Arthrogryposis

Definition. Arthrogryposis multiplex congenita (AMC) is a descriptive term for conditions with multiple congenital joint contractures affecting two or more body areas. It is a clinical finding, not a specific diagnosis, so the task is to find the cause behind it. The contractures are present from birth, often in all four limbs, and the condition is non-progressive.

Frequency. AMC affects about 1 in 3000 live births. Amyoplasia is the most common recognisable type, at about 30% of cases.

The common pathway. Any condition that decreases fetal movement leads to contractures. Whatever the cause, every form of arthrogryposis shares this final pathway: fetal akinesia during the critical period of joint development.

Soft tissue, not bone. The pathology is periarticular. Fibro-fatty replacement of muscle, capsular fibrosis and ligamentous shortening fix the joint, while the articular surfaces and ossification centres are usually structurally normal at birth. Bony deformity is secondary and develops with growth against unbalanced soft tissues.

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The pattern in classic amyoplasia. Each joint has a typical position, a structure that holds it there, and a place in the order of priorities:

Pathomechanics. Muscle is deficient rather than merely tight, so the deforming forces are weak but a corrected position is also poorly maintained by active power. This dual problem, a rigid contracture with weak motors, is why recurrence is high and why surgery aims for a balanced, braceable position rather than a normal arc of motion.

Normal joint development. Joints form through cavitation of mesenchyme in weeks 8-12, and movement is essential for the joint cavity to form. Fetal movement shapes the articular surfaces and prevents contractures, and muscles develop in response to neural input and use.

Timing and duration. What the akinesia produces depends on how long it lasts and when it begins:

Why the contractures are fixed. Lack of fetal movement leads in turn to failure of joint cavity formation (ankylosis), capsular and ligamentous contracture, muscle fibrosis and shortening, and secondary bony deformity. That is why the contractures are rigid and resistant to simple stretching.

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The causes. Intrinsic genetic or neuromuscular causes and extrinsic maternal or mechanical factors converge on fetal akinesia. Neurogenic causes are the most common.

• Neurogenic - anterior horn cell dysfunction (amyoplasia, a vascular insult), spinal muscular atrophy, myelomeningocele, CNS malformations
• Myopathic - congenital myopathies (nemaline, central core), congenital muscular dystrophy, maternal myotonic dystrophy
• Mechanical restriction - oligohydramnios (renal agenesis, PROM), multiple pregnancy, uterine anomalies, amniotic bands
• Connective tissue disorders - diastrophic dysplasia, Larsen syndrome

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Amyoplasia. Amyoplasia is caused by anterior horn cell ischaemia in early gestation. Its symmetric involvement suggests a vascular aetiology; anterior horn cells are particularly vulnerable to hypoxia. The result is muscle hypoplasia or aplasia with fatty replacement, while sensory function is preserved, the posterior horn being spared.

A vascular disruption. Amyoplasia is thought to follow an early vascular insult, the same compromise that depletes anterior horn cells and replaces muscle with fibro-fat. That insult produces a recognised cluster of associated anomalies elsewhere, and they change both the newborn work-up and the way the child is handled. The newborn with amyoplasia needs a whole-child screen, not just a limb assessment. Hall's defining series of 560 individuals sets the anomalies out:

The midline frontal capillary haemangioma belongs to the same vascular picture.

The orthopaedic point. The amyoplasia limb is osteopenic and fragile, so it must be handled and cast gently, and a fresh "deformity" at birth may actually be a fracture. Screen the abdomen for an atresia or wall defect and count the digits. Despite the cluster of anomalies, amyoplasia is sporadic, with negligible recurrence risk.

AMC is sorted into amyoplasia, distal arthrogryposis and the syndromic forms.

Amyoplasia (classic AMC). The most common recognisable type. It is sporadic, with no inheritance, and the limb involvement is symmetric. Intelligence is normal: do not assume cognitive impairment; these children are typically intelligent and motivated. Its features:

• Upper limb - internal rotation, elbow extension and wrist flexion, the "waiter's tip"
• Lower limb - hip dislocation (30%), knee flexion or extension, clubfoot
• Face - micrognathia, depressed nasal bridge
• Skin - dimpling, lack of creases

Distal arthrogryposis. Primarily the hands and feet, with the proximal joints relatively spared. Inheritance is autosomal dominant across many types (DA1, DA2A Freeman-Sheldon and others), and intelligence is normal. The hands show ulnar deviation, camptodactyly and thumb-in-palm, the feet clubfoot or vertical talus, and treatment focuses on hand and foot function.

