Cataract a common characteristic in both forms of the disease
The prevalence of the disease differs, ranging from 1:8000 in Caucasians to 1:20000 in Asian and Africans, and 1:500 in the Canadian region of Quebec.4,5 In 1918, Fleischer described the anticipation phenomenon in myotonic dystrophy type 1.6,7 In 1992, it was discovered that an unstable expansion of CTG trinucleotide in the 3' untranslated region of dystrophya myotonica protein kinase (DMPK) gene on chromosome 19 is responsible for DM1.8,9
The anticipation phenomenon is related to increasing disease severity and to decreasing the age of the onset, depending on a number of inherited unstable CTG repeats becoming more in future generations. The number of CTG triplet repeats in healthy people ranges from 5 to 34. Individuals with 35 to 49 repeats present no clinical features, but their children could inherit a larger number. This condition is called permutation or mutable normal. Full penetrance allele over 50 repeats is associated with the disease.10 Clinical manifestation of the disorder is variable from mild, in late adult onset, to fatal, in the congenital form, especially if the repeats are over 2000.11
Myotonic dystrophy type 2 (PROMM or DM2) is another disease in the group of myotonic dystrophies. The disease was first described in 1994 in patients whose clinical picture resembles DM1 but expansion of the number of CTG repeats in DMPK gene was not found.16,17 Recently has been discovered that an unstable (CCTG)n expansion in intron 1 of zinc finger protein 9 gene (ZNF9) on chromosome 3 is responsible for DM2.18–21 Myotonic dystrophy type 2 like DM1 is a clinically heterogenous disorder and many clinical signs were reported in the affected individuals such as proximal myotonic myopathy, cataracts, facial muscles involvement, nonspecific abnormalities on muscle biopsy, normal cardiac condition, deafness.22,23
The prevalence of DM2 is estimated to about 1/100000. The most affected families are German or Polish in origin.24 According to Ricker K. (1999), the frequency in German population of DM2 is equal to DM1.25
The purpose of the study described in this article was to evaluate the frequency and clinical characteristics of neuroophthalmological signs in both types of patients with DM1 and DM2.
Thirty-three patients with DM1 - 16 women and 17 men, aged 15 to 62 years at the time of examination (mean age 37.5 years) - and four patients with DM2 - three women and one man, aged 52 to 57 years (mean age 54.5 years) - were enrolled in the study.
All patients had wellestablished myotonic dystrophy phenotype and additional genetic verification.
The neuroophthalmological examination of all patients included bestcorrected visual acuity, slitlamp biomicroscopy, direct and indirect ophthalmoscopy, and evaluation of ocular motility. The photographs of cataracts were taken using the slit-lamp biomicroscope SL 990 type 5 X (CSO, Florence, Italy). Full neurological examination, electromyography (EMG), MRI, echocardiography, electrocardiography, pulmonary function test, hormone blood tests, and molecular genetic analysis were performed to verify the diagnosis.