Rhinometry and young children open-mouth posture in Alan M. Gross, PhD," Gloria D. Kellum, PhD, b Tracy Morris, MS, c Diane Franz, BA, c Catherine Michas, BS, ~ Monica L. Foster, MA, = Michael E. Walker, BA, = and F. W. Bishop, DDS, MS d University and Oxford, Miss. A biracial sample of 348 elementary school children who were in the first grade were assessed for open-mouth posture (OMP) in the natural environment. In addition, rhinometry was performed on 296 of the children. Means were computed for percent OMP and cross-sectional nasal airway. Results indicated that, in general, these children exhibited relatively high rates of OMP. Boys displayed significantly greater OMP than girls, and black children showed significantly larger cross-sectional nasal areas than white children. A significant correlation between OMP and nasal area only was evident for children exhibiting OMP during more than 80% of the observation intervals. The implications of the findings were discussed. (AM J ORTHOO DENTOFAC ORTHOP 1993;103:526-9.) Recently, there has been renewed interest in the impact of environment On dentofacial develop- ment. Experimental and clinical evidence suggest that dental, muscular, and skeletal relationships can be sig- nificantly influenced by changes in the local envi- ronment, t The environmental factor most widely viewed as influential in the development of dentofacial anomalies is open-mouth posture (OMP) associated with mouth- breathing. Long-term mouthbreathing is frequently characterized by an open-mouth posture, a nose that appears to be flattened, nostrils that are small and poorly developed, a short upper lip, and a voluminous and pouting lower lip. These persons can display retroclined incisors, a narrow v-shaped upper jaw with a high, narrow palatal vault, a Class II skeletal relationship, enlarged tonsils, 2-s a posterior crossbite, and a tendency to open bite. 6 It is argued that these morphologic ad- aptations are a result of an alteration in activity of spe- cific craniofacial muscles and structures involved in mouthbreathing. Although it has long been suggested that mouth- breathing is related to nasal airway impairment, it is only lately that human experimental evidence has been obtained supporting this position. By using a pressure flow technique to estimate nasal cross-sectional area and inductive plethysmography to assess nasal oral' This research was supported in part by Grant DEO8641 from the Nnt. 'Professor, Department of Ps~,-chology. ~Professor, Department of Communicative Disorders. ~Doctoral candidates, Department of Psychology. aDirector, One Health Center, Oxford, Miss. Copyright 9 1993 by the American Association of Orthodontists. 0889-5406193151.00 + 0.10 811137289 526 breathing, Warren et al. 7 reported adult nasal airway size to be related to nasal-oral respiration. However, they also found that approximately 12% of their sample were classified as mouthbreathers despite having an adequate nasal airway. Warren et al. 8 have recently examined the relation- ship between nasal cross-sectional size and nasal airway in a sample of 102 children ages 6 to 15 years. It was reported that nasal cross-sectional size increased with age across the years 6 to 14. However, before age 8 years, the number of children considered to be primarily nasal breathers versus primarily oral breathers was ap- proximately equal. Moreover, it was noted that after 8 years of age the majority of children were nasal breathers. The work of Warren and his colleagues suggests that nasal breathing is subject to developmental con- siderations, both physical and behavioral. Clearly, nasal cross-sectional size influences the ability to breathe through the nose. The observation that a percentage of both children and adults with adequate nasal airways are predominantly oral breathers also indicates that learning influences mode of respiration. Moreover, the evidence-indicates that OMP and its associated features (e.g., low, forward tongue position), regardless of whether it develops as a result of an impaired nasal airway, may be an important variable functionally re- lated to dentofacial development. 9 The purpose of the present investigation was to examine further the relationship between the nasal cross-sectional size and the mouth posture in a large biracial sample of children. Each child was assessed for nasal cross-sectional size, and mouth posture was examined with a systematic observational coding sys- tem. It was expected that children displaying predom-