THE DIFFERENT TYPES OF CMT
Currently, there are 4 main categories of CMT, each with many genetic subtypes. Each type and subtype of CMT are unique, with some being more common than others, and the disorder impacting people differently.
CMT CATEGORIES
A diagnosis of CMT will have a type and usually a subtype associated with it, with these two components helping identify the symptoms a person may experience in the future, the potential severity and inheritance pattern of the disease. It is also worth noting that while a person's subtype can be determined through genetic testing, a person's symptoms can be a combination of the different types.
CMT TYPE 1
Accounting for ~55% of all CMT cases, CMT-1 is the most common type of CMT, with 66% of those cases being substype CMT-1A.
CMT TYPE 2
Type 2 accounts for about 30% of all dominant CMT cases and is similar to Type 1, with the main differences being when the disease becomes prominent as well as the level of severity of the disease with symptoms usually less severe than Type 1.
CMT TYPE 3
CMT Type 3 is no longer used and is instead now referred to as Dejerine-Sottas syndrome)
CMT TYPE 4
Type 4 instances of CMT make up ~5% of cases, with severity ranging from mild to severe. Symptoms can also manifest in other areas of the body beyond the peripheral limbs, such as the eyes and ears.
CMT TYPE 5, 6, 7
Types 5, 6 and 7 are classifications no longer used, having first been classified in 1975. Today they are named after the associated symptoms, with Type 5 being referred to as CMT with pyramidal features, as patients suffer from a loss of movement in lower limbs, and Type 6 being labelled as CMT with optic atrophy.
CMT-X
Between 10 - 20% of CMT patients have Type X, with over 90% of Type X patients having subtype X1. Symptoms are similar to Types 1 and 2, with males usually being more severely impacted than females.
CMT SUBTYPE INDEX
CMT Types can be further refined into subtypes, which each have distinct symptoms and usual onset age, however, it is worth noting that there are always exceptions that may experience different symptoms earlier or later than the based on clinical models. Below is a table sourced from the Neuromuscular Disease Center, Washington University, referencing Lawson et al 2010.
Disorder | Gene | Locus | Usual onset | Early or distinct symptoms | Tendon Reflexes | NCVs |
|---|---|---|---|---|---|---|
CMT 1A | PMP-22 Duplication | 17p11 | 1st decade | Distal weakness, Commonest form | Absent | 15 to 20 M/s |
CMT 1B | P0 | 1q22 | 1st decade | Distal weakness, More severe | Absent | <20 M/s |
CMT 1C | LITAF | 16p13 | 2nd decade | Distal weakness | Reduced | 16 to 25 M/s |
CMT 1D | EGR2 | 10q21 | 2nd decade | Distal weakness, Ptosis | Absent | 26 to 42 M/s |
CMT 1F | NF-68 | 8p21 | 1 to 40 yrs | Distal weakness, Ataxia | Reduced | Axon loss |
CMT 1 | Fibulin-5 | 14q32 | 3rd to 6th decade | Distal weakness | Reduced | Axon loss |
CMT 1 | PMP2 | 8q21 | 1st & 2nd decade | Distal weakness | Absent | 15 to 22 M/s |
CMT X (S-D*) | GJB1 | Xq13 | 2nd decade | Distal weakness, Hearing loss, Encephalopathy | Absent distal | 25 to 40 M/s |
HNPP | PMP-22 Deletion | 17p11 | 3rd decade | Focal episodic weakness | Normal | Entrapments |
Dejerine-Sottas (HMSN 3, CMT 3) | PMP-22, 8q23, EGR2 | 17p11, 8q23, 10q21 | 2 yrs | Severe weakness | Absent | <10 m/s |
CMT RESOURCES
For further information on the mechanisms of CMT, please see here or check out the resources below that offer a wide range of detailed information. Please be aware that the sites below are hosted by third parties and are in no way affiliated with CMT Central.
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