Amyoplasia is a non-progressive condition characterized by multiple joint contractures present at birth. It is the most frequently occurring form of Arthrogryposis Multiplex Congenita (AMC), often called classic arthrogryposis. The name “amyoplasia” means non-development of muscle, describing the pathology where muscle tissue is replaced by fibrous and fatty tissue. This structural change causes the stiffness and fixed positioning of joints observed in newborns.
Clinical Presentation of Amyoplasia
The physical characteristics of Amyoplasia are distinct, typically involving symmetrical contractures in all four limbs. The limbs often appear tubular and featureless due to reduced muscle bulk, and skin dimpling may be visible over affected joints. In the upper body, shoulders are commonly internally rotated and held close to the body (adduction).
Elbows are frequently fixed in an extended position, limiting bending. The forearms are often pronated, and the wrists and fingers are typically flexed. These upper limb contractures limit the use of the arms for feeding and self-care.
In the lower extremities, presentation is variable. Hips may be flexed, abducted, and externally rotated, or extended and dislocated. Knees may be fixed in either a bent (flexed) or straight (extended) position. Rigid clubfoot (talipes equinovarus) is the most frequent foot deformity.
The contractures are fixed and severe, preventing a full range of motion. Despite the physical findings, children with Amyoplasia typically have normal intellectual development. Associated features can include a midline facial hemangioma and sometimes abdominal wall defects like inguinal hernias or gastroschisis.
Etiology and Underlying Causes
Amyoplasia is a sporadic condition, meaning it occurs randomly and is not typically inherited. The primary mechanism is reduced fetal movement (fetal akinesia) during the first trimester. This lack of movement prevents joints from developing a full range of motion.
Developing muscle fibers become hypoplastic without mechanical stimulation. This muscle tissue is progressively replaced by dense fibrous and fatty tissue, leading to fixed contractures. The specific cause of fetal akinesia remains uncertain, but it is not linked to a single genetic mutation.
Theories focus on factors that compromise neurological or muscular development. One hypothesis suggests impaired blood circulation early in pregnancy causes low blood pressure and oxygen deprivation. This may damage anterior horn cells in the spinal cord, leading to muscle weakness and fetal akinesia.
Non-genetic factors contributing to reduced movement include mechanical restriction within the uterus, such as uterine abnormalities or reduced amniotic fluid. Higher frequency in one of a pair of identical twins suggests vascular compromise in the shared placenta may be a factor.
Diagnostic Process
Diagnosis of Amyoplasia is primarily clinical, established through a physical examination of the newborn. The presence of multiple, non-progressive joint contractures affecting at least two body areas, especially symmetrical four-limb involvement, strongly suggests the diagnosis. Characteristic limb positioning and generalized lack of muscle development support the assessment.
Prenatal detection may occur during routine ultrasound if decreased fetal movement or abnormal limb positioning is noted. However, a definitive diagnosis is typically made after birth. Postnatal evaluation includes X-rays to assess bony structure and rule out other skeletal dysplasias.
MRI or electromyography (EMG) may evaluate the extent of muscle replacement by fibrous tissue and distinguish Amyoplasia from myopathic or neurogenic forms of arthrogryposis. Genetic testing is performed to exclude genetic forms of AMC, confirming its sporadic nature. The absence of central nervous system involvement is a key distinction.
Therapeutic Management and Long-Term Care
Management of Amyoplasia begins immediately after birth and requires a multidisciplinary team, including orthopedic surgeons, physical therapists, occupational therapists, and physiatrists. The focus is maximizing functional independence and mobility by improving joint range of motion and strengthening existing musculature.
Non-surgical interventions form the foundation of early treatment. Intensive physical and occupational therapy involve gentle, daily stretching and manipulation to lengthen shortened soft tissues. Early intervention prevents contractures from becoming rigid.
Splinting and casting maintain the range of motion gained through stretching. Serial casting, especially for rigid deformities like clubfoot, involves applying a series of casts changed periodically to gradually stretch the joint into a functional position. Long-term bracing and night splinting prevent contractures from recurring as the child grows.
Surgical intervention is often necessary for severe contractures that do not respond to conservative management. Procedures include soft tissue releases, such as tendon lengthening or capsulotomies, to improve flexibility. For fixed deformities, bony procedures like osteotomies may be performed to realign the limbs.
Surgical correction of foot deformities is prioritized to establish a stable, plantigrade foot for walking. Upper extremity surgeries are often performed early to improve hand function for self-care. Assistive devices and adaptive equipment, such as specialized orthoses, utensils, and mobility aids, support self-sufficiency and participation.