Syndactyly: Background, Problem, Epidemiology

9/26 11:24:08

Background

Syndactyly is the most common congenital malformation of the limbs, with an incidence of 1 in 2000-3000 live births.^{[1, 2, 3]}Syndactyly can be classified as simple when it involves soft tissues only and classified as complex when it involves the bone or nail of adjacent fingers. It is a shared feature of more than 28 syndromes, including Poland, Apert, and Holt-Oram syndromes. Syndactyly is a failure of differentiation in which the fingers fail to separate into individual appendages. This separation usually occurs during the sixth and eighth weeks of embryologic development. The root words of the term syndactyly are derived from the Greek words syn, meaning together, and dactyly, meaning fingers or digits.

Classification

Classification of syndactyly is based on the severity of the clinical presentation.

The mildest form is simple syndactyly, which refers to fingers joined only by soft tissue.
Incomplete simple syndactyly is when the soft-tissue union is only partial and does not extend to the fingertips (see images below). Dorsal view of a hand demonstrating simple incomplete syndactyly between the left long finger and ring finger. Note the incidental café-au-lait spot. Palmar view of hand with syndactyly. The level of the syndactyly, just proximal to the proximal interphalangeal (PIP) joint, can be clearly seen on the palmar view.
When the soft tissue union extends to the fingertip, the condition is referred to as complete simple syndactyly (see images below). Dorsal view of the hand of a 1-year-old child with a complete simple syndactyly. Note that both the long finger and the ring finger have distinct nail plates with a trough separating them. Palmar view of the hand of a 1-year-old child with a complete simple syndactyly.
Complex syndactyly refers to fingers joined by bone or cartilaginous union, usually in a side-to-side fashion at the distal phalanges.
The most severe form is classified as complicated syndactyly, which refers to fingers joined by bony fusion other than a side-to-side fashion and can include bony abnormalities, such as extra, missing, or duplicated phalanges and abnormally shaped bones, such as delta phalanges (see image below). Abnormalities in the musculotendinous and neurovascular structures may also be present. Radiograph of the left hand of a patient with Apert syndrome (type III). Note the complicated syndactyly with osseous union in the distal phalanges of all the fingers. Symphalangism is present between the proximal and middle phalanges, without the formation of a proximal interphalangeal (PIP) joint in the ring, long, and index fingers.

In simple syndactyly, the third web space between the long finger and the ring finger is the area most commonly involved, followed by the fourth, second, and (rarely) first web spaces. Bilateral involvement is found in 50% of patients.

Syndactyly can be an isolated finding, or it can be found in association other abnormalities, such as polydactyly, cleft hands, ring constrictions, or craniofacial syndromes (e.g., Apert syndrome).^[4]

Associated syndromes

Syndactyly often can be associated with other syndromes, particularly the craniofacial syndromes, of which Apert syndrome is the best known. Another syndrome is Poland syndrome, in which the pectoralis muscle abnormality is found in association with symbrachydactyly and/or other anomalies of the ipsilateral upper extremity.^[5]Constriction band syndrome can be associated with syndactyly, but the etiology of the syndactyly is different.

Apert syndrome

Apert syndrome, or acrocephalosyndactyly, is a rare anomaly, occurring in 1 per 160,000-200,000 live births.^{[6, 7]}Besides the characteristic facial anomalies of Apert syndrome, it is also defined by the associated upper- and lower-limb anomalies. The anomalies in the hands are mirror images of each other and are characterized by the following 4 common features: (1) radial deviation of a short thumb as a result of an abnormally shaped proximal phalanx (ie, delta phalanx); (2) complex syndactyly of the index, long, and ring fingers; (3) symbrachyphalangism of the central segments of the index, long, ring, and small fingers; and (4) simple syndactyly of the web space between the ring and small fingers.

The web space between the thumb and the index finger is variable, and the extent of syndactyly at this web space serves as the basis for the classification of Apert syndactyly into 3 types:

