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
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CMT RESOURCES

 

For further information on the different categories and subtypes 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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CMTA

Charcot-Marie-Tooth Association

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MDA

Muscular Dystrophy Association

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GARD

Genetic and Rare Diseases Information Center