Chiari malformations

What is Chiari malformation?

Hans Chiari was a German pathologist who, between 1891 and 1896, described various anomalies of the caudal cerebellum and brain stem from post-mortem studies.¹

He first described an abnormality of elongated peg-like cerebellar tonsils that are displaced into the upper cervical canal through the foramen magnum. This is now called the Chiari type I malformation. Abnormalities of the skull and upper vertebrae are common in this type.

Five years later he reported a complex congenital malformation of the brain, nearly always associated with myelomeningocele. It is now called the Chiari type II, or Arnold-Chiari malformation.² This condition includes downward displacement of the medulla, fourth ventricle and cerebellum into the cervical spinal canal, as well as elongation of the pons and fourth ventricle, probably due to a relatively small posterior fossa. About 25% have distension of the central or paracentral canal of the cervical spinal cord, causing hydromyelia or syringomyelia.

He also described a single case of cervical spina bifida with herniation of the cerebellum through the foramen magnum, now called Chiari III malformation.³

Some authors have added a severe form of cerebellar hypoplasia without displacement of brain through the foramen magnum and called it Chiari IV malformation.

A case of acquired Chiari malformation secondary to skull hyperostosis has been described.⁴

How common are Chiari malformations? (Epidemiology)

Chiari type I

Retrospective investigation of brain MRIs has found a prevalence of 1 in 1,280 individuals.⁵ It tends to present earlier if there is syringomyelia.

Chiari type II (Arnold-Chiari malformation)

Only found in association with myelomeningocele.⁶

Chiari malformation symptoms (presentation)

Chiari type I⁵

The presentation of this condition depends upon the severity of the herniation and associated other features. The hydrodynamics of the cerebrospinal fluid (CSF) are upset, possibly causing caudal to cranial flow and this is a contributor to symptoms. A variety of symptoms can occur:

• Suboccipital headaches are common.

• Pain behind the eyes, visual disturbances, diplopia and photophobia can occur.

• Bilateral hearing loss has been reported.⁷

• Dizziness, vertigo, disturbance of hearing and nystagmus may develop.

• Compression of the hindbrain can cause weakness, paraesthesia, ataxia, cranial nerve palsies, dysphagia, dysphasia, palpitations, syncope, apnoea and sudden death.

• Problems of the spinal cord present in about 95% of patients with syringomyelia and two thirds of those without it. Symptoms include central cord syndrome, impaired sensation, impaired motor control, gait disturbance, torticollis, bowel and/or bladder symptoms, neuropathic joints, trophic phenomena and pain.

• Various abnormalities of the skull and upper vertebrae are found in 25-50%.

Chiari type II⁶

• This tends to present much earlier with significant morbidity and mortality in the neonatal period.

• A third of patients with meningomyelocele develop brain stem dysfunction by the age of 5 years and a third of these die in infancy.

• Cranial nerve and brain stem dysfunction are the most serious features.

• There are two types of presentation in type II. One involves infants and the other older children. Infants rarely present before a few weeks old but life is threatened by involvement of cranial nerves IX and X and compression of the respiratory centre.

Presentation in infancy

• Inspiratory stridor.

• Dysphagia or nasal regurgitation.

• Aspiration from bilateral abductor vocal cord paralysis or central neural dysfunction, or both.

• Respiratory distress.

• Episodes of apnoea.

• Weak cry.

• Scoliosis.

• Quadriparesis.

• Opisthotonic posture.

Presentation in childhood or adolescence

• Syncope.

• Nystagmus.

• Spastic quadriparesis with upper motor neurone (UMN) features.

• Recurrent pneumonia from aspiration.

• Gradual loss of function.

Presentation of types III and IV have been ignored as they are much rarer.

Investigations

• MRI:
  

  • The investigation of choice is MRI.⁸

  • MRI now means that myelography is not required. Injection of a contrast and enhancing agent such as gadolinium may help to clarify the anatomical abnormalities.

  • The use of cine MRI is sometimes necessary to elucidate whether an abnormality seen on static films is causing a significant physiological obstruction to CSF flow.⁹

• Plain X-ray of the skull and cervical spine will show skeletal abnormalities like scoliosis.

• CT scan demonstrates caudal displacement of the fourth ventricle.

• Ultrasound scan:
  

  • Only useful whilst the fontanelle is open.

  • May be a useful initial investigation but should be followed by more accurate assessment with MRI.

  • Ultrasound is useful for prenatal screening in Chiari types I and II.

Chiari malformation treatment and management

Non-surgical

All patients, whether they are suitable for surgery or not, should be considered for rehabilitative therapy and psychological support.

Surgical

Chiari type I¹⁰
The main issues to be determined in the management of a patient with a Chiari I malformation are:

• What is the anatomical malformation?

• What are the main disabling symptoms?

• How does the malformation relate to physical symptoms?

A decision on whether to proceed to surgery should be based on the following approach:

• A multidisciplinary assessment is helpful, with neurologists, neurosurgeons and rheumatologists all giving an opinion about the patient.

• A decision regarding surgery should be taken on reviewing these opinions together with the imaging results.

• Patients most likely to benefit from surgery are those with reproducible neurological signs - eg, nystagmus, absent gag reflex, and demonstrably abnormal CSF flow in the hindbrain on cine MRI.⁹

• Posterior fossa decompression with suboccipital craniectomy is the gold standard operative procedure.¹¹ Adding duraplasty is sufficient to treat symptoms without the need for further surgical procedures.

• Chiari-related syringomyelia can be treated successfully with surgery.¹²

Chiari type II¹³
Studies suggest that early surgery in type II malformation results in a better outcome. Prompt decompression of a meningomyelocele in patients under the age of 2 years can lead to a significant reduction in mortality and morbidity.¹⁴

Prognosis

Some patients are asymptomatic and remain so for the length of their lives. There are reports of spontaneous resolution of Chiari type I.¹⁵

Prognosis is favourable in Chiari I malformations with improved diagnostic and surgical techniques. Surgery for Chiari I malformations gives improvements in 73% of people in less than one year, 79% at 1-3 years.¹⁰ The prognosis for Chiari II malformations depends on the severity of the malformation, associated neurological abnormalities and complications.¹³