Head circumference of infants related to body weight.

The measurement of the maximum circumference of the head is a part of the routine physical examination of any baby, just as much as examination of the heart, chest, and abdomen, as examination of the hips for subluxation, of the urine for phenylpyruvic acid, and ofthe hearing after the age of 3 or 4 months, The importance of measuring the head circumference lies partly in the early detection of hydrocephalus, which is now amenable to surgical procedures, and partly in the early diagnosis of mental subnormality. When there are indications for a careful developmental assessment, measurement of the maximum head circumference is a vital part of the examination. Though the maximum head circumference is of great importancein developmental and neurological assessment, there are several difficulties in its interpretation. A large head may be merely a familial feature. It is rarely due to megalencephaly, and still more rarely to hydranencephaly. The diagnosis of the obvious cause, hydrocephalus, is normally supported by other clinical signs, such as a bulging fontanelle or undue separation of the sutures. In the very young baby with a spina bifida, air studies may demonstrate a fairly considerable degree of hydrocephalus before its presence is revealed by an excessive head size or excessive increase in its size (Lorber, 1961). A small head is not strictly synonymous with microcephaly. It may, for instance, be a familial feature. A true microcephalic head is characterized not only by the small head circumference, but by its shape, and in particular by the excessive tapering off toward the vertex. Microcephaly is usual in children suffering from mental subnormality (unless there be hydrocephalus, megalencephaly, or hydranencephaly), and if a mentally subnormal child has developed normally during the early months, the head will by then have attained much of its eventual size. By the age of 9 months the brain has reached half its adult size, and by the age of 2 years, three-quarters its adult size. To put it another way, if mental subnormality dates from birth or before birth, the head is usually small. The later in the first year at which the mental subnormality develops, the less will be the evidence of microcephaly. If it develops after the first year there will be no significant evidence of microcephaly. A broad head (brachycephaly), with a flat occiput, may have an average head circumference, but it is nevertheless commonly associated with mental subnormality. The head in generalized craniostenosis may be unduly small, but craniostenosis is distinguished from microcephaly by palpation of the sutures and by radiological examination. Serial measurements are more important than isolated measurements. A changing relationship between the head circumference and its placing on the percentile head chart may well indicate the development of hydrocephalus or microcephaly (Fig. 1). On the other hand, a placing above or below the upper or lower percentile respectively will be seen to be normal when serial measurements indicate that the position in relation to the percentile is not changing (Fig. 2). A minor difficulty in interpretation of the head circumference lies in the fact that the younger the baby (or foetus) the larger the head size in relation to the size of the rest of the baby. A major difficulty is the obvious fact that at any given age a large baby is likely to have a larger head than a small baby, and vice versa. It is necessary, therefore, to relate the size of the head to the size of the baby and a convenient index of the size of the baby is his weight. Up to the present, we have plotted the head circumference on a chart which gives the 10th, 50th, and 90th percentiles, and his weight against a weight chart which gives the same percentiles for the relevant sex at the age in question. One can then determine whether his head size corresponds with (i.e. occupies the same relative percentile position as) his weight. This method