Relative size of the eye and orbit: an evolutionary and craniofacial constraint model for examining the etiology and disparate incidence of juvenile-onset myopia in humans.

The principal aim of this research is to provide a new model for investigating myopia in humans, and contribute to an understanding of the degree to which modern variation and evolutionary change in orbital and overall craniofacial morphology may help explain the common eye form association with this condition. Recent research into long and short-term evolution of the human orbit reveals a number of changes in this feature, and particularly since the Upper Paleolithic. These include a reduction in orbital depth, a decrease in anterior projection of the upper and lower orbital margins, and most notably, a reduction in orbital volume since the Holocene in East Asia. Reduced orbital volume in this geographic region could exacerbate an existing trend in recent hominin evolution toward larger eyes in smaller orbits, and may help explain the unusually high frequency of myopia in East Asian populations. The objective of the current study is to test a null hypothesis of no relationship between a ratio of orbit to eye volume and spherical equivalent refractive error (SER) in a sample of Chinese adults, and examine how relative size of the eye within the orbit relates to SER between the sexes and across the sample population. Analysis of the orbit, eye, and SER reveals a strong relationship between relative size of the eye within the orbit and the severity of myopic refractive error. An orbit/eye ratio of 3 for females and 3.5 for males (or an eye that occupies approximately 34% and 29% of the orbit, respectively), designates a clear threshold at which myopia develops, and becomes progressively worse as the eye continues to occupy a greater proportion of the orbital cavity. These results indicate that relative size of the eye within the orbit is an important factor in the development of myopia, and suggests that individuals with large eyes in small orbits lack space for adequate development of ocular tissues, leading to compression and distortion of the lithesome globe within the confines of the orbital walls. The results of this study indicate that future research examining the etiology of juvenile-onset myopia, and particularly its correlation with ancestry, sex, age, and intelligence, should consider how the eye interacts with the matrix of structural and functional components of the skull during both ontogenetic and evolutionary morphogenesis.