eMedicine Specialties > Radiology > Pediatrics

Chiari I Malformation

Author: Nasir H Siddiqi, MD, Assistant Professor of Radiology, Washington University; Attending Radiologist, Vascular and Inerventional Radiology, Mallinckrodt Institute of Radiology, Washington University
Coauthor(s): Lakshmana Das Narla, MD, Director of Pediatric Radiology, Associate Professor, Department of Radiology, Medical College of Virginia
Contributor Information and Disclosures

Updated: May 4, 2009

Introduction

Background

In 1891, Hans von Chiari described certain hindbrain abnormalities as postmortem findings in infants. These came to be known as Chiari malformations. Four types of Chiari malformations are described in the literature: types I, II, III, and IV. (Chiari malformation types II, III, and IV are distinct from type I and are not discussed in this article.)^1,2

CMI is not directly associated with other congenital brain malformations, specifically myelomeningocele. Myelomeningocele is a feature of Chiari II malformation. However, craniovertebral malformations are common in patients with CMI.

Distention with cerebrospinal fluid (CSF) of the central canal of spinal cord (ie, hydromyelia) or paracentral cavities (ie, syringomyelia) is present in approximately 25% of patients with CMI. The cervical cord is the most common site of syringohydromyelia.

Pathophysiology

Evidence suggests that Chiari I malformation (CMI) may in fact be a mesodermal disorder. Underdevelopment of the occipital somites of the para-axial mesoderm produces a diminutive, overcrowded posterior cranial fossa (PCF). Tonsillar herniation occurs secondarily as a result of mechanical factors.^3,4,5

A small PCF was identified in morphometric studies of patients with CMI, compared with the supratentorial volume in the same patients and the PCF volume in control subjects. A diminutive PCF underlies the development of acquired CMI in patients with craniosynostosis, hypophosphatemic rickets, achondroplasia, Paget disease, and acromegaly.

The association of CMI with cranial and vertebral anomalies is well known. CMI is not directly associated with other neuroectodermal abnormalities. When present, neural abnormalities are attributable to the effects of tonsillar herniation.

CMI may have a genetic basis, as suggested by (1) its association with known genetic disorders such as achondroplasia, Hadju-Cheney syndrome, and Klippel-Feil syndrome and (2) reports of familial aggregation and concordance among monozygotic twins and triplets. Findings from pedigree analyses in some families suggest an autosomal dominant inheritance pattern with reduced penetrance or an autosomal recessive inheritance pattern.

Acquired CMI is reported to develop after lumboperitoneal or ventriculoperitoneal shunt placement, in addition to the conditions mentioned above. Radiologic findings in patients with acquired CMI cannot be distinguished from those in congenital form of the malformation.

Altered CSF flow

Altered CSF dynamics may play a pathophysiologic role. CSF flow is characterized by systolic and diastolic CSF displacements related to the phases of the cardiac cycle. An increase in the cerebral blood volume with the systolic arterial pulse produces a wave of CSF displacement that descends into the cervical subarachnoid space from the basilar cisterns (craniocaudal displacement). Net cerebral venous outflow and the elastic recoil of the spinal dura during diastole cause CSF displacement in the reverse direction (caudocranial). Respiration also affects CSF flow pulsation. During inspiration, the increased venous return causes the spinal epidural veins to collapse and promotes craniocaudal CSF flow; during expiration, distention of the epidural veins promotes caudocranial flow.

As the herniated tonsils fill the foramen magnum in the setting of CMI, CSF flow is reduced at the craniovertebral junction, and a compensatory pulsatile descent of the cerebellar tonsils is observed during systole. This combination can effectively plug the CSF pathway at the foramen magnum.

Anterior indentation of the medulla and cord secondary to basilar invagination may further contribute to the obstruction of CSF flow at the foramen magnum. This effect exaggerates the pulsatile systolic wave in the spinal subarachnoid space. During diastole, rapid recoil of the brain stem and tonsils disimpacts the foramen magnum and allows normal CSF diastolic pulsation.

Given the small PCF-CSF space, decreased CSF compliance is expected. Along with altered CSF dynamics, this effect can play a role in causing headaches and other symptoms that mimic an orbital pseudotumor, Ménière disease, bulbar compression, or syringomyelia.

Patients with CMI remain asymptomatic for prolonged periods. Arachnoidal scarring and adhesions are suggested to build up at the foramen magnum as the cerebellar tonsils rub against bone over the years. These arachnoidal adhesions may increase the compression of the hindbrain and spinal cord and further interfere with CSF flow at the foramen magnum and thus cause symptoms. These adhesions are often discovered at the time of surgery; however, many patients remain symptomatic despite adequate surgical decompression of the foramen magnum.⁶

Many patients report trauma, such as whiplash injuries and direct blows to the head and neck, as the precipitating event for symptom onset. Such trauma may accentuate tonsillar impaction or result in subarachnoid hemorrhage that destabilizes a marginally compensated CSF system.

Syringohydromyelia

Syringohydromyelia probably is secondary to pathologic CSF dynamics. The exaggerated pulsatile systolic wave in the spinal subarachnoid space drives the CSF through anatomically continuous perivascular and interstitial spaces into the central canal of the spinal cord. More severe obstruction occasionally can cause hydrocephalus.

