Chiari Malformation Type Ii

Description

Chiari malformation type II (CM2), also known as the Arnold-Chiari malformation, consists of elongation and descent of the inferior cerebellar vermis, cerebellar hemispheres, pons, medulla, and fourth ventricle through the foramen magnum into the spinal canal. CM2 is uniquely associated with myelomeningocele (open spina bifida; see 182940) and is found only in this population (Stevenson, 2004). It is believed to be a disorder of neuroectodermal origin (Schijman, 2004).

For a general phenotypic description of the different forms of Chiari malformations, see Chiari malformation type I (CM1; 118420).

Clinical Features

Lindenberg and Walker (1971) described the Arnold-Chiari malformation in 2 successively born daughters of nonconsanguineous parents. Both children had associated hydrocephalus and lumbar meningomyelocele.

In a review, Stevenson (2004) noted that CM2 is associated with polygyria, partial or complete agenesis of the corpus callosum, subnormal intelligence, ventricular abnormalities, and hydrocephalus. Symptoms in infants include inspiratory stridor, prolonged expiratory apnea with cyanosis, neurogenic dysphagia, and feeding difficulties. Other features include hypotonia, opisthotonus, nystagmus, and weak cry. Older children may experience upper extremity weakness, spasticity, ataxia, and occipital headache. Syringomyelia (186700) may also be present.

History

The term 'Chiari malformation' is used in recognition of the work of Hans van Chiari, a Viennese pathologist who practiced medicine in Vienna, Prague, and Strasbourg in the late 19th century. Chiari (1891) described a 17-year-old woman with elongation of the cerebellar tonsils and medulla into the spinal cord, what is now referred to as the Chiari type II malformation. However, the Chiari II malformation was probably first described by Cleland (1883) in a child with spina bifida, hydrocephalus, and anatomic alterations of the cerebellum and brainstem (Stevenson, 2004). Arnold (1894) described a single patient with myelomeningocele and herniation of the cerebellum into the cervical canal.

Animal Model

Using Cre-recombinase-activated markers in transgenic mice, Matsuoka et al. (2005) mapped a cryptic neural crest-mesoderm boundary inside the neck and shoulder girdle skeleton in which cellular distributions of neural crest and mesoderm correspond precisely to muscle attachment scaffolds to the shoulder girdle. The study challenged the 'ossification model' and corroborated the 'scaffold model' of vertebrate neck and shoulder evolution. The skeleton that Matsuoka et al. (2005) identified as neural crest-derived is specifically affected in human Klippel-Feil syndrome (118100), Sprengel deformity (184400), and Arnold-Chiari malformation. Loss or dysplasia of post-otic neural crest (PONC)-derived basicranial (clivus) bone attachments for the internal pharynx and larynx constrictors and ensuing widening of the foramen magnum are the primary mechanical cause of the Arnold-Chiari malformation. In this case PONC respecification from endochondral attachment bone to connective tissue is the likely cause of cryptic basicranial instability and early death.