Patients with amyoplasia congenita do not have evidence of brain, cardiac, or genitourinary tract abnormality. Bowel atresia and gastroschisis have been reported. The cause of amyoplasia congenita is unknown. Fetal spinal cord disruption due to systemic hypotension producing anterior horn cell ischemia is the most likely explanation. Amyoplasia congenita is a sporadic condition. This is an important condition to recognize because the chances of recurrence are low and the prognosis is good. Muscle biopsies may show evidence of myopathy and neuropathy. Amyoplasia congenita is the final diagnosis in about one-third of neonates with arthrogryposis.

Spinal Cord Abnormalities
Traumatic spinal cord injury usually occurs during delivery. It should be suspected if no cause for arthrogryposis is found and there are no signs of brain or brainstem involvement. Magnetic resonance imaging of the spine is indicated. Arthrogryposis of the lower extremities has been reported with lumbosacral meningocele (Figure 163.1) and with sacral agenesis.

A	B

Figure 163.1.— [A] Arthrogryposis of the lower extremities (after surgery). [B] MRI of the spine demonstrating a lumbosacral meningocele.

Infantile Spinal Muscular Atrophy
Arthrogryposis multiplex congenita occurs in 10% to 20% of neonates with infantile spinal muscular atrophy. Deoxyribonucleic acid studies for infantile spinal muscular atrophy should be performed in neonates with unexplained arthrogryposis. The prognosis of arthrogryposis due to infantile spinal muscular atrophy is poor. Autosomal recessive and X-linked inheritance have been reported.

Infantile Neuronal Degeneration
Infantile neuronal degeneration can only be diagnosed by performing an autopsy. The autopsy findings reveal anterior horn motor neuron atrophy and degenerative changes in the Clarke’s column; corticospinal tracts; spinocerebellar and spinothalamic tracts; Purkinje cell layer; and dentate and ventral thalamic nuclei. It presents with EMG findings of infantile spinal muscular atrophy and delayed sensory and motor conduction. Deoxyribonucleic acid study for Werdnig-Hoffmann disease is normal.

Focal Spinal Muscular Atrophy
Arthrogryposis multiplex involving only the upper or the lower extremities occurs with congenital focal cervical or lumbar spinal atrophy.