Syndromic AMC. Here the contractures are part of a broader syndrome, drawn from the neurogenic, myopathic and connective-tissue causes above. In the neurogenic group (SMA, myelomeningocele, CNS malformations) intelligence is often impaired. Prognosis depends on the underlying syndrome, and multidisciplinary care is essential.

Abnormal bone points to a syndrome. Structural skeletal abnormalities signal syndromic arthrogryposis rather than classic amyoplasia. Primary bone changes are not features of classic amyoplasia and should trigger syndromic investigation. PITX1-related mandibular-pelvic-patellar syndrome, for example, produces limb-length discrepancy, abnormal pelvis and patellae, knee contracture, carpal/tarsal abnormalities and equinovarus.

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History. The questions that place the child in a category:

• Pregnancy - decreased fetal movements, oligohydramnios
• Birth - breech presentation, complications
• Family history - any inheritance pattern
• Development - milestones, intelligence

Examination. Examine every joint against the pattern in the anatomy table. The elbow may be held in flexion rather than extension, the hip may be dislocated or have a flexion contracture, the foot may show a rigid clubfoot or a vertical talus, and there may be a congenital knee dislocation. Then look beyond the limbs: the facies and skin, the spine for scoliosis, which is common, and a neurological assessment of muscle bulk, tone and reflexes.

Differential diagnosis. Arthrogryposis is a sign, not a diagnosis. The task is to place the child into a prognostic and genetic category.

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The trap. The amyoplasia upper limb is classically described as the "waiter's tip": an internally rotated, adducted shoulder, extended elbow, pronated forearm and flexed wrist. That is exactly the posture of an obstetric brachial plexus (Erb's, C5-6) palsy, and getting it wrong sends the child down the wrong pathway entirely.

The discriminator is the joints. In amyoplasia they are stiff, a fixed contracture limiting passive as well as active motion. In Erb's palsy the limb is floppy with full passive motion, a paralysis rather than a contracture.

Why it matters. The management diverges completely. Erb's palsy is a nerve problem, treated with physiotherapy; it often partially recovers, and microsurgical nerve reconstruction is for those that do not. Amyoplasia needs contracture management: stretching, serial casting and later tendon transfers such as triceps-to-biceps. A waiter's tip arm that is bilateral, stiff and accompanied by clubfeet is amyoplasia until proven otherwise, and should never be labelled a birth palsy.

The diagnosis is clinical, resting on the multiple congenital contractures. Investigations aim to find the underlying cause:

• Genetic testing - gene panels for distal arthrogryposis, SMA and others
• MRI brain and spine - if a neurogenic cause is suspected
• EMG/NCS - to differentiate myopathic from neurogenic
• Muscle biopsy - if a myopathy is suspected
• Ophthalmology review - for associated eye anomalies
• Cardiac echo - for associated cardiac anomalies

Orthopaedic imaging. A hip radiograph or ultrasound assesses dislocation, a spine radiograph looks for scoliosis, and foot radiographs assess the clubfoot.

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Principles. AMC involves many joints, so prioritise them for function: elbows, knees, hips and feet. Function comes before appearance.

Stretching still gains range. The contractures are often rigid, yet early stretching and serial casting can still gain useful range. Escalate selectively, when residual deformity prevents function, bracing, hygiene or comfortable positioning.

After clubfoot surgery. Cast for 6-8 weeks, then brace long term (AFO, UCBL) and monitor for recurrence.

After elbow surgery. Immobilise in flexion in a splint for 6 weeks, then regain range gradually, with long-term physiotherapy.

Amyoplasia. Outcomes are excellent with appropriate management and comprehensive multidisciplinary care. Most children walk, and intelligence and lifespan are normal.

Syndromic AMC. The outcome depends on the underlying condition.

Function. Children adapt remarkably and achieve independence.

Global epidemiology: Arthrogryposis affects roughly 1 in 3,000 to 1 in 5,000 live births worldwide; amyoplasia is the most common recognisable form (Hall 2014, N=560). Distribution is broadly similar across regions; apparent variation reflects ascertainment rather than true geography.

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