Introduction 4 6 19 37 50 61 6 4 6 6 37 48 50 51 61 6 34 35 48 62 3 20 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: asymmetry, plagiocephaly, torticollis, posture, scoliosis, (differential) 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 musculoskeletal system, obstetric complications, brachial plexus palsy/lesion, clinical syndromes (Grisel, Sandifer), congenital anomalies and syndromes, gastroesophageal reflux, developmental dysplasia of the hip, paroxysmal torticollis, (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 4 6 34 35 37 48 50 51 1 Table 1 Disorders related to symptomatic asymmetry (SA) from the literature search Disorders with known incidence Incidence/1,000 1. Developmental dysplasia of the hip (DDH) 40 2. Perinatal fracture of the clavicle 35 3. Congenital muscular torticollis (CMT) 20 4. Obstetric brachial plexus palsy 4 5. Central nervous system disorders 2 6. Craniosynostosis/lambdoid suture 0.03 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)  24 33 36 51 1 Developmental dysplasia of the hip 11 50 4 11 48 5 56 Perinatal fracture of the clavicle 49 28 40 49 54 54 Congenital muscular torticollis 10 11 19 58 10 12 19 39 11 32 10 12 19 58 10 18 15 6 37 48 51 61 Obstetric brachial plexus palsy 31 49 52 52 31 Central nervous system disorders 3 7 45 57 25 45 46 43 3 7 51 Craniosynostosis 36 24 33 36 37 42 24 33 42 24 33 36 50 51 The following three categories are groups of disorders. Clear incidence rates could not be found. Congenital abnormalities or malformations 3 7 22 38 64 2 64 60 Disorders in sensory systems 26 47 63 63 23 Acquired asymmetry, non-musculoskeletal 27 8 13 29 55 7 19 29 9 17 21 3 7 12 19 20 30 2 Table 2 Signs and symptoms of acquired symptomatic asymmetry disorders with a low incidence Signs and symptoms Hints for disorders General history Heavy pain 13 Vomiting/drowsiness 30 Lethargy/irritability 12 30 27 Trauma 27 Seizures/convulsions 17 Acute onset 13 29 55 Stridor, dyspnea 60 Reflux 17 Fever 13 Specific examination Sunset phenomenon Increased intracranial pressure Bulging anterior fontanel 27 Abnormal course Increasing head tilt 13 12 30 Recurrent episodes 9 21 Discussion 6 15 34 35 37 62 1 3 7 4 6 37 41 14 15 37 61 16 44 53 59 3 7 15 19 61 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.