Chiari Malformation

Chiari malformations are typically caused by a structural defect in the brain and spinal cord that occurs during fetal development (referred to as primary or congenital CM). Often the opening at the base of the skull is narrow or small in relation to the size of the cerebellum, which may cause the developing brain to push downward through the foramen magnum.

CMs can also develop later in life due to traumatic brain injury, infection, or disease (called acquired or secondary CM). Primary CM is much more common than secondary CM.

Some studies suggest CMs affect women more often than men. CMs sometimes appear to run in families, so researchers are looking into possible hereditary factors.

CMs are classified based on their severity and the part of the brain that extends into the spinal canal:

• Type I – This is the most common type of CM, which occurs when the lower part of the cerebellum—called the cerebellar tonsils—extends through the foramen magnum.
• Type II – This type of CM happens when the cerebellum and brainstem protrude through the foramen magnum. Type II CM is usually accompanied by a myelomeningocele — a form of spina bifida that happens when the spinal cord and backbone do not close before birth. In these cases a portion of the spinal cord is typically exposed in a sac through an opening in the spine.
• Type III – This is the most serious form of CM. It occurs when a portion of the cerebellum and brainstem stick out through an abnormal opening in the skull.
• Type IV – In this rare form of CM, the cerebellum is incomplete or underdeveloped, which may cause parts of the skull and spinal cord to be exposed.

Many people with a Type I CM do not show symptoms. However, the severity of one’s symptoms do not necessarily correspond to how far the cerebellar tonsils extend below the opening in the skull. Symptoms of a CM vary from person to person and may change depending on factors like compression and CSF pressure.

Chiari malformation symptoms include headache (most commonly headaches that occur after coughing, sneezing or straining), neck pain, muscle weakness or numbness, dizziness, and problems with fine motor skills, hand coordination, hearing, and balance. People with a CM may also experience difficulty swallowing, breathing or speaking, sensitivity to light, blurred vision, ringing or buzzing in the ears, sleep issues, and vomiting.

Individuals with a myelomeningocele—as in the case of a Type II CM—usually experience partial or complete paralysis of the area below the spinal opening. CMs are often associated with other conditions, such as hydrocephalus, spina bifida, syringomyelia (a fluid-filled cyst in the spinal cord), tethered cord syndrome, and spinal curvature.

There is currently no definitive test to determine if a baby will be born with a CM. Some CMs are visible on an ultrasound before birth. Certain birth defects like spina bifida are often associated with CMs, so babies born with these defects are typically checked for malformations. Some people do not discover they have a CM until adolescence or adulthood, usually by accident while being examined for a different condition. 

To make a diagnosis of CM, a doctor may test memory, cognition, and balance (functions of the cerebellum) as well as evaluate reflexes, sensation, and motor skills (functions controlled by the spinal cord). Imaging tests can also help diagnose a CM or confirm abnormalities that are associated with CMs.

CMs that do not cause symptoms or interfere with daily activity may only need regular monitoring by a physician. Medication can help relieve symptoms like headache and pain. In many cases, surgery is the only treatment option that will improve or stabilize symptoms and stop additional damage to the central nervous system—though any paralysis one experiences prior to treatment is typically permanent.

The most common surgery to treat a CM is posterior fossa decompression. This procedure involves removing a portion of bone at the bottom of the skull (craniectomy) or at the top of the spinal canal (spinal laminectomy). This creates more space for the cerebellum, relieves pressure on the spinal cord, and allows for the normal flow of CSF. 

In some cases, a surgeon will perform a procedure to remove the portion of the cerebellar tonsils that extends through the foramen magnum (electrocautery). The cerebellar tonsils do not have a recognized function and can be removed without creating any known neurological issues.

Infants with a myelomeningocele often require surgery to close the opening in the spine. This type of surgery is most effective when performed prenatally, when the baby is still in the womb.

Current research efforts are investigating specific aspects of brain development that may lead to CM, including genetic mutations and mechanisms of the midbrain-hindbrain (MHB) boundary. Other studies are focusing on improved brain imaging for diagnosis and the effects of surgery for CM treatment. By learning more about CMs and human brain development, researchers will be able to develop new diagnostic tests and better treatment options for related brain and spinal conditions.