Unidad Regiomontana de Diagnostico, Monterrey, N.L., Mexico.

Definition: Congenital myotonic dystrophy (CMD) is a disease manifested by hypotonia, muscle atrophy, mental retardation, feeding difficulties and facial diplegia. It is an inherited autosomal dominant multisystemic disease, with an incidence of 1:8000 in adults1.

Case Report: A 22-year-old woman with no family history of malformations or genetic disorders, due to polyhydramnios was referred to our ultrasound unit at 29 weeks. Ultrasonography revealed polyhydramnios with an amniotic fluid index of 35, absence of fetal movements, bilateral talipes equinovarus, facial diplegia, and a high resistance index of umbilical artery of 0.81. The following images were obtained:

Three weeks later the patient underwent another examination, showing persistence of the findings mentioned before. This time scalp and upper torso edema was noticed and fetal stomach was no longer visible. Fetal echocardiography was normal. Posturing of the extremities was the same than it was three weeks before. Fetal position in uterine cavity was also the same. The mother was never aware of fetal movements. These images were obtained:

Last examination was performed at 34 weeks. Polyhydramnios had increased, fetal position was the same seen in previous scans, and lunar deviation of the hands was documented in 3D reconstructions. Doppler analysis of umbilical cord showed absent diastolic flow of umbilical artery, and umbilical vein pulsations were present. Facial diplegia was more evident.

Considering these findings, interruption of pregnancy was decided. The infant was severely affected with absence of swallowing and respiratory movement, and died immediately after delivery.

History: This disorder was first described by Vanier in 19602. Most patients are identified as having CMD only after delivery. Available reports of CMD are retrospective reviews, and there is only one recent report of ultrasound prenatal diagnosis of this disease. Fried et al.3 in 1975 observed that infants with neonatal myotonic dystrophy have thin ribs, talipes, polyhydramnios, and reduced fetal movements. Rudnik-Schoneborn et al.4 in 1998 reviewed the obstetric histories of 67 gestations. Preterm labor was a major problem in gestations with CMD fetuses, as was polyhydramnios. Stratton an Patterson5 in 1993 found bilateral effusions and scalp and upper torso edema by ultrasound examination at 30 weeks gestation, polyhydramnios was also present. Ito et al.6 (1996) studied the movements of two fetuses with CMD, and found them extremely hypotonic and neither displayed any gross movement, respiratory movement or swallowing postnatally. Esplin et al.1 (1998) evaluated ultrasound findings of 4 fetuses with CMD and included abnormal posturing of the hands, equinovarus deformity of the lower extremity, and camptodactyly with ulnar deviation of the digits. In 2002 Mashiach et al.7 described the facial appearance of neonates with CMD comprising ptosis and a ¨tent-shaped¨ mouth, indicating facial diplegia.

Etiology: Mutation responsible of CMD is an abnormal CTG triplet repetition in the myotonin protein kinase gene, mapped on chromosome 19q13.3, which undergoes expansion in these patients.8
Sonographic Findings: CMD must be considered in patients with polyhydramnios, absent or reduced fetal movements, diplegia, scalp and torso edema, abnormal posture of the hands, clubfoot or equinovarus deformity of the lower extremities.

Differential diagnosis:

• Idiopathic polyhydramnios.
• Fetal hypomobility caused by a central nervous system anomaly.
• Non-immune hydrops fetalis.

Associated anomalies:  There are no reports of associations with other anomalies.

Prognosis: Respiratory difficulties are frequent and are often fatal. Those that survive the neonatal period initially follow a static course, eventually learning to walk but with significant mental retardation en 60 to 70% of cases. By age 10 they develop myotonia and in adulthood develop the additional complications described for the adult onset disease.

Management:  Women with a familial history of myotonic dystrophy or ultrasonographic evidence of hypotonia, including positional abnormalities of the extremities should be offered deoxyribonucleic acid testing for the myotonic dystrophy mutation, and treated in a tertiary center, since infants with this disorder will very probably require admission to the intensive care unit .

References
1- Esplin MS, Hallam S, Farrington PF, Nelson L, Byrne J, Ward K. Myotonic dystrophy is a significant cause of idiopathic polyhydramnios. Am J Obstet Gynecolog 1998; 179: 974-7
2- Vanier M. Myotonic dystrophia in childhood. Br. Med J 1960; I: 1284-8
3- Fried, K.; Pajewski, M.; Mundel, G.; Caspi, E.; Spira, R. :
Thin ribs in neonatal myotonic dystrophy. Clin. Genet. 7: 417-420, 1975.
4- Rudnik-Schoneborn, S.; Nicholson, G. A.; Morgan, G.; Rohrig, D.; Zerres, K. Different patterns of obstetric complications in myotonic dystrophy in relation to the disease status of the fetus. Am. J. Med. Genet. 80: 314-321, 1998.
5- Stratton, R. F.; Patterson, R. M. DNA confirmation of congenital myotonic dystrophy in non-immune hydrops fetalis. Prenatal Diag. 13: 1027-1030, 1993.
6- Ito, T;Tanikawa, M; Miura, H;Teshima, N;Kadowaki, K;Nagata, N;Makio, A;Terakawa, N. The movements of fetuses with congenital myotonic dystrophy in utero. Journal of Perinatal Medicine 24: (3) 277-282, 1996
7- Mashiach R.; Rimon E.; Achiron R. Tent-shaped mouth as a presenting symptom of congenital myotonic dystrophy. Ultrasound in Obstetrics and Gynecology. 20: (3) 312
8- Brook, J. D.; McCurrach, M. E.; Harley, H. G.; Buckler, A. J.; Church, D.; Aburatani, H.; Hunter, K.; Stanton, V. P.; Thirion, J.-P.; Hudson, T.; Sohn, R.; Zemelman, B.; Snell, R. G.; Rundle, S. A.; Crow, S.; Davies, J.; Shelbourne, P.; Buxton, J.; Jones, C.; Juvonen, V.; Johnson, K.; Harper, P. S.; Shaw, D. J.; Housman, D. E. : Molecular basis of myotonic dystrophy: expansion of a trinucleotide (CTG) repeat at the 3-prime end of a transcript encoding a protein kinase family member. Cell 68: 799-808, 1992.