What Is a Chiari Malformation?

Rare defect causes brain tissue to protrude into the spinal canal

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A Chiari malformation is a rare condition in which brain tissues at the back of the skull protrude into the upper part of the spinal canal. It is mostly caused by a congenital birth defect but can also occur later in life as a result of an injury, infection, or disease.

Depending on the extent of tissue displacement, symptoms can range from headaches and poor coordination to hearing loss, seizures, and death. Some people may have no notable symptoms.

Chiari malformation is diagnosed with imaging studies but can sometimes be seen on physical examination. Treatment depends on the severity of the defect, with some people only requiring symptomatic treatment and others needing surgery to relieve pressure on the brain.

Chiari malformation is named after an Austrian pathologist named Hans Chiari who published detailed descriptions of the condition from 40 postmortem exams conducted between 1891 and 1896.

Types of Chiari Malformation

Chiari malformations are a complex group of defects with different causes, symptoms, and outcomes. They involve the displacement of a part of the brain known as the cerebellum through an opening of the base of the skull called the foramen magnum.

The cerebellum a major structure at the back of the brain responsible for the coordination of voluntary movements, while the foramen magnum allows for the passage of the spinal cord to the brain.

When Chiari malformations occur, one or both lobes of the cerebellum, called the cerebellar tonsils, will slip through this opening and extend into the upper spinal canal.

In some cases, the brainstem (an adjacent structure responsible for respiration, heart rate, blood pressure, and other vital functions) will also be involved. The displacement of tissues can place extreme pressure on these structures, disrupting their normal functions.

Chiari malformations that occur during fetal development are referred to as congenital or primary malformations, while those that develop later in life as a result of injury or illness are called acquired or secondary malformations. Primary malformations are much more common than secondary ones.

Chiari malformations are further classified by the severity of the defect. There are five classifications—delineated on a scale of I to IV—that doctors use to direct the appropriate course of treatment and predict the likely outcome (prognosis).

  • Type I: The herniation (bulging) of one or both cerebellum tonsils will extend into the upper spinal canal by more than 5 millimeters (roughly 1/4 inch). In some cases, a portion of the brainstem may be involved.
  • Type II: Also known as the Arnold-Chiari malformation, the herniation is more profound and involves both the cerebellum and brainstem. It a more serious form of the defect usually accompanied by spina bifida.
  • Type III: The herniation of brain tissues will cause occipital encephalocele, a sac-like protrusion extending outward from the base of the skull. In addition to the cerebellum and brainstem, a portion of the occipital lobe (the visual processing part of the brain) may be involved.
  • Type IV: This rare defect does not involve herniation but rather occurs when the cerebellum and brainstem fail to develop properly during fetal development. This places both structures into an incorrect position in relation to the foramen magnum.
  • Type V: This extremely rare condition is characterized by the absence of the cerebellum and the herniation of the occipital lobe into the foramen magnum. Type V Chiari malformation has only been described twice in the medical literature, both cases of which occurred in newborns with spina bifida.

Chiari Malformation Symptoms

The symptoms of Chiari malformations can vary by the severity of the defect and may not present in all cases. Generally speaking, newborns, infants, and toddlers will experience more symptoms (and have more rapid deterioration of neurological function) than older children and adults.

Although the symptoms are relatively consistent between the various types, not everyone with a Chiari malformation will have the same symptoms or severity of symptoms.

