614 different from the physiological asymmetry [6]. A “posi- tional preference” is defined as a condition in which the infant’s head is turned toward one side most of the time and active movement to the other side is restricted [4, 6]. About 25% of these infants (approximately 5,000 a year in the Netherlands) are referred to pediatric physical therapists [6]. Asymmetry in infancy is a clinical condition with a wide variation in appearances (shape, posture, and movement), etiology, localization, and severity. From the referred infants, the asymmetry is either idiopathic or symptomatic, and originates ante- and/or postpartum [37, 48, 50, 51, 61]. In case of an idiopathic asymmetry (IA), the etiology is uncertain; environmental factors play a major role in the development of the asymmetry [6, 34, 35, 48, 62]. In SA, an underlying disorder, disease, or dysfunction causes the asymmetry. The majority of cases will be idiopathic, but an initial presentation of positional preference might be a symptom of a serious underlying problem. In the last decade, many studies on the appearances of IA have been published. If the focus in diagnostics and pattern recogni- tion is on IA, there is a chance that an SA will be missed [3, 20]. When examining young infants with a positional preference, differentiating SA from IA is necessary to determine the prognosis and to choose appropriate inter- vention strategies. This review will address the following question: which diagnoses, incidence rates, signs, and symptoms are described in the literature and are thought to cause a symptomatic asymmetrical posture or movement pattern in infants during their first six months of life? Methods Search strategy This review is based on a comprehensive literature search on SA. The following strategy is used: peer-reviewed literature on this topic in journals with a science citation index was searched, as well as clinical textbooks from the various clinical specialties. Computerized bibliographic databases were searched (PubMed, Pedro, Cinahl, and Cochrane Controlled Trials Register), and related papers and their references. General keywords used were: asym- metry, plagiocephaly, torticollis, posture, scoliosis, (differ- ential) diagnosis, and screening. The search then focused on specific diagnoses that might cause asymmetry in infancy condition with the keywords: etiology, tumors, disorders (related to) vision, hearing, central nervous, or musculo- skeletal system, obstetric complications, brachial plexus palsy/lesion, clinical syndromes (Grisel, Sandifer), congen- ital anomalies and syndromes, gastroesophageal reflux, developmental dysplasia of the hip, paroxysmal torticollis, Eur J Pediatr (2008) 167:613–619 (birth) trauma, and clavicle fracture. Finally, we focused on the incidence and prevalence. The search was limited to citations that included: “all infants, birth–23 months,” had an abstract, were written in English, and the search terms were in the title or abstract. The year of publication was not restricted. When more papers on the same subject were found, the most current studies were chosen. Only diagnoses that could be observed in infants in the first six months of life were included. Unique case reports and innocuous abnormalities that require no specific treatment were not included. Results The prevalence and/or incidence of the various medical diagnoses leading to SA was not always documented. Some disorders had no consistency in their reported incidence rates. The majority of children with a positional preference or asymmetry during the first six months of life are diagnosed with an IA [4, 6, 34, 35, 37, 48, 50, 51]. Table 1 shows a selection of the most frequently detected disorders causing an SA. In the last decade, discussion on positional preference leading to deformational plagiocephaly (DP) has increased substantially. A relatively high number of hits found during a search in January 2008 within PubMed resulted in the following: asymmetry (811), plagiocephaly (206), torticollis (225), posture (405), and scoliosis (623). However, when combined with “differential diagnosis” or “screening,” the result decreased to less than 20 each. Differential diagnosis from craniosynostosis was often described [24, 33, 36, 51]. The main designs were retrospective or prospective descrip- Table 1 Disorders related to symptomatic asymmetry (SA) from the literature search Disorders with known incidence 1. Developmental dysplasia of the hip (DDH) 2. Perinatal fracture of the clavicle 3. Congenital muscular torticollis (CMT) 4. Obstetric brachial plexus palsy 5. Central nervous system disorders 6. Craniosynostosis/lambdoid suture Remaining groups of disorders 7. Congenital abnormalities or malformations Musculoskeletal Chromosomal 8. Sensory systems Ocular disorders Hearing disorders 9. Acquired asymmetry postpartum in one of the remaining systems (non musculoskeletal) Incidence/1,000 40 35 20 4 2 0.03