Universlty of Manitoba, ln Partîal Fulfiìlment - MSpace at the ...

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The value of such col lected series is limited by heterogeneìty of the
data,gnd correlation wìth naternal age and parlty. Stevenson et al.
(1966) therefore collected data from twenty-four centres simultaneously
and appl ied a correction for maternal age per thousand births. They
found that anencephaly, spîna bifida, encephalocele and hydrocephalus
occu!- th!'eughout the vlorld, though at very dî'fferent frequenc?es. The
hÌghest values were shown by Belfast and Alexanclría,with high levels
ln Helbourne, Bombay and Mexico City. Some towns in eastern lJales have
recently been shown to have an incidence as high as 12 per thousand
births of anencephaly and spina bîfida cystica combined (Laurence, 1976) .
2.2.2 Temporal Fluctuat¡ons ln lncidence
ln areas where records are available for a long periodr,gradual
changes in incídence mây be detectable. Rogers and Morris (197.|) found
that mortal íty from spina bifida in England and kales showed a steady
lncrease between 1848 and 1920, wirh a sharper r.ise between lgZO and ,l942,
followed by a declíne until the present (apart from rr"l i". peak in
"
1954 ).The recent fall in mortal ity might partly result from the improved
prognosis due to early closure, but the peaks must have some sepa!.ate
significancå. A dramatîc epidemic of anencephaly and spina bifida
occurred in Birl in between 1946 and 1950 (Gesenius, 1952). These postwar
European peaks, however, were not seen in New England.where a r+ther
uníform incidence of anencephaly and spìna bif¡da between 1890 and 1920,
and sharp increase between 1920 and 1932,have been followed by a ãteady
decl ine (Macl'lahon and Yen, 1!/t).
Edwards (1958) found that the overall reductîon in anencephaly and
spina biflda ln Bl rmingham and in Scotland since 1939 has not been
accompanîed by a similar fall in the íncÍdence of congenital hydrocephalus,