Amyoplasia is a rare congenital disorder characterized by multiple joint contractures of the arms and legs. These contractures result in the wasting of skeletal muscle, which can be replaced by a mixture of dense fat and fibrous tissue. The contractures can be improved with early physical therapy and splinting, however, surgery is often necessary for affected patients.
Amyoplasia, meaning "absent muscle development," is also referred to as amyoplasia congenita. It the most common form of arthrogryposis multiplex congenital (AMC). AMC is a term used to describe a condition where multiple joint contractures are present at birth. Arthrogryposis is derived from the Greek word meaning "with crooking of joints," and AMC can be translated to mean "curved joints, multiple, evident at birth." It occurs in about one out of every 3,000 live births. There are more than 150 types of AMC. Amyoplasia accounts for 40% of AMC cases.
The most striking feature of amyoplasia is the multiple joint contractures, which appear between birth and a few months of age. These joint contractures may affect upper extremities, lower extremities, or both. As a result of these contractures, muscles will often atrophy and become replaced by fat and fibrotic tissue. Additionally, joints can become encased in thickened, fibrotic tissue. More severe cases of amyoplasia may involve other internal organ abnormalities or central nervous system conditions. Individuals affected with amyoplasia are most often of normal intelligence, although they may demonstrate delays in gross and fine motor skills.
Amyoplasia results when a fetus is unable to move sufficiently in the womb. Mothers of children with the disorder often report that their baby was abnormally still during the pregnancy. The lack of movement in utero (also known as fetal akinesia) allows extra connective tissue to form around the joints and, therefore, the joints become fixed. This extra connective tissue replaces muscle tissue, leading to weakness and giving a wasting appearance to the muscles. Additionally, due to the lack of fetal movement, the tendons that connect the muscles to bone are not able to stretch to their normal length and this contributes to the lack of joint mobility as well.
The fetal akinesia in amyoplasia is thought to be caused by various maternal and fetal abnormalities. In some cases, the mother's uterus does not allow for adequate fetal movement because of a lack of amniotic fluid, known as oligohydramnios, or an abnormal shape to the uterus, called a bicornuate uterus. There may also be a myogenic cause to the fetal akinesia, meaning that fetal muscles do not develop properly due to a muscle disease (for example, a congenital muscular dystrophy). Similarly, connective tissue (i.e., tendon) and skeletal defects may contribute to the fetal akinesia and be the primary cause of amyoplasia. Additionally, malformations may occur in the central nervous system and/or spinal cord that can lead to a lack of fetal movement in utero. This neurogenic cause is often accompanied by a wide range of other conditions. Other causes of fetal akinesia may include a maternal fever during pregnancy or a virus. There is no single factor that is consistently found in the prenatal history of individuals affected with amyoplasia and, in some cases, there is no known cause of the disorder.
Amyoplasia is a sporadic condition that occurs due to lack of fetal movement in the womb. There is no specific gene that is known to cause the disorder. It is thought to be multifactorial, meaning that numerous genes and environmental factors play a role in its development. The recurrence risk is minimal for siblings or children of affected individuals. There have been no reports of recurrent cases of amyoplasia in a family.
Amyoplasia occurs in approximately one in 10,000 live births. There are no reports of the condition being more common in specific ethnic groups or geographical regions.
Signs and symptoms
Delivery of infants with amyoplasia may be difficult and they may deliver in breech presentation. It is possible for limb fractures to occur during a traumatic delivery. However, in general, infants with amyoplasia are most often full term, average weight, and healthy.
