Plagiocephalometry: a non-invasive method to quantify asymmetry of the skull; a reliability study

Deformational plagiocephaly (DP) in newborns and very young children is a common problem in daily practice. The intrarater and interrater reliability of plagiocephalometry (PCM), a new, non-invasive, inexpensive instrument to assess and quantify the asymmetry of the skull, is evaluated at the outpatient Department of Physical Therapy of the Bernhoven Hospital at Veghel, The Netherlands. Using a thermoplastic material to mould the outline of the infant's skull, a reproduction of the skull shape is performed on paper, allowing for accurate cephalometric measurements. Fifty children (aged 0-24 months), with or without positional preference of the head, and with or without DP, were measured three times by two separate, experienced pediatric physical therapists. Intraclass correlation coefficients (ICC) regarding the measurements of the drawn lines were all above 0.92 (intrarater reliability) and 0.90 (interrater reliability). The ICCs of the plagiocephaly indicators ear deviation (ED), antero-sinistra-antero-dextra (ASAD), postero-dextra-postero-sinistra (PDPS) and oblique diameter difference (ODD) were 0.88, 0.57, 0.92 and 0.96, respectively, for the intrarater reliability and 0.90, 0.65, 0.94 and 0.96, respectively, for the interrater reliability. The ICCs of the two indices oblique diameter difference index (ODDI) and cranial proportional index (CPI) were 0.97 and 0.96, respectively, for the intrarater reliability and 0.95 and 0.92, respectively, for the interrater reliability. The limits of agreement according to Bland Altman, comprising 95% of the differences between two measurements (2 sd), were 4.3 mm (ED), 5.9 mm (ASAD), 3.0 mm (PDPS), 3.4 mm (ODD), 2.7% (ODDI) and 4.5% (CPI) for the intrarater reliability, and 3.7 mm (ED), 5.2 mm (ASAD), 2.4 mm (PDPS), 3.3 mm (ODD), 2.9% (ODDI) and 5.8% (CPI) for the interrater reliability.

We conclude that PCM is an easy-to-apply, non-invasive and reliable measurement instrument to assess skull asymmetry with good clinical accuracy and low application costs. PCM might serve as an instrument to be used in all levels of care for children with DP, and might provide information concerning the natural course of DP, as well as the assessment of the effects of conservative treatment strategies on DP.