Type I is the most common and the least severe of the three types. The thumb is foreshortened and associated with a radial clinodactyly as a result of a delta phalanx of the proximal phalanx. Although it is separate from the index finger, the first web space is shallow. A complex syndactyly includes the index, long, and ring fingers because of osseous or cartilaginous union of the distal phalanges. Simple syndactyly of the small and ring fingers is present; this syndactyly may be complete or incomplete. The distal interphalangeal joint of the small finger is well formed and functional. Type I hands are often referred to as spade hands (see images below). Dorsal view of hand of a 6-month-old patient with type I Apert syndrome. Note that the thumb is separate. Radial view of hand of a 6-month-old patient with type I Apert syndrome. The index fingernail is separate, and the syndactyly is complete. The small fingernail is also separate from the rest of the hand. A simple syndactyly between the small finger and the rest of the hand is present. Note that the broad thumb and the abnormal curvature of the thumb is due to the presence of a delta phalanx seen in the x-ray below. Radiograph of hand of the 6-month-old patient with type I Apert syndrome demonstrates the distinctive characteristics of Apert syndrome. In the small finger, the distal interphalangeal (DIP) is formed. Although a rudimentary proximal interphalangeal (PIP) joint is present, clinically this joint is stiff. In the ring, long, and index fingers, no PIP joint is present, and symphalangism is present between the middle and the proximal phalanges. The distal phalanges of the ring and long finger are fused. In the thumb, the proximal phalanx is shaped abnormally and is referred to as the delta phalanx. Synostosis of the ring and the small finger metacarpals is present.
Type II is more severe and is characterized by a simple incomplete or complete syndactyly of the thumb with the index ray, without any osseous union. The thumbnail matrix remains separate from the index fingernail. The hand has a large, concave palm. The bony union of the distal phalanges of the index, long, and ring fingers are more extensive than in type I. The ring finger–small finger syndactyly remains simple but complete. Type II hands have been referred to as mitten hands or spoon hands (see image below). Apert type II hand. Note the complete syndactyly between the ring and the small fingers. The patient's hand was complicated by a chronic paronychia and skin maceration preoperatively.
Type III is the most severe form and, fortunately, the least common. A tight osseous or cartilaginous union is present between all 5 fingers. All 5 nailplates are conjoined, and they sometimes have longitudinal ridges, which indicate separate underlying distal phalanges. The thumb is indistinguishable from the index ray. The small finger, although joined by a common fingernail, does not have an osseous union at the distal phalanx and remains a simple but complete syndactyly. Usually, metacarpal synostosis of the small and ring finger rays is present. Type III hands have been termed rosebud or hoof hands (see first 2 images below). Radiographs are difficult to obtain and interpret because of the overlap of osseous structures (see third image below). ^[6] Apert syndrome (type III), dorsal view. Apert syndrome (type III), volar view. Radiograph of the left hand of a patient with Apert syndrome (type III). Note the complicated syndactyly with osseous union in the distal phalanges of all the fingers. Symphalangism is present between the proximal and middle phalanges, without the formation of a proximal interphalangeal (PIP) joint in the ring, long, and index fingers.

As stated, the goal of treating complex syndactyly is the surgical release of the fingers to increase the functionality of the hand. Timing of the surgery is critical because the child requires multiple operations for other abnormalities of the cranium, midface, and orbits associated with Apert syndrome.

Poland syndrome

Poland syndrome is a sporadic congenital anomaly characterized by the absence of the sternal head of the pectoralis major muscle, along with hypoplasia and/or aplasia of the breast or nipple, with deficiency of the subcutaneous fat and axillary hair. There can be associated abnormalities of the rib cage and the ipsilateral upper extremity. It was first described by Alfred Poland in 1841 as a medical student, when he reported the absence of the sternocostal portion of the pectoralis major muscle during a cadaver dissection. In the hand and fingers, anomalies include symphalangism, syndactyly with hypoplasia, brachydactyly, or aplasia of the fingers. Two variations of hand anomalies with syndactyly in patients with Poland syndrome can be seen in images below.

Poland Syndrome: Dorsal view of a left hand in a patient with Poland Syndrome with brachydactyly (short fingers) and adactyly (missing fingers) with associated simple incomplete syndactyly between the ring and the small finger

Volar view of the preceding hand in a patient affected with Poland syndrome.

Right hand of a patient with Poland syndrome. Note the incomplete syndactyly between the hypoplastic right index and long fingers.

Volar view of the index and long finger in incomplete syndactyly in a patient with Poland syndrome.

Constriction band syndrome

The syndactyly found in constriction band syndrome (also known as amniotic band syndrome) is not the result of failure of differentiation during embryogenesis. In this condition, the fingers are already formed, and because of the injuries due to the constricting amniotic bands, the fingers heal together at the site of injury, causing postinjury syndactyly.^[8]The extent of involvement may be mild, with only a rudimentary small skin bridge connecting the 2 fingers (see images below).

Dorsal view of the right hand of a 1.5-year-old patient with constriction band syndrome. The fingers can still be identified individually. Note the presence of a fistula tract between the affected fingers, particularly between the long finger and ring finger.

Palmar view of the right hand of a 1.5-year-old patient with constriction band syndrome. The fingers can still be identified individually. Note the presence of a fistula tract between the affected fingers, particularly between the long finger and ring finger.

Right hand of a 1.5-year-old patient with constriction band syndrome. Radiograph demonstrates that the level of the amputation occurred at the proximal interphalangeal (PIP) joints.