Joint contractures will be evident either at the time of birth or in the first few months of life. The primary joints involved are the foot, hip, knee, wrist, elbow, and shoulder. Typically, the contractures will be symmetrical, occurring on both the right and left side of the body. The majority (60–84%) of cases involve all four limbs. Less involve only the lower limbs and even fewer involve only the upper limbs. Often, the involvement of the lower limbs is more extensive than that of the upper limbs. Upper limb involvement may include internal rotation of shoulders, hyperextended elbows, flexed wrists, or "policeman tip" hands (thumb-in-palm). Lower limb involvement may include hip flexion and abduction contractures, dislocated hips, knee flexion or extension contractures, congential dislocation of the knee, and foot deformities (i.e., clubfoot). The affected joints will demonstrate limited range of motion. There is diminished muscle mass and limbs may be spindle-shaped (narrower at the ends when compared to the middle). Additionally, there is often a lack of skin creases seen over the affected joints and webbing across the elbow and/or knees may occur. Individuals with amyoplasia have normal sensation, although deep tendon reflexes may be decreased or absent. Cognition and speech are usually normal in individuals with amyoplasia.
Other conditions can be associated with amyoplasia as well. For example, patient often have growth retardation are of small stature compared to the general population. Scoliosis is also fairly common and occurs in approximately 30% of affected individuals. Lung hypoplasia is frequently a problem and leads to recurrent respiratory infections in some patients. Facial abnormalities are common, including capillary hemangioma (strawberry birthmark), micrognathia (small jaw), and small, upturned nose. Amyoplasia is occasionally accompanied by genital abnormalities, such as undescended testes, inguinal hernia, and hypoplastic (underdeveloped) external genitalia. Abnormalities in the abdominal wall may be observed as well, for example, gastroschisis, a defect of the ventral abdominal wall in which the internal abdominal organs protrude out of the abdomen.
There are no tests available to definitively diagnose amyoplasia prior to or after birth. The condition may be suspected prenatally if limb deformities are seen on ultrasound (i.e., clubfoot) or if decreased fetal movement is noted. Generally, the diagnosis of amyoplasia is made by ruling out other disorders that cause joint contractures. This is often done via muscle biopsies, blood tests, computed tomography (CT) scans, chromosomal studies, and clinical findings.
Treatment and management
Treatment and management of amyoplasia should involve a multidisciplinary team of health care providers, including pediatrics, neurology, orthopedic surgery, genetics, physical therapy, and occupational therapy. The main goal of treatment is to improve function, not to improve cosmetic appearance.
Generally, it is important to focus on the elbow and wrist in the upper extremity, as contractures in these joints are more problematic than those in the shoulder. Particular attention should be paid to the upper extremity. Due to the emphasis that parents place on encouraging their child to walk, the importance of the function of the arms is often overlooked. For the lower extremity, it is important to pay attention to all joints, however, it is recommended that deformities of the feet are treated first, followed by the knees and then the hips.
Most often, intervention begins immediately after birth with physical therapy and range of motion exercises designed to improve flexibility in muscles and joints. Once the joint is positioned adequately by these exercises, splinting is used to maintain the gains in range of motion. If the joint cannot be positioned adequately with range of motion exercises, casting or soft-tissue release surgery with subsequent casting may be necessary.
In addition to physical therapy, surgery is often necessary for patients with amyoplasia. Muscle transfer is a surgical procedure that involves moving muscles from one location in the body to another location where they might perform better. This is an option for affected patients. However, if muscles are nonfunctional or limited in function as they often are in amyoplasia, this procedure may not be effective. Osteotomy is the surgical cutting of a portion of the bone to correct deformity and may be necessary in some cases of amyoplasia. However, due to the possibility of the recurrence of a bone deformity, this procedure should be postponed until an
Amyoplasia is not a progressive disorder and it does not worsen with age. Usually the outlook is very good, especially with early intervention via physical therapy, mobilization, and other treatment. Overall function has been shown to be related to family support, patient personality, education, and early efforts to encourage independence. In rare cases, survival can be poor, particularly if other conditions are associated (i.e., central nervous system disorders). However, most people with amyoplasia are able to lead productive, independent adult lives with minor modifications to daily activities.
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Mary E. Freivogel, MS, CGC