Upper Extremity Conditionsin the Neonate

Introduction

Several complex congenital anomalies are seen in the upper extremities. This chapter will provide an introduction and overview of several of these anomalies that are seen relatively more frequently. The Oberg, Manske, and Tonkin Classification of congenital hand and upper limb anomalies provides a framework for placing conditions in one of three groups based on limb embryogenesis: malformations, deformations, and dysplasias. Malformations represent the majority of congenital upper extremity differences (74%–90% of total diagnoses), examples of which include syndactyly, polydactyly, thumb hypoplasia, radial/ulnar/central longitudinal deficiencies, and symbrachydactyly. Deformations are subdivided into constriction ring sequence, trigger digits, and not otherwise specified. Dysplasias are subdivided into hyperplasia such as macrodactyly and tumorous conditions such as vascular tumors/malformations, connective tissue tumors, and skeletal tumors. ^, A comprehensive list of anomalies and associated syndromes is described according to the Oberg, Manske, and Tonkin Classification ( Table 71.1 ).

Congenital abnormalities of the upper extremities can occur in isolation or with associated syndromes and can be sporadic or inherited. There are numerous congenital upper extremity anomalies that occur as a result of changes in limb embryogenesis. They are often clinically evident at birth and therefore may be recognized and diagnosed by the primary care team. However, specialist referral to pediatric hand surgeons within plastic surgery or orthopedic surgery is recommended for management of congenital upper extremity pathology.

Embryology

Limb bud formation begins at approximately 4 weeks’ gestation and is complete at approximately 8 weeks (days 26–52). The majority of limb abnormalities take place during this time. The upper limbs develop approximately 24 hours sooner than the lower limbs, because the embryo forms cranially to caudally. Both the paraxial mesoderm and parietal plate mesoderm participate in limb development. Limbs initially form as paddle-like structures (appearing at day 33), flattened mediolaterally and hugging the body. As the limb develops and takes on a more recognizable structure, a delicate balance of differential cell growth and selective cell death must be obtained.

Fibroblast growth factor 10 is a signaling molecule that migrates from the lateral plate mesoderm across formerly mesodermal tissue, now undifferentiated mesenchyme, to ectodermal tissue to initiate limb formation (limb bud appears at day 26). Bones of the limb originate from somatic lateral plate mesoderm. All bony structures in the limb, except the clavicles, originate via endochondral ossification, a process by which bone invades a preformed cartilaginous model with bone precursors (osteoblasts). The paraxial mesoderm forms bundles, called somites, along the length of the body. Somitic cells may differentiate into somitic myotome, eventually resulting in limb muscle. A portion of the myotome will divide into two factions: a dorsal muscle mass and a ventral muscle mass. The masses will surround bone in the limb on either the dorsal or ventral sides as their names suggest. They will transition to myoblasts (muscle precursor cells) and eventually mature muscle cells. This process provides an explanation as to the different innervations of the two muscle groups. Motor axons from the neural tube diverge into anterior and posterior ventral rami and take the path of least resistance, avoiding dense connective tissue. Somatic fibers trigger myoblast differentiation. Sensory axons take a permissive path; they follow the path motor neurons took because it offers the least resistance, with minimal tissue interference. These developmental steps explain why the sensory and motor neurons are located in the same place within limbs.

The development of limbs can be thought of as occurring in three spatial axes: proximal-distal, cranial-caudal, and dorsal-ventral. Along the edge of the distal limb bud is a ridge of ectodermal tissue called the apical ectodermal ridge (AER). It is believed that the AER gives a signal to differentiate proximal and distal structures; however, the exact mechanism is unclear. The progress zone model, a timing-­based model, and the early allocation and progenitor expansion model, a prespecification model, are believed to be possible explanations for proximal-distal differentiation. ^, Cranial-caudal axis formation is centered around a chemical located on the caudal side of the developing limb, sonic hedgehog (SHH). The area expressing SHH is called the zone of polarizing activity. SHH from the zone of polarizing activity radiates across the developing limb bud in every direction from the caudal side. The signal is weaker further from the origin, resulting in a morphogenetic gradient. The thumb develops on the cranial side, where there is greater not-caudal character. Finally, the dorsal-ventral axis involves Wnt signaling, although this pathway is poorly understood. This axis is not the result of a gradient. Instead, through the Wnt signaling pathway, a signaling molecule, Lmx-1, is expressed on the dorsal side of the limb, causing the mesenchyme to adopt dorsal characteristics while the ventral side expresses a signaling molecule, En-1, whose product essentially stops the dorsal signaling by blocking the Wnt signaling pathway.

Segmental separation occurs to achieve the definitive structure of the limb. Constriction of the presumptive wrist and elbow, flattening of the hand plate in a dorsal-ventral fashion, and division of the AER and apoptosis in the interdigital spaces are all consequences of selective cell death of the limb. Finally, limb rotation gives rise to the adult dermatome structure.

Pathophysiology

Limb development is tightly controlled and regulated. Molecular and biochemical alternations occur in a precise and transient manner as to trigger the next stage in a cascade of events through limb development. Therefore it is fitting that even small alterations affecting morphologic processes in limb development can result in abnormal structure. Teratogens, genetic components, and metabolic poisons can all adversely affect the delicate process of limb development leading to the conditions described in this chapter.

The exact cause of syndactyly remains unclear. Four areas of caudal divisions of the AER from apoptosis result in a typical five-digit hand. Deviations from the typical programmed cell death that yields five distinct digits can result in syndactyly. Digits in the hand become apparent on days 41 to 43 and are fully separated by day 53 in a typical hand. Apoptosis is mediated by bone morphogenic protein 4. ^, Disturbance of these molecular mechanisms may result in atypical hand plate formation. Syndactyly has been reported in some cases to be transmitted in an autosomal-dominant manner with incomplete penetrance. ^,

Polydactyly can be a product of disruptions in the cranial-caudal axis, leading to additional caudal divisions and subsequently extra digits. Polydactyly has shown an autosomal dominant form of transmission, and genetic defects have been noted in chromosome 2. Polydactyly has also been linked to chromosome 7, where the regulatory element of SHH is located.

Thalidomide was previously given to pregnant women for relief of nausea and morning sickness; however, it has been shown to be associated with a high incidence of skeletal deformities including radial longitudinal deficiency when given to women between days 38 and 45 of fetal development. For this reason, it is no longer used. Other teratogens have also been associated with radial longitudinal deficiency, including fetal alcohol syndrome.

Clinical Features, Evaluation, Management, Long-Term Outcomes, and Conditions

Syndactyly

Clinical Features

Syndactyly refers to an anomalous connection between two or more digits. It is the second most common congenital malformation of the hand. ^, Overall prevalence of syndactyly ranges from 1 in 1000 to 1 in 2500 live births. ^{, ,} It is more common in males than females and occurs with equal frequency unilaterally and bilaterally. Syndactyly occurs most commonly between the ring and middle fingers (40%–50%), second most commonly between the ring and small fingers (25%–28%), and least commonly between the index and middle fingers or the thumb and index finger. Syndactyly of the fingers is typically an isolated condition. However, it may be associated with other conditions such as syndactyly of the toes, polydactyly, and club foot and with syndromes such as Apert syndrome ( Fig. 71.1 ), Poland syndrome, and acrosyndactyly. ^,

Syndactyly is classified according to whether the connection between two or more digits is partial, creating a webbed digit, or fully extending to the tip (incomplete versus complete, respectively), whether the connection involves skin only or bone also (simple versus complex, respectively), and whether there is abnormal bone structure with rudimentary bones, missing bones, extra bones, abnormal joints, or more than one synostoses (complicated). ^,

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