Environment vs. Genetics: A Study of Family Head Shape

Jennifer Calcaterra, M.S.; Jeanne K. Pomatto, B.O.C.; Timothy R. Littlefield, M.S.


Introduction

Deformational plagiocephaly is a permanent condition caused by external forces applied consistently to an infant's malleable skull. The progression of this condition typically follows a pattern. A small area of flattening begins in the occipital-parietal region due to external pressures placed on the head in utero or shortly after birth. As the head rests on that spot for extended periods of time, the flattened area will gradually increase. As the flat spot increases, the ipsilateral ear and forehead shift forward, creating a parallelogram-shaped deformity [1,2]. This unique head shape is used to diagnose deformational plagiocephaly.

Figure 1

In recent years, however, the head shape associated with plagiocephaly has taken on a new configuration that presents not only with asymmetry, but also with excessive cranial width (Figure 2 ). In combination with the increased biparietal width, many of these infants have a shortened skull base and increased posterior head height. The purpose of this study was to investigate whether this ‘evolution' of head shape was genetically or environmentally induced.

Figure 2

Methods

A prospective, IRB approved study was performed in five centers across the country from January 2003 through April 2003. The five centers were located in Arizona (Phoenix), Florida (Miami), New Jersey (Paramus), North Carolina (Charlotte) and Texas (Dallas). All families entering one of the five clinics were asked to participate in this study if they met the following criteria: (1) the infant was diagnosed with positional plagiocephaly; (2) both biological parents were present; and (3) the infant had not previously been treated for plagiocephaly.

Sleep positions after birth, ages and anthropometric measurements (cranial length [g-op], cranial width [eu-eu], circumference) were collected on each family member. Cephalic indices were calculated ([eu-eu / g-op] x 100). Family photographs were obtained (Figure 3 ).


Figure 3

Results

Thirty-nine families participated in this study. The sample infant population (n = 43) consisted of 38 males (88.4%) and 5 females (11.6%) with a mean age of 7.8 months (Range: 5 to 13 months). Thirtyseven (86.0%) of the infants were back sleepers, while the remaining six were split between sleeping on their side or in multiple positions. Fifty (64.1%) parents slept on their stomachs as infants, 24.4% were unsure of their sleeping positions and the remaining were side and back sleepers.

Forty-three percent of the infants had cranial widths at least two standard deviations above age and gender specific norms (mean: 2.61, range: -1.42 to +14.95). Almost ten percent (9.3%) had widths greater than 10 standard deviations above normal. By the age of 8 months, 11.6% had widths that were already greater than one of their parents. Fourteen percent of the infants had a cephalic index greater than 100, indicating that their heads were wider then they were long. Mean infant cephalic index was 88.5, which was statistically greater than their age and gender specific norms (P=0.0000, a=0.05) [3] (Figure 4 ). Mothers and fathers, on the other hand, had mean cephalic indices of 74.2 and 75.1 respectively, which fell within published norms [4].

Figure 4

Discussion

Infants currently presenting with de-formational plagiocephaly have significantly wider and shorter heads than those of the previous generation. Because many of these infants had widths greater than one of their parents at eight months of age, this change in head shape cannot be a genetic factor. Rather, this evolution is environmentally induced and can be directly related to the switch to supine sleeping.

Two deformational variants were observed. Some infants presented with excessive cranial width, while others had an increase in both width and posterior head height. Infants who presented with exceedingly wide craniums plotted higher on the pediatric percentile growth charts for cranial circumference; whereas, infants with wide and tall heads plotted lower on the same charts. Consequently, the percentile growth curves used by pediatricians for plotting head circumference may not be reliable for this new back sleeping population.

The switch to supine sleeping has resulted in a number of unintended and unanticipated consequences. These consequences include excessive cranial width and shortening of the skull base, which can be associated with jaw misalignment [8-11] and malocclusion. In severe cases, these consequences can cause narrowing of the airway, snoring and breathing issues [12-13]. Furthermore, these infants are delayed in reaching their developmental milestones (i.e. rolling, crawling, sitting) [14-18], and some lag in head control and exhibit trunk and upper extremity weakness.

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