AMC disease, formally known as arthrogryposis multiplex congenita, is a rare condition where a baby is born with stiff, contracted joints in multiple areas of the body. It affects roughly 1 in 3,000 to 5,200 live births. The condition itself is nonprogressive, meaning the joint contractures present at birth don’t worsen on their own over time, though secondary complications can develop as a child grows.
What Causes Joint Contractures Before Birth
AMC isn’t a single disease with one cause. It’s an umbrella term covering many different conditions that all share the same result: joints that are locked in fixed positions at birth. The common thread is reduced fetal movement during pregnancy. When a developing baby doesn’t move enough in the womb, the joints, muscles, and surrounding tissues don’t develop normally. Connective tissue and fatty tissue can replace muscle, and joints stiffen into place.
The reasons a fetus might not move enough vary widely. In the most common form, called amyoplasia, the leading theory is that impaired blood flow to the fetus early in pregnancy causes low oxygen levels that damage the nerve cells controlling muscle movement. This leads to underdeveloped muscles that get replaced by fat and connective tissue. Other causes include abnormalities in the muscles themselves, problems with the nervous system, physical crowding in the uterus, or maternal illness during pregnancy.
The Three Main Types
Doctors generally classify AMC into three broad groups based on what else is going on in the body beyond the joint contractures.
Amyoplasia is the most common form, accounting for about one-third of all cases. It occurs randomly with no family history. Babies with amyoplasia tend to have more severe muscle weakness and joint stiffness compared to other types, and their motor function is more significantly affected. Muscle strength, rather than how far a joint can bend, turns out to be the bigger factor in how well these individuals eventually move.
Distal arthrogryposis is the second most common form. Unlike amyoplasia, it’s often inherited in an autosomal dominant pattern, meaning a child needs only one copy of the affected gene from one parent. The contractures primarily affect the hands and feet. A hallmark sign is flexed, overlapping fingers at birth. Muscle weakness is present in about 44% of these patients, making it generally milder than amyoplasia. Several subtypes exist, with varying additional features beyond the joint involvement.
Syndromic AMC involves joint contractures alongside problems in other body systems, including the brain. This group includes a large number of genetic syndromes and chromosomal abnormalities. Children in this category may have intellectual disability, central nervous system dysfunction, or organ involvement beyond the muscles and skeleton.
What It Looks Like at Birth
The severity of AMC ranges enormously. Some babies have only a few affected joints with near-normal range of motion, while in the most severe cases nearly every joint is involved, including the jaw and spine.
Leg joints are affected more often than any other, followed by the arms. Babies with AMC are commonly born with a recognizable pattern of features: shoulders that slope inward, extended elbows, curled wrists and fingers, dislocated hips, stiff knees, and feet that point downward and inward (clubfoot position). The spine may curve sideways. The skin over affected joints often lacks normal creases because the joint has never bent.
Complications That Develop Over Time
While the contractures themselves don’t get worse, the body’s growth can create new problems. Scoliosis develops in a significant number of patients. In one study of 88 people with AMC, 18 had scoliosis, most commonly a double curve running through the mid and lower spine that extended down to the pelvis. These curves tended to become rigid and fixed early in life, with worsening pelvic tilt as the curves progressed. Nearly all patients with scoliosis in that study also had significant hip contractures or hip dislocations.
Hip dislocation is common more broadly in AMC, not just among those who develop scoliosis. Severe spinal curvature can eventually compromise lung function if it restricts the chest cavity, which is one reason doctors monitor the spine closely.
How AMC Is Treated
There is no cure for AMC, but early and consistent treatment makes a substantial difference in how much function a person gains. The approach is always individualized because no two cases look the same.
Physical therapy is the foundation and typically begins in infancy. Treatment involves gentle, progressive stretching of all affected joints, strengthening exercises targeting the weakest muscles, and work on motor development milestones like sitting and walking. Therapists also use splints and braces to hold joints in corrected positions after stretching. For young children, therapy often incorporates mat exercises, balance training, and gait practice. Older children and adults may add hydrotherapy, occupational therapy for daily living skills, and other techniques.
The critical point that comes up repeatedly in the research is that rehabilitation needs to be lifelong. It’s not something that ends after childhood. Ongoing therapy helps maintain the range of motion and strength gains achieved through early intervention, and it reduces the likelihood of needing surgery.
Surgery becomes an option when contractures are too severe to respond to stretching and bracing alone. Common procedures address hip dislocations, clubfoot, severe knee contractures, and scoliosis. Spinal fusion may be recommended for progressive scoliosis to protect lung function. Surgery and physical therapy work best in combination, with therapy continuing before and after any procedure.
Long-Term Mobility and Independence
A long-term follow-up study tracked 65 patients with AMC into adulthood, ranging in age from 22 to 65. The results paint a realistic picture of what mobility looks like across the spectrum. Of those 65 adults, 29 were independent walkers without assistive devices, 8 walked outside using crutches, 9 could walk indoors but used a wheelchair outdoors, and 18 used a wheelchair full-time. Among the group, 22 were able to drive a car, and 30 had fully independent mobility needing only modified shoes and no splints.
These numbers reflect the wide range of severity within AMC. People with milder forms, particularly distal arthrogryposis, tend to have better functional outcomes than those with amyoplasia. Specific life expectancy data for AMC is limited, but the condition itself is not considered life-shortening in most forms. The oldest patient in the follow-up study was 65, and many adults with AMC live full, independent lives with appropriate support and adaptive strategies developed over years of therapy.
Genetic Testing and Diagnosis
AMC is usually apparent at birth based on the physical appearance of the joints. Prenatal ultrasound can sometimes raise suspicion when a fetus shows limited movement or joints held in unusual positions, though this finding alone isn’t definitive.
Genetic testing plays an increasing role, especially for distal arthrogryposis and syndromic forms where a specific gene mutation may be identified. Several genes have been linked to different types of AMC. Some forms follow autosomal recessive inheritance, meaning both parents carry one copy of a mutated gene without showing symptoms themselves. This pattern is more common in families with consanguinity (parents who are related). Other forms, particularly distal arthrogryposis, follow autosomal dominant inheritance with variable expressivity, meaning the severity can differ significantly even within the same family. Amyoplasia, the most common type, occurs sporadically with no identifiable genetic pattern.