CMT1 and CMT2 are the two basic CMT Types that doctors are able to clinically diagnose. CMT is divided into three types of neuropathy: demyelinating, axonal, and intermediate. Each of these three are determined by their related nerve conduction characteristics, how the nerves conduct (transmit) a signal. Clinically, demyelinating CMT, as determined by nerve conduction studies, is classified as CMT1. Axonal CMT, as determined by nerve conduction, is classified as CMT2. Intermediate CMT exhibits nerve conduction characteristics that don’t comport with either demyelinating or axonal CMT, the nerve conduction profile is somewhere in between, it’s intermediate. Intermediate does not refer to disease severity or progression.
Doctors trained in recognizing the electrodiagnostic abnormalities of CMT and the nuanced differences between demyelinating CMT and axonal CMT can use NCS data to tell whether CMT is either demyelinating (CMT1) or axonal (CMT2), but intermediate CMT is exceedingly difficult to elecctrodiagnostically differentiate (NCS is a type of electrodiagnostic test). Because of this difficulty, doctors typically will not clinically diagnose somebody as having intermediate CMT, instead favoring either a CMT1 or a CMT2 clinical diagnosis.
How can I tell if I have CMT1? Doctors can tell the difference between demyelinating CMT and axonal CMT by NCS results. Demyelinating CMT nerve conduction exhibits velocities less than 38meters/sec (the speed at which the nerve transmits a signal), slightly reduced amplitudes (signal strength), prolonged latencies (how quickly the nerve responds to an input signal), fairly uniform findings in all nerves tested, and there might be absent sensory nerve responses. When NCS results meet these criteria, and the person meets the appropriate diagnostic picture, CMT1 is the clinical diagnosis.
Once the underlying responsible gene mutation is identified, the diagnosis is transitioned to the related subtype, such as CMT1H, for example. CMT1 is a group of 11 subtypes that are each demyelinating as determined by their nerve conduction characteristics. However, there are currently 30 demyelinating subtypes. Therefore, a clinical CMT1 diagnosis might not remain a CMT1 subtype diagnosis should CMT genetic testing identify the genetic cause.
How can I tell if I have CMT2? Axonal CMT nerve conduction exhibits velocities faster than 38 meters/sec (the speed at which the nerve transmits a signal), significantly reduced amplitudes (signal strength), prolonged latencies (how quickly the nerve responds to an input signal), there can be significant variability between the nerves tested, there can be evidence of conduction block (different conduction results at different segments of a nerve), and there can be absent sensory nerve responses. When NCS results meet these criteria, and the person meets the appropriate diagnostic picture, CMT2 is the clinical diagnosis.
Once the underlying responsible gene mutation is identified, the diagnosis is transitioned to the related subtype, such as CMT2C, for example. CMT2 is a group of 37 CMT subtypes that are each axonal as determined by their nerve conduction characteristics. However, there are currently 127 axonal subtypes. Therefore, a clinical CMT2 diagnosis might not remain a CMT2 subtype diagnosis should CMT genetic testing identify the genetic cause.
