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

Abstract

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 hemi spheres 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 con ditions 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 social classes of Scotland (Edwards, 1958), associa tions with birth order and maternal age (Ingalls, Pugh, and MacMahon, 1954; Record, 1961), and seasonal variations in incidence (Edwards, 1958), 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 experim ntal animals through a variety of pro c dures. Hydrocephalus has been obtained through vitamin A deficiency in rats and rabbits. Folic acid antagonists and vitamin E deficiency have been re ported to lead to hydrocephalus, cranium bifidum, spina bifida, and exencephaly in several species. Giroud a d Martinet (1954) reported the production of 54 pe cent, anencephalic offspring when female rats were treated with 60,000 IU vitamin A for 3 days beginning with the 8th day of pregnancy. Hyperand hypothermia and pressure changes have been shown to lead to a variety of central nervous system defects. K lter 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 th roughly explored. The determination of incidence relative to environ mental factors and parental characteristics may be of cen ral importance in studies of the aetiology and genetics of any trait. The difficulty of assembling from clinical records case series of sufficient magni tude for analytic purposes and the selection biases inherent in hospital samples would suggest the appro priateness of utilizing demographic methods. Unfor tun tely, few data are available on the occurrence of congenital malformations in population groups, and no n tional statistics are published on the frequencies among live and stillbirths. Scotland is the only country for which stillbirths are classified by cause in suitable detail for distinguishing among the most com o defects. However, a series restricted to still births is of little value for the study of spina bifida a d hydrocephalus and only partially adequate for an nc phalus which is not compatible with postnatal life beyond a few hours or days. Neonatal and infant 153