Syndrome With Asymmetrical Weakness in a Patient With Charcot-Marie-Tooth Disease Type 1A.
Ahmed K, Elachola M, Alsararatee H, Pugh N
Cureus 2025; (17(10)):e94525 doi:10.7759/cureus.94525.
PMID: 41098931 - 11
Antisense oligonucleotides offer hope to patients with Charcot-Marie-Tooth disease type 1A.
Shy ME
The Journal of clinical investigation 2018; (128(1)):110-112.
PMID: 29199996 - 12
Peripheral Myelin Protein 22 Gene Mutations in Charcot-Marie-Tooth Disease Type 1E Patients.
Jung NY, Kwon HM, Nam DE, et al.
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PMID: 35886002 - 13
High glucose level as a modifier factor in CMT1A patients.
Secchin JB, Leal RCC, Lourenço CM, et al.
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PMID: 32347995 - 14
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PMID: 26349404
This page explains the diagnostic process and testing for CMT1A for educational purposes only. Always consult a neurologist or genetic counselor for formal diagnosis, test interpretation, and medical advice.
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