Syringohydromyelia has been suggested to result from pulsatile flow directed down the central canal of spinal cord from the fourth ventricle with obstructed channels. However, most syringes are separated from the fourth ventricle by an occluded or stenotic segment of the central canal. CSF flow abnormalities improve after decompressive neurosurgery; the symptoms resolve, and the syrinx cavity collapses.

Syringomyelia was observed in only 14% of patients in a pediatric case series. Spinal MRI results were available in only a minority of patients; therefore, this figure may represent an underestimation of the presence of syringomyelia. In addition, sufficient time may not have elapsed for syringomyelia to develop.

Frequency

United States

The true incidence of Chiari I malformation (CMI) is not known. Before the availability of MRI, CMI was rarely diagnosed. In one study, a rate 0.6% was reported in all age groups, and a rate of 0.9% was reported in a study of only pediatric patients. Therefore, CMI is more common in both the adult and pediatric populations than was recognized previously.

Mortality/Morbidity

See Clinical Details.

Sex

A female predominance was reported in some large case series, with a male-to-female ratio of 2:3.

Age

Chiari I malformation (CMI) is predominantly a congenital abnormality. Prior to MRI, CMI was observed almost exclusively in those aged 10-30 years, hence the term adult-type Chiari malformation.

Patients are generally 10 months to 65 years of age. In one series, the mean patient age at diagnosis was 39 years (range, 6-60 y). In another series, the mean patient age at the time of symptom onset was 24.9 ± 15.8 years. Patients with syringomyelia presented at a slightly younger age (24.7 ± 16.6 y).⁷ Among the patients, 37% reported lifelong symptoms such as headaches and clumsiness, and 24% reported that trauma was the precipitating event.

Anatomy

The anatomic landmarks of this disorder are depicted in Image 1.

Presentation

A variety of symptoms have been reported in the literature, including the following:

• Suboccipital headaches
• Ocular symptoms, including retro-orbital pain, visual disturbances, photophobia, and diplopia
• Otoneurologic symptoms, including dizziness, vertigo, hearing disturbances, oscillopsia, nystagmus, and synkinesis
• Hindbrain compression symptoms, including weakness, paresthesia, ataxia, cranial nerve palsies, dysphagia, dysphasia, palpitations, syncope, apnea, and sudden death
• Syringomyelia symptoms, including central cord syndrome, impaired sensation, impaired motor control, gait disturbance, torticollis, bowel and/or bladder symptoms, neuropathic joints, trophic phenomena, and pain
• Spinal cord dysfunction, which is present in as many as 94% of patients with syringomyelia and 66% without syringomyelia

Patients who have Chiari I malformation (CMI) and a syrinx almost always present with symptoms referable to the syrinx. If the syrinx extends into the medulla, bulbar symptoms predominate. In pediatric patients, the most common symptoms are headache, neck pain, and ataxia. Usually, the degree of tonsillar ectopia and the presence and severity of neurologic symptoms and signs are directly correlated. Posttraumatic neurologic complications in patients with CMI are well described. Minor trauma can result in significant symptoms in the setting of pediatric CMI.

The natural history of CMI is not understood clearly. Many patients are asymptomatic and probably remain so all of their lives. The age of patients at symptom onset is variable. In symptomatic patients, whether the malformation is causative or incidental is not always clear. Many patients have serious clinical sequelae. Rarely, the malformation resolves spontaneously.

Usually, patients with clinical signs of CMI have greater mean tonsillar herniation (ie, 12.3 mm) compared with that of patients with incidentally discovered CMI (ie, 7.3 mm). Herniations greater than 12 mm are almost always symptomatic.

Tonsillar ectopia does not explain the entire clinical picture. The extent of tonsillar ectopia is not correlated with the patient's age at onset, presence of syringomyelia, or symptoms such as headache. As many as 31% of patients with a tonsillar herniation of 5 mm or greater may be asymptomatic. In one series, tonsillar ectopia of 25 mm was discovered incidentally. Tonsillar herniation may be asymmetric without corresponding lateralizing symptoms or signs. The extent of tonsillar descent is not correlated with the Karnofsky score or the incidence of syringomyelia. Asymptomatic children with tonsillar ectopia of greater magnitudes may be at higher risk for neurologic symptoms later in life.

Preferred Examination

MRI is the imaging modality of choice except in patients in whom MRI is contraindicated. MRI demonstrates the abnormal CSF flow and configuration and position of the brain and spinal cord.

Brainstem auditory evoked potentials is widely used during posterior fossa decompression  to assess functional integrity of the central auditory system during surgery. Improvement has been shown to occur primarily during bony decompression.⁹

Limitations of Techniques

MRIs may not reliably demonstrate abnormal findings of the skeleton associated with Chiari malformations. Radiography-based imaging modalities such as conventional radiography or CT are preferred for this indication.

Radiologic findings in patients with acquired CMI cannot be distinguished from those in congenital form of the malformation.

Differential Diagnoses

Chiari II Malformation

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• Chiari Malformation (Neurosurgery)
• Chiari II Malformation (Radiology)