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SNOMEDCT: 253185002; ORPHA: 268882; MONDO: 0007316;
Chiari malformation type I (CM1) is the protrusion of the cerebellar tonsils through the foramen magnum, defined radiologically as tonsillar descent of 5 mm or more. CM1 is associated with syringomyelia (see 186700) in up to 80% of cases. Although many individuals with CM1 are asymptomatic, the malformation can cause headaches, ocular disturbances, otoneurologic disturbances, lower cranial nerve signs, cerebellar ataxia, or spasticity. Onset of symptoms is usually in the third decade of life (Speer et al., 2003).
Since many cases of CM1 are asymptomatic, prevalence estimates may not be accurate. However, a retrospective investigation of brain MRIs reported the prevalence of CM1 to be 1 in 1,280 individuals (Meadows et al., 2000).
Four classic types of Chiari malformation have been described. Type I is descent of the cerebellar tonsils into the cervical canal and is usually not associated with hydrocephalus. In Chiari type II (CM2; 207950), there is descent of the cerebellar tonsils, cerebellar inferior vermis, and portions of the cerebellar hemispheres into the spinal canal along with displacement of the brainstem and fourth ventricle. Type II is the most frequent form of Chiari malformation and is associated with spina bifida (see 182940) and hydrocephalus. Type III is characterized by caudal displacement of the medulla and herniation of part of the cerebellum in an occipital or cervical meningocele. Part of the hindbrain may also be herniated. Hydrocephalus is associated in 50% of cases and is due to aqueductal stenosis or Dandy-Walker malformation. Type IV refers to hypoplasia or aplasia of the cerebellar hemispheres and alterations of the pons with marked dilatation of the fourth ventricle, cisterna magna, and basal cisterns. Whereas Chiari types II, III, and IV are considered to be primarily neural in origin resulting from neuroectodermal anomalies, Chiari type I is considered to result from a mesodermal defect (Schijman, 2004).
Two additional types of Chiari malformation have been described. 'Chiari type 0' has been defined as syringohydromyelia without cerebellar tonsillar herniation, but with distortion of the contents of the posterior fossa. These patients have resolution of symptoms with decompression surgery (Iskandar et al., 1998; Tubbs et al., 2001). Caudal migration of the brainstem and cerebellar tonsils often associated with syringomyelia but without spina bifida has been referred to as 'Chiari type 1.5' (Schijman, 2004; Tubbs et al., 2004).
Adams et al. (1941) have been credited with the first detailed clinical description of Chiari type I malformation, although confusion regarding the nomenclature existed up to the 1970s (Schijman, 2004).
Bentley et al. (1975) described 2 families, each with 2 affected sibs. In the first family, a 54-year-old woman presented with a 25-year history of loss of pain and temperature sensation and weakness of the left arm. Myelography showed Chiari type I malformation with slight dilatation of the cervical cord suggestive of syringomyelia. Her younger sister presented with loss of pain and temperature sensation of both hands, right Horner syndrome and impaired corneal response, and extensor plantar responses. Syringomyelia was diagnosed clinically; myelography was not performed. In the second family, a brother and sister had adult onset of paresthesias, segmental muscle weakness and wasting, and lower limbs hyperreflexia. Myelography in the brother showed obstruction at the foramen magnum suggestive of CM1; both patients were diagnosed clinically with syringomyelia. Bentley et al. (1975) noted the association of syringomyelia with CM1 and basilar impression (109500; see also Spillane et al., 1957), and suggested autosomal dominant inheritance of a developmental abnormality of the base of the skull during embryogenesis.
Gimenez-Roldan et al. (1978) reported a father, daughter, and son with Chiari malformation type I associated with cervical syringomyelia. Clinical features included loss of pain sensation in the hands, muscle weakness of the forearms and hands, scoliosis, hyperreflexia of the lower limbs, segmental sensory loss, and enlarged sagittal diameter of the cervical canal. Another family member had basilar impression of the skull without neurologic abnormalities. Autosomal dominant inheritance was suggested.
Coria et al. (1983) described a family in which members in 3 generations showed Chiari I malformation in association with marked occipital dysplasia causing a small and flat posterior fossa, as demonstrated by plain skull films. Herman et al. (1990) reported 2 affected sibs.
Stovner et al. (1992) described CM1 in 2 adult monozygotic female twins, their mother, and possibly in 2 of their 4 daughters. The twins were discordant for the extent of herniation of the cerebellar tonsils; syringomyelia was present in only 1 twin. The twin with syringomyelia had neonatal pertussis with severe coughing, which the authors postulated may have caused a craniospinal pressure gradient that increased the impaction of the cerebellar tonsils through the foramen magnum and precipitated the development of syringomyelia. Adult symptoms began in connection with the first pregnancies, suggesting a relation to physical strains and hormonal changes. Although the occipital squama appeared somewhat short in some of the patients, no other bony abnormalities were found on MRI. Stovner et al. (1992) classified the disorder in this family with autosomal dominant craniocervical malformations. In a follow-up report of the same family, Stovner (1992) noted that the mother and twin daughters had migraine-like headaches, often associated with nausea and photophobia. Other features suggested a relation to the CM1 malformation, including precipitation by coughing, bending forward, heavy lifting, and neck extension.
