Eur J Pediatr (2008) 167:613–619 tions of the neck, associated with gastroesophageal reflux) [9, 17, 21]. The most alarming causes of asymmetry are related to neurological syndromes, such as syringomyelia, epilepsy, high intracranial pressure, postencephalitic syn- dromes, or life-threatening tumors of the central nervous system [3, 7, 12, 19, 20, 30]. These disorders are mainly described in case studies, without proven incidence rates. The signs and symptoms of these non-musculoskeletal causes are described in Table 2. Discussion Asymmetry in infancy is a condition with a high prevalence in infants in the first six months of life. In the majority of cases, the origin is idiopathic and is often related to environmental factors [6, 15, 34, 35, 37, 62]. This review addresses the possible causes, incidence rates, and symp- toms of symptomatic asymmetries due to an underlying disorder, dysfunction, or disease. Not all of the incidence rates could be found, while some inconsistencies were observed in the current literature. The disorders with a high prevalence are well described in epidemiologic studies. The rarer diseases were, most of the time, documented in case reports without incidence rates. The incidence rates mentioned in the studies are inconsis- tent because of different opinions regarding the operation- alization and assessment of the SA. Frequently, psychometric properties of instruments and concepts have not been described or evaluated. Variations in incidence rates (e.g. CMT) are inevitable, considering the variety in inclusion criteria and diagnostic tests used in the studies. The sequence in estimated incidence rates, as proposed in Table 1, is open to debate. A clear description of signs and symptoms was not always presented in the literature. The variety in the etiology of asymmetry is considerable. The level of evidence of the included studies varies. Literature of more than 10 years ago mainly described underlying causes of SA, in particular, non-muscular torticollis [3, 7]. They still turned out to be useful in establishing criteria for differential diagnostic screening and are widely cited in current studies. However, an update regarding new developments in studies on infant asymmetry is needed. The exponential increase of plagiocephaly in the last decade, related to the recommendations to put babies on their back to sleep, is reflected in the objectives of recent studies [4, 6, 37, 41]. They mainly focused on IA and its predispositions, with little attention to SA. A number of recent papers described features to distinguish craniosy- nostosis from DP. Although craniosynostosis has a very low incidence, craniofacial clinics are deluged with infants with DP [14, 15, 37]. One of the positive effects of this 617 situation is that authors from this background described useful clinical diagnostic criteria for craniosynostosis. van Vlimmeren et al. [61] stated in their review on diagnostic strategies, that asymmetry in infancy is a diagnosis with a large spectrum of features and a multifac- torial etiology without consensus on definition, nomencla- ture, or classification. In the present review, a classification by virtue of etiology is proposed. The dichotomy, symptom- atic versus idiopathic, is often used in medicine [16, 44, 53, 59] and fits well with this health problem, since a large number of children have unexplained asymmetry. Although flow diagrams for diagnostic strategies are presented in some reviews [3, 7, 15, 19, 61], clear clinical diagnostic criteria that could be used were not mentioned. The criteria found in the present review might be considered in a future study. An expert validation, such as a Delphi study with clinical experts, could be a next step towards establishing clinical diagnostic criteria as warning flags in young infants with IA or SA. This review presents an overview of the most common disorders underlying SA in infants less than six months of age. We have discovered that the literature does not provide a comprehensive analysis of the incidence, characteristics, signs, and symptoms of SA. Knowledge of the presented clues is important in the clinical decision making with regard to young infants with asymmetry. The endpoint of this review may be a starting document for the creation of a protocol, but it needs additional studies in order for it to become a valid and useful screening instrument. Acknowledgments We like to thank Raoul H. H. Engelbert, PhD, PCS, PT, Associate Professor, Amsterdam School for Health Professions, Amsterdam, the Netherlands, for his substantive comments. 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