Chiari Malformation Symptoms by Type
Type Symptoms
Type I Headache, back pain, leg pain, hand numbness or weakness, neck pain, dizziness, unsteady gait (usually seen during early childhood), tinnitus (ringing in the ears)
Type II

Noisy breathing, dysphagia (difficulty swallowing), sleep apnea, pulmonary aspiration (breathing food into the lungs), arm weakness, loss of fine motor skills, vision problems, hearing loss, scoliosis (abnormal curvature of the spine, usually seen in older infants and children), and paralysis below the waist due to spina bifida

Type III Headache, fatigue, muscle weakness and stiffness, slurred speech, unsteadiness, loss of ability to feel cold and/or heat, neck or spinal spasms, vertigo, sleep apnea, vocal cord paralysis, dysphagia, nystagmus (uncontrolled eye movements), hydrocephalus ("water on the brain"), torticollis (a neurological disorder causing the head to tilt downward), seizures, and numerous other severe neurological problems
Type IV Not compatible with life
Type V Not compatible with life

Causes

There are several different causes of a Chiari malformation. Most often, it is caused by structural defects in the brain, spinal cord, and skull that occur during fetal development. Less commonly, the condition is acquired later in life through traumatic injury or illness.

Primary Chiari Malformations

In the past, primary Chiari malformations were reported in less than one of every 1,000 live births. However, with the advent of advanced imaging tools, the defect is more commonly detected in children, some of whom may not have symptoms or have generalized, unexplained symptoms. 

Current studies suggest that between 0.5% and 3.5% the population may have a type I Chiari malformation, the mildest form of the defect.

Broadly speaking, there are a number of mechanisms that can give rise to primary Chiari malformations:

  • Reduced posterior fossa size: The posterior fossa is the space within the skull that houses and protects the cerebellum and brainstem. With type I Chiari malformations, the posterior fossa will be abnormally small and essentially "force" brain tissue into the upper spinal canal. With type II and type III malformations, the space within the posterior fossa will be even smaller.
  • Spinal cyst formation: The pressure placed on the compressed cerebellum and brainstem can cause cerebrospinal fluid to leak into the upper spinal canal. This can cause the formation of a fluid-filled cyst called a syrinx. As the syrinx enlarges, it can compress the spinal cord and brainstem, causing an array of neurological problems.

The cause of this cascade of events is poorly understood. Most research today suggests that the neural tube (the embryonic precursor to the central nervous system) will fail to close sometime during the course of fetal development. When this occurs, defects like Chiari malformations, spina bifida, and anencephaly can develop. At the same time, it is not uncommon for the base of the skull to flatten during the latter half of pregnancy, reducing the size of the posterior fossa.

Genetics are believed to play a central role in the development of primary Chiari malformations. The exact genetic mutations have yet to be identified, but researchers believe that alternations of chromosomes 9 and 15 are likely suspects.

Mutations of these chromosomes are closely tied to hereditary connective tissue diseases, like Ehlers-Danlos syndrome, that commonly occur in people with Chiari malformations.

It is unclear if Chiari malformations can be passed through families. Early studies suggested that 12% of people with a Chiari malformation have a close family member with the defect, but other studies have not found such a clear association.

There is also evidence that the deficiency of certain vitamins during pregnancy, particularly vitamin A and vitamin D, can contribute to the defect. Both are essential to the growth of bone and may lead to underdevelopment of the posterior fossa in women with severe deficiencies.

Although vitamin deficiency is not the sole cause of a Chiari malformation (and taking maternal vitamins won't likely prevent the defect), it is the one risk factor that is modifiable during pregnancy.

Secondary Chiari Malformations

Secondary Chiari malformations are uncommon occurrences but ones that can be acquired through disease and injuries. Generally, any condition that takes up space in the posterior fossa can increase intracranial pressure and lead to a secondary Chiari malformation.

Examples include:

Diagnosis

A Chiari malformation is diagnosed with imaging studies but can sometimes be observed in children with a type III malformation and occipital encephalocele.

Imaging studies commonly used in the diagnosis of a Chiari malformation include:

  • Magnetic resonance imaging (MRI): An imaging technique that employs powerful magnetic and radio waves to create highly-detailed images, particularly of soft tissues.
  • Computed tomography (CT): An imaging technique involving a series of X-rays that are compiled into "slices" to create a three-dimensional representation of an internal structure of the body.
  • Ultrasonography: A non-invasive technique using sound waves to create images of internal structures without exposure to ionizing radiation.