Colombo and Cislaghi (1993) reported 2 affected Italian sisters. One had Chiari type I malformation and syringomyelia, and the other had syringomyelia and Klippel-Feil syndrome (see 118100). The authors hypothesized that genetic factors played a role in the etiology of abnormalities of these hindbrain structures.
Zakeri et al. (1995) reported a brother and sister with CM1 and syringomyelia as well as mild scoliosis. Both had symptom onset as adults, including upper and lower limb numbness and muscle weakness and atrophy. Zakeri et al. (1995) provided a detailed review of the literature and noted that families consistent with autosomal recessive and autosomal dominant inheritance had been reported. The authors cited several early reports of familial syringomyelia (Redlich, 1916; Barre and Reys, 1924; van Bogaert, 1929). In an accompanying commentary, Hoffman (1995) suggested that all familial cases of syringomyelia have a primary defect in the region of the foramen magnum resulting in Chiari malformation type I or basilar invagination, and that syringomyelia results from disordered CSF dynamics occurring in this area. Accordingly, some affected members of a family may have isolated CM1 whereas others go on to develop syringomyelia.
Cavender and Schmidt (1995) reported monozygotic triplets who exhibited variable degrees of tonsillar herniation. One sib presented with CM1 and associated syringomyelia, whereas the other 2 sibs had 4 mm and 2.5 mm of tonsillar herniation, respectively. The findings suggested a common hereditary etiology. Stovner and Sjaastad (1995) reported 2 adult sibs with segmental hyperhidrosis and sensory loss who were both found to have CM1. The brother also had a cervical syrinx. Atkinson et al. (1998) reported female identical twins with CM1 and syringomyelia. The daughter of 1 of the women had CM1 without syringomyelia. Detailed radiographs of the mother and daughter showed occipital dysplasia and crowding of the posterior fossa contents.
Milhorat et al. (1999) presenting clinical and radiographic findings of 364 patients with CM1. The mean age at symptom onset was 25 years. Associated anomalies included syringomyelia (65%), scoliosis (42%), and basilar invagination (12%). Forty-three patients (12%) reported a family history of CM1 and/or syringomyelia. Clinical features included headaches, pseudotumor-like episodes, a Meniere-like syndrome, lower cranial nerve signs, and spinal cord disturbances. MRI showed obliteration of the retrocerebellar CSF space, tonsillar herniation, and varying degrees of cranial base dysplasia. Volumetric calculations showed a significant reduction in the posterior fossa cavity with normal brain volume. Milhorat et al. (1999) noted that some symptomatic patients had tonsillar herniation less than 5 mm and that clinical manifestations likely resulted from CSF disturbances.
Speer et al. (2000) identified 31 pedigrees in which 2 or more individuals had CM1 associated with syringomyelia. Imaging analysis of family members revealed that 21% of asymptomatic first-degree relatives also had CM1/syringomyelia. The pattern of inheritance was most consistent with autosomal dominant transmission. In no case was syringomyelia identified without attendant CM1. Speer et al. (2000) suggested that the phenotype of the familial disorder should be classified as 'CM1 with or without syringomyelia.' Furthermore, since isolated syringomyelia is so rare, reports of 'familial syringomyelia' likely represent familial CM1 with associated syringomyelia.
Yabe et al. (2002) reported a Japanese mother and son with CM1 and syringomyelia who first developed symptoms at ages 54 and 35 years, respectively. Clinical features included gait disturbance, paresthesias in the lower limbs, segmental sensory loss, nystagmus, and extensor plantar responses.
Tubbs et al. (2004) reported 11-year-old twin boys with syringomyelia. One brother also had Chiari malformation type 1 and Klippel-Feil anomaly; the other had the so-called Chiari type 0 malformation without tonsillar herniation. Both brothers underwent successful posterior fossa decompression surgery. Tubbs et al. (2004) concluded that frank tonsillar ectopia is not necessary to produce syringomyelia, and that there is a continuum of space-volume relationships in the posterior fossa. The authors theorized that cases reported as 'familial syringomyelia' may actually represent Chiari type 0 with secondary syringomyelia.
Mavinkurve et al. (2005) reported 2 adolescent sisters with Chiari I malformation and syringomyelia. Each was asymptomatic and found to have scoliosis on routine medical examination. The older sib had lateral gaze nystagmus which resolved completely after cervicomedullary decompression surgery. Both girls had near resolution of the syringomyelia following surgery.
