Diagnosing CMT and Understanding Your Tests

The journey to a Charcot-Marie-Tooth (CMT) diagnosis often begins with a physical exam where a doctor looks for classic signs like high foot arches (pes cavus), curled toes (hammer toes), or a loss of deep tendon reflexes ^[1][2][3]. However, to confirm the diagnosis and determine the specific type of CMT, specialized tests are required.

The Electrical Tests: EMG and NCS

To see how your nerves are functioning, doctors use two tests that measure the electrical activity in your muscles and nerves.

1. Nerve Conduction Study (NCS): This test measures how fast and how strong an electrical signal travels through your nerves ^[3]. It involves placing small sensors on your skin and giving the nerve a tiny electrical pulse (which feels like a quick “tap” or “tingle”).
2. Electromyography (EMG): This test checks the health of your muscles and the nerves that control them ^[3][4]. A very thin needle is inserted into the muscle to record its electrical activity while you relax and then gently contract the muscle.

The “38 m/s” Cutoff

In the world of CMT, the number 38 meters per second (m/s) is a critical marker when measuring the upper extremity motor nerves (like the median or ulnar nerve in the arm) ^[3][5]. (Leg nerves are naturally slower and often more degraded, so arm nerves are used for this classification).

• Below 38 m/s: This usually points toward CMT Type 1 (Demyelinating), where the “insulation” (myelin) is damaged, causing the signal to slow down ^[3][5].
• Above 38 m/s: This often points toward CMT Type 2 (Axonal), where the “wire” (axon) is damaged. In this case, the signal speed might be near normal, but the signal strength is often very low ^[3][6].

The Genetic “Gold Standard”

While electrical tests can tell you how the nerve is damaged, genetic testing is the only way to find out why ^[7][8]. Using a Next-Generation Sequencing (NGS) panel, doctors can look at over 100 known CMT genes at once ^[9][10]. This is the “gold standard” because it identifies the exact mutation, which is essential for understanding your personal prognosis and qualifying for future clinical trials ^[11][8].

Please note that waiting for genetic test results can take several weeks to a few months ^[7]. This delay is normal and does not mean anything is wrong with your sample.

CMT vs. CIDP: Why the Distinction Matters

It is vital to distinguish CMT from Chronic Inflammatory Demyelinating Polyneuropathy (CIDP).

• CIDP is an autoimmune condition where the body attacks the nerves. Unlike CMT, CIDP is often treatable with immunotherapy like steroids or IVIg ^[12][13].
• The Risk of Misdiagnosis: Because the symptoms look similar, some CMT patients are misdiagnosed with CIDP and given years of expensive, ineffective treatments ^[12][14].

Doctors look for specific “red flags” that suggest CIDP rather than CMT, such as asymmetric weakness (affecting one side more than the other) or a conduction block (where the electrical signal stops completely at one point) ^[12][15].

Your Completeness Checklist

Ensure the following information is documented in your medical record to support an accurate diagnosis:

• [ ] Full Clinical History: Detailed notes on when symptoms started and any family history of foot issues ^[3].

• [ ] NCS/EMG Report: Including specific conduction velocities (m/s) and signal amplitudes ^[3][16].

• [ ] Genetic Testing Results: A copy of the laboratory report identifying the specific gene mutation ^[7][8].

• [ ] Exclusion of Mimics: Documentation ruling out diabetes, B12 deficiency, and CIDP ^[7][12].

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