Based on the findings—including the degree of herniation, the position of the brainstem and occipital lobe, and the presence of syrinxes in not only on the top of the spine but the bottom as well—doctors can classify the Chiari malformation by type and direct the appropriate treatment.

In some cases, intracranial pressure will need to be measured either by inserting a catheter into the fluid-filled space in the brain (called the ventricle) or placing a subdural screw in the skull to measure pressure with electronic sensors. This is especially important in people with a secondary Chiari malformation caused by a head injury or intracranial hypertension.

Treatment

The treatment of a Chiari malformation can vary by the severity of the disease. In there are no symptoms, the doctor may take a "watch-and-wait" approach and simply monitor the condition with a routine MRIs. If symptoms are relatively mild, they may be treated with pain medications or other drugs.

The treatment of a Chiari malformation is directed by multiple factors, including a person's age and general health, the relationship of the malformation to physical symptoms, the likelihood of disease progression, and the impact of surgical interventions on a person's quality of life.

Surgery

Generally speaking, surgery is indicated when a Chiari malformation causes falls, dysphagia with aspiration, sleep apnea, or the formation of syrinxes. The aim of surgery to relieve pressure on the cerebellum, brain stem, and spinal cord.

The surgery commonly used in individuals with a Chiari malformation is called decompressive surgery (also known as a laminectomy). It involves the removal of the lamina—the back portion of a spinal bone that helps separate the bones—from the first (and sometimes second or third) vertebra of the spine.

Doing so allows the bones to slide down over the other, relieving pressure on the brain. A portion of the occipital bone at the base of the skull may also be removed in a procedure known as a craniectomy.

The covering of the brain, called the dura mater, may be opened so that a patch can be inserted to widen the space and reduce pressure. The procedure, known as duraplasty, is effective in relieving symptoms in 76.9% of cases. If crowding still occurs, a portion of the cerebellar tonsils may be removed using electrocautery.

For persons with syrinx or hydrocephalus, a shunt (tube) may be inserted into the subarachnoid space between the skull and brain to drain excess cerebrospinal fluid.

Children with a severe form of spina bifida called myelomeningocele (in which the spinal cord develops outside of the body) may require surgery to reposition the spine and close the opening in the back. This is usually done prenatally while the baby is still in the womb.

Prognosis

The prognosis of a Chiari malformation can vary by the type involved, the general health of the individual, and the type and severity of symptoms.

Children with type I Chiari malformations who undergo decompressive surgery generally have excellent results and may be able to enjoy a high quality of life and a normal lifespan.

According to a 2015 study in the Journal of Neurosurgery: Pediatrics, of 156 children treated with decompressive surgery, over 90% experienced the improvement or resolution of symptoms without the need for duraplasty.

Infants with symptomatic type II Chiari malformations tend to have poorer outcomes, particularly if myelomeningocele is involved. Most studies suggest that these children have a three-year mortality rate of roughly 15% irrespective of the treatment strategy.

Newborns with type III Chiari malformation are at high risk of in-hospital death. Those who survive will have severe neurological problems that may require around-the-clock care for the duration of the child's life (including tube feeding and assisted breathing).

Type IV and V Chiari malformations, both of which are exceptionally rare, are not considered compatible with life, with newborns rarely living more than a few days. Stillborn births are also common.

The prognosis of secondary Chiari malformation can vary significantly based on the underlying cause, with some fully recovering and others left with lasting impairment.

A Word From Verywell

Receiving the news that your child has a Chiari malformation can be devastating. It is important, however, to remember that a Chiari malformation is not one thing. There are many different types, and not all children with the same types have the same symptoms or outcomes.

When faced with the diagnosis, take the time to learn what it means, and ask as many questions as you need to make a fully informed choice. If you cannot get the answers you need, do not hesitate to seek a second opinion, ideally with a pediatric neurosurgeon experience in Chiari malformations.

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