Szewka et al. (2006) reported 3 families with CM1. In 1 family, a mother, son, and possibly the mother's father were affected. The mother had chronic migraine-like headaches and unsteady gait. Her son presented at age 19 months with abnormal gait, hypertonicity on the right, spastic gait, and extensor plantar responses. In the second family, 15-month-old monozygotic twin boys were affected, mainly showing swallowing difficulties. In the third family, an 11-year-old boy presented with urinary urgency and left hand numbness and was found to have a CM1 and syringomyelia. A first cousin had CM1 and syringomyelia, whereas 2 additional family members had a crowded posterior fossa.
George and Page (2006) reported 2 adult sibs with CM1, 1 of whom had associated syringomyelia. A 49-year-old man presented with diplopia, nystagmus, left arm and thigh pain, change in gait, and occipital headaches. MRI showed CM1 with the cerebellar tonsils herniated to C2. Surgical decompression was successful. His 52-year-old sister presented with weakness, shaking, and fasciculations of the left upper limb, hyperreflexia of the lower limbs, and decreased pin-prick sensation in the C2-C8 distribution. MRI showed CM1 with extensive syringomyelia extending from C1 to the mid-dorsal region. Decompression surgery was partially successful.
Several sets of affected monozygotic twins have been reported (Stovner et al., 1992; Cavender and Schmidt, 1995; Turgut, 2001), suggestive of a genetic component.
Robenek et al. (2006) reported a sister and 2 maternal half brothers with syringomyelia; the 2 half brothers also had CM1. All had adult onset of symptoms. The authors suggested autosomal dominant inheritance.
Boyles et al. (2006) examined brain MRIs from 99 individuals from 35 families with CM1. Statistical analyses of 10 measurements obtained from the posterior fossa showed that posterior fossa volume was highly heritable, while herniation was not heritable. The findings supported the cramped posterior fossa theory of CM1 pathogenesis and suggested a genetic basis for the condition.
Using neuroimaging, Nishikawa et al. (1997) found that 30 patients with Chiari type I malformation had a significantly smaller bony posterior fossa compared to controls. The exocciput and supraocciput were smaller and the tentorium was steeper, but there was no difference in posterior fossa brain volume. Nishikawa et al. (1997) postulated that the etiology of CM1 involves underdevelopment of the occipital bone, perhaps due to abnormal development of the occipital somite originating from the paraxial mesoderm, resulting in overcrowding in the posterior fossa. Basilar invagination was associated with a more severe downward herniation of the hindbrain.
Speer et al. (2000) postulated that an underlying gene responsible for CM1/syringomyelia may have pleiotropic effects that influence posterior fossa volume, other skull bone abnormalities, the extent of cerebellar tonsil herniation, and the formation of syringomyelia.
Aydin et al. (2005) reported cranial MRI of 60 adult patients with CM1 and tonsillar herniation of at least 5 mm below the foramen magnum. All measurements of the posterior fossa were reduced in patients compared to controls, except the anteroposterior diameter of the foramen magnum. Syringomyelia was associated in 46 patients. The findings suggested that the bony components of the posterior fossa are not fully developed in these patients, supporting the concept that CM1 is a disorder of the paraxial mesoderm.
Levine (2004) proposed a novel theory of the pathogenesis of syringomyelia associated with lesions at the foramen magnum. Obstruction of the subarachnoid space at the foramen magnum leads to transient abrupt changes of CSF pressure with increased pressure rostral to the block and decreased pressure caudal to the block. These changes produce corresponding changes in transmural venous pressure, causing compression of vessels above the site of compression at the foramen magnum and dilation of vessels below. This leads to mechanical stress on the spinal cord resulting in tissue breakdown and partial disruption of the blood-spinal cord barrier, allowing a plasma infiltrate to accumulate in the spinal cord forming a syrinx.
By genomewide linkage analysis of 23 families comprising 71 individuals with CM1, Boyles et al. (2006) identified a candidate locus on chromosome 15q21.1-q22.3 (maximum 2-point nonparametric exponential lod score of 3.33 at rs744318).
By genomewide linkage analysis of 23 families comprising 71 individuals with CM1, Boyles et al. (2006) identified a candidate locus on chromosome 9q22.31 (maximum multipoint parametric lod score of 3.05 between rs1000735 and rs2895201). The results delineated an 8.5-cM region with multipoint lod scores above 2.0 between rs1573231 and rs1412488. There were no significant 2-point lod scores in this region.
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 (Schijman, 2004). In a series of cases of posterior fossa alterations, Chiari (1896) likely described the distinct Chiari I malformation (Schijman, 2004).
Occipital bone hypoplasia with foramen magnum obstruction and secondary syringomyelia similar to human Chiari type I malformation is a common condition in the Cavalier King Charles spaniel. Rusbridge and Knowler (2004) reported pedigree information spanning 24 generations from 120 affected dogs. The study showed that 8 great-grandparents of all affected dogs could be traced to 2 female ancestors. The disease appeared to be more severe and have an earlier onset with increased inbreeding. Selection for coat color is believed to have influenced the development of occipital bone hypoplasia as well as idiopathic epilepsy in these dogs.
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