Eur J Pediatr (2008) 167:613–619 tive studies and reviews. In diagnoses with a low incidence, the studies were predominantly case reports. All diagnoses were classified according to the Interna- tional Classification of Diseases [1]. The results of the literature search are presented below, starting with the diagnosis with the highest incidence rate. Developmental dysplasia of the hip Developmental dysplasia of the hip (DDH) has a high rate of co-morbidity with congenital muscular torticollis (8– 20%) [11] and, to a lesser extent, with postural torticollis or scoliosis [50]. The reported incidence rate in the Nether- lands ranged from 3 to 4% of all newborns [4, 11, 48], with 80% being unilateral [5, 56]. The clinical signs which are described include asymmetry in hip abduction and leg length and/or asymmetrical skin folds in the inguinal and upper thigh region. The strong association with other asymmetries warrants a thorough screening on the signs of developmental dysplasia of the hip in infants with an asymmetry. Perinatal fracture of the clavicle A fracture of the clavicle during birth may induce a positional preference in the first weeks of life and, as such, may cause an asymmetry. A perinatal fracture can be an option in the differential diagnostics of asymmetry during the first weeks of life. A co-incidence with other perinatal injuries (like brachial plexus injury) was described by Perlow et al. [49]. The obstetric brachial plexus lesion is described separately. The incidence varies between 0.1 and 3.5% [28, 40, 49, 54], and these fractures usually consolidate within 3 weeks without complications. The clinical presentation can be asymptomatic. When symp- tomatic, signs include: decreased or absent movement and pain, or tenderness on movement of the arm on the affected side and palpable irregularity along the clavicle [54]. Congenital muscular torticollis Congenital muscular torticollis (CMT) is frequently de- scribed in the literature. Unilateral fibrosis or thickening of and tightness in the sternocleidomastoid muscle can cause a characteristic posture of the head and restricted neck movements. The etiology of the pseudo-tumor or mass is unclear [10, 11, 19, 58]. A compartment syndrome due to intra-uterine malposition is the most frequently mentioned etiological hypothesis [10, 12, 19, 39]. An association with birth trauma and breech presentation is mentioned, but the evidence is weak, since CMT is also seen in infants born via a cesarean section [11, 32]. A pseudo-tumor can be palpated in the second or third week after birth. Incidence 615 rates of CMT vary between 0.3 and 2% [10–12, 19, 58]. Ultrasound screening soon after birth has indicated that the incidence rate could be 3.9%. This screening method tends to be especially sensitive in detecting occult cases of fibrosis [10]. Fibroids of the uterus and other intrauterine tumors are described as a possible etiology to an atrophy of the sternocleidomastoid muscle of the child [18]. This phe- nomenon can cause a unilateral muscular dysfunction and a strong imbalance between both muscles. The distinction between CMT and postural torticollis is not always clear [15]. A CMT is primarily a condition with a structural fibroid shortening of the sternocleidomastoid muscle, visible and palpable in the first weeks of life, as opposed to a postural torticollis that occurs secondary to a positional preference and a DP [6, 37, 48, 51, 61]. Obstetric brachial plexus palsy Palsy of the brachial plexus during delivery is caused by traction or compression of the plexus during labor. In most cases, the upper brachial plexus is affected; in 15% of the patients, hand function is also impaired. The described incidence is 0.1–0.4% [31, 49, 52]. The extent of the neural damage becomes evident during the first six months of life [52], although in severe cases, the inactivity of the extremity is observed from birth onwards. Timely recogni- tion of severe cases is important, since neurosurgical intervention can enhance future capacities. Between 20 and 25% of the infants experience persistent functional impairments [31]. Central nervous system disorders Cerebral palsy (CP) syndromes, in particular, spastic unilateral CP, are neurological disorders that can cause asymmetry [3, 7]. Serious disorders of the central nervous system are generally easy to recognize, but a CP may also be discrete with subtle features. Early diagnosis, before the age of six months, might be difficult [45, 57]. The neurodevelopmental (motor) behavior is an important issue in early recognition: persistent infantile reflexes and abnormal muscle tone, motor delay, abnormal spon- taneous movement patterns (especially “general move- ments”), and poor postural control are more or less predictors of CP [25, 45, 46]. In a review on the epidemiology of CP, the world-wide prevalence of all types of CP is estimated at 0.2% [43]. Hypotonia and developmental delay were also men- tioned as causes for developing positional preference and DP. A neurological disorder might be the underlying problem, but it is not always diagnosed at this young age [3, 7, 51].