Declining incidence of central nervous system anomalies in New York State.

Anencephalus, spina bifida, and hydrocephalus are the most common of the major structural anomalies of the central nervous system recognizable at birth. Anencephalus is characterized by absence of the cranial vault of the skull. The cerebral hemispheres are completely missing or are reduced to small masses attached to the base of the skull. Spina bifida is a dorsal defect of the vertebral column ranging in severity from minor bony defects, visible only by x ray, to massive clefts of the spinal column. Meningocele, meningomyelocele, and myelocele are part of the same embryological process. Congenital hydrocephalus is characterized by enlargement of the head and separation of the cranial bones caused by abnormal production, circulation, or absorption of the cerebrospinal fluid (Potter, 1952). Spina bifida in the thoracic and upper cervical region is a frequent concomitant of both hydrocephalus and anen-cephalus. Stevenson (1956) reported that the Arnold-Chiari syndrome by itself may be the cause of hydro-cephalus, and Penrose (1957) found it to be in frequent association with anencephalus, thereby providing a morphological link between the conditions and justifying concurrent analysis. The mechanisms and aetiologies underlying the production of the three deformities are incompletely understood. Epidemiological studies of incidence have demonstrated the existence of wide geographical variations, the bases of which remain unclear. The markedly increased frequencies of cases in the lower are consistent with the operation of environmental causes. The low familial incidence and the discord-ance between monozygotic and conjoined twins reported by Penrose argue against a simple genetic hypothesis. Few consanguinity studies have been completed. Abnormal karyotypes have not been demonstrated by cytological examination. All three malformations have been produced in experimental animals through a variety of procedures. Hydrocephalus has been obtained through vitamin A deficiency in rats and rabbits. Folic acid antagonists and vitamin E deficiency have been reported to lead to hydrocephalus, cranium bifidum, spina bifida, and exencephaly in several species. Giroud and Martinet (1954) reported the production of 54 per cent. anencephalic offspring when female rats were treated with 60,000 IU vitamin A for 3 days beginning with the 8th day of pregnancy. Hyper-and hypothermia and pressure changes have been shown to lead to a variety of central nervous system defects. Kalter and Warkany (1954), in their extensive review, provide a lengthy list of agents which have proved to be teratogenic in experimental situations. The per-tinency of these results for man remains to be more thoroughly explored. The …