The extent of involvement may be more severe, with a complete soft-tissue fusion in association with amputated fingers (see images below).

Dorsal view of left hand of a 1.5-year-old patient with constriction band syndrome. The left hand is more severely involved than the right, with all of the fingers being nearly indistinguishable from one another. Note the presence of pits between the fingers where normal webs would be. The most prominent one is between the ring and the small fingers. They often represent fistulas between the dorsal and the volar surface of the hand and are often the only thing remaining of the previous normal web space.

Volar view of left hand of a 1.5-year-old patient with constriction band syndrome. The left hand is more severely involved than the right, with all of the fingers being nearly indistinguishable from one another. Note the presence of a prominet fistula between the small finger and ring finger.

Radiograph of left hand of a 1.5-year-old patient with constriction band syndrome. The level of amputation is through the midportion of the proximal phalanges of the involved fingers.

Occasionally, epithelialized sinuses or fistula tracts can be found usually proximal to the level of the syndactyly (see images below). The finger distal to the constriction ring is usually atrophic, or it has been amputated in utero as a result of ischemia. These findings in the fingers may be isolated or in association with other constrictions on the proximal arm, leg, or face.

Right hand of a 1.5-year-old patient with constriction band syndrome. Radiograph demonstrates that the level of the amputation occurred at the proximal interphalangeal (PIP) joints.

If the syndactyly is minimal, standard techniques and skin grafts may be used to release the syndactyly (see image below).

Image of the left hand of a 1.5-year-old patient with constriction band syndrome after a second reconstructive procedure following release of the second and fourth web spaces. The syndactyly partially recurred because of a skin-graft loss. During the second-stage operation to separate the long finger and ring finger, the recurrence in the second and fourth web spaces were re-revised. Thus, sutures are present on both sides of the ring finger and long finger.

If sinus tracts or fistulas exist within the syndactyly, these tracts can often be released to reveal epithelialized web spaces, which do not require skin grafts (see image below). The markings made prior to the syndactyly then require modification.

Palmar view of the right hand of a 1.5-year-old patient with constriction band syndrome after a previously staged syndactyly release. No skin graft was needed for the release between the long and ring fingers. The patient also underwent revision of the web space between the index and long fingers. Next

Problem

Unreleased syndactyly can significantly impair finger and hand function. The impairment is worse when the syndactyly is complete, is complex, or involves the border digits with fingers of uneven lengths, such as the ring and small fingers or the thumb and index finger (see images below).

Complete simple syndactyly of the ring and small fingers. Note the ring finger proximal interphalangeal (PIP) joint flexion deformity due to the complete syndactyly between the border digits.

Complete simple ring and small finger syndactyly. Note the ulnar deviation of the ring finger due to the syndactyly of the small finger.

Epidemiology

Frequency

Syndactyly is the most common congenital abnormality of the hand, with a rate of 1 per 2000-3000 births.^{[9, 10, 11, 12]}The male-to-female ratio is 2:1.

Etiology

Several genetic abnormalities have been associated with syndactyly.^{[13, 14, 15]} Approximately 10-40% of cases are familial with variable penetrance. Syndactyly can occur as part of a syndrome or as sporadic events that are nonhereditary and nonsyndromic. One report indicates that there is an association of syndactyly with smoking during pregnancy.^[16]

Presentation

Although many patients with syndactyly have been evaluated by multiple specialists and referred by their primary care pediatrician, the hand surgeon should also obtain a thorough prenatal, postnatal, and familial history. In addition to the hand being examined, the cranium, face, torso, and lower extremities should be examined for anomalies.

The hand evaluation should proceed systematically.

Note and document the number of digits present, the level of web involvement, the length of the finger, and the appearance of the fingernails.
Often, photographing or drawing a picture of the hands during the initial visit is helpful.
Passively move the fingers to determine bony union; differential motion occurs only if no underlying bony union is present. Fusions of the fingernails often are associated with bony union, and a broad fingernail also may indicate a hidden polydactyly.
The extent of anomaly of tendons and neurovascular structures reflects the complexity of the syndactyly. In a simple complete or a complex syndactyly involving only the distal phalanx, the underlying tendon and neurovascular structures are usually normal. However, in an individual with brachysyndactyly or complicated syndactyly, the bifurcation of the nerves and digital vessels may be located more distally, or only one side may be present.
Always obtain radiographs to help identify any other anomalies, such as bony synostosis, a delta phalanx, or symphalangism.

Indications

In itself, a minor incomplete syndactyly is not an indication for surgery if the only issue is its incongruous appearance. However, a syndactyly that prevents full range of motion in the involved fingers warrants surgical release to increase functionality of the fingers. (see images below). As with any operation, exceptions to the rule exist (see Contraindications).