Myotonic Dystrophy type 2 (DM2)
DM2 was previously named “proximal Myotonic Myopathy” or “PROMM” and shares many of the clinical and genetic features of DM1.
It is probably more common in central Europe and the USA than the rest of the world. DM2 is an important diagnosis to consider in patients who have proximal muscle weakness around the shoulders and pelvis or a “limb-girdle weakness”.
DM2 is an autosomal dominant genetic disorder which means that, on average, it is passed on to half of the children of an affected parent.
Unlike DM1, the size of the repeated DNA expansion (see The Science: DM type 2) does not relate to the age of onset or disease severity in DM2. The disease does not tend to be worse in children of affected patients unlike in DM1 when children are often more severely affected compared to their parents. Children affected at birth or a “congenital form” has not been reported in DM2.The test for DM2 involves taking a blood sample which is analysed for the number of CCTG repeats. The exact number of repeats can be difficult to calculate and so the test result often comes back as “affected” or “not affected”.
Clinical features of DM2
DM2 is a multi-system disorder characterised by an inability to relax muscles once they have contracted or “myotonia” and muscle weakness. DM2 is generally a milder condition than DM1.The clinical onset of DM2 is typically in the third or fourth decade, with the most commonly presented symptoms being muscle weakness, stiffness and pain. The weakness typically affects proximal muscles around the shoulders and pelvis causing problems with climbing stairs, brushing and drying hair as well as getting out of a chair. Cardiac conduction defects, posterior sub-capsular cataracts and diabetic changes are also common. The effects of DM2 on the brain are also less severe than DM1.
Management of DM2
The management of patients with DM2 is less clearly described than in DM1 because of the relatively low frequency of DM2. The screening recommendations for DM1 should also be considered to be applied to DM2 in spite of the lack of formal evidence. This includes cardiorespiratory, ocular and endocrine screening as well as discussion of bowel symptoms and genetic counselling. DM2 patients less commonly require walking aids than in DM1.
DM2 is a similar disease to DM1 in that it affects many organs including muscle and is caused by a similar genetic problem but affects a different gene. Even though less is known about DM2 than DM1, DM2 shares enough similarities in its clinical and molecular features that similar principles of management can be applied.
Information provided by Dr Chris Turner Consultant Neurologist, National Hospital of Neurology & Neurosurgery, London.