The prevalence is estimated to be approximately 1 in 1000. CM affects individuals of every race and ethnicity. Some studies show that females are affected more than males. However, the exact prevalence is unknown because many affected individuals are asymptomatic, undiagnosed, or misdiagnosed. It is difficult to determine the true frequency in the general population.

CM is primarily caused by structural abnormalities of the brain and spinal cord during fetal development, also known as congenital or primary CM. It can be a result of a genetic mutation or a lack of adequate vitamins or nutrients. Acquired or secondary CM occurs later in life if the spinal fluid is excessively lost due to a traumatic injury or disease. Affected individuals can also be asymptomatic. CM can be detected accidently from a diagnostic imaging test for another unrelated reason.

The cerebellum in the brain is responsible for motor movements and controls balance. In CM, the cerebellum and the base of the skull is directly above an opening in the skull that allows the spinal cord through; the hole is called the foramen magnum.  CM is when the lower part of the cerebellum, the tonsils, dips down into the foramen magnum, and extends into the upper spinal canal. When the cerebellum pushes down into the spinal canal, it causes pressure on the brain and blocks the flow of cerebrospinal fluid (CSF) leading to an accumulation of CSF; causing an array of symptoms. The CSF is a clear liquid that surrounds and cushions the brain and spinal cord.

The severity of the disorder and the manifestation of symptoms depends on the type of CM. The most common symptom of CM is headache, especially after coughing, sneezing, or other straining actions.

Other symptoms may include, but varies between affected individuals, are:

• Neck pain/ stiffness

• Choking/ swallowing difficulties

• Hearing problems/ ringing in the ears

• Dizziness

• Issues with balance and coordination

• Vision problems

• Weakness/ numbness in the limbs

• Scoliosis

• Loss of bowel and bladder control

• Insomnia

• Depression

Some affected individuals may be asymptomatic, and others can have very serious neurological issues. Symptoms can vary depending on the compression of the cerebellar tonsils on the nerves and the buildup for CSF pressure.  

There are several conditions that is associated with CM.

Syringomyelia or hydromyelia is common, where a cavity or cyst forms in the spinal cord filled with CSF. Syringomyelia is the leading cause of CM; it is related to the abnormal flow of CSF and increased pressure. Hydromyelia is related to an abnormal widening of the spinal canal filled with CSF.

Hydrocephalus is an excessive buildup of CSF, commonly associated with type II.

Spinal bifida is the incomplete development of the spinal cord, commonly associated with type II.

Tethered cord syndrome is when the spinal cord abnormally attaches to the tissues at the bottom of the spine. The spinal cord cannot move freely, causing damage to the nerves and muscles of the lower body and legs. This commonly occurs in type II.

Spinal curvature occurs when the spine bends to the left or right, known as scoliosis, or bend forward, known as kyphosis. It is commonly associated with type I.

There are five types of CM, type 0 to type IV. Type III and type IV are extremely rare.

Type 0 CM is minimum or no descent of the cerebellar tonsils. These individuals have slightly abnormal to near-normal flow of CSF. They often have syringomyelia. Headaches occur as well.

Type I CM is the most common form, the tonsils extend into the foramen magnum. Many do not manifest symptoms, so it is not typically diagnosed until later in life, in adolescence or adulthood. It is often by accident for another condition.  It does not usually cause neurological issues, but neurological symptoms can occur. Some symptoms that can occur are headache, neck pain, weakness/ numbness, dizziness, balance problems, and vision programs.

Type II CM is more severe than type II, the cerebellum and the brain stem tissue extend into the foramen magnum.  It appears during infancy or early childhood and can cause life-threatening issues.

They often have myelomeningocele, the most severe form of spinal bifida. Spinal bifida is a birth defect where the spinal cord does not develop completely and properly. In myelomeningocele, the spine, spinal cord, and spinal canal do not form or close normally. Myelomeningocele causes an open spinal cord at birth leading to partial or complete paralysis of the area at the spinal opening.  A sac filled with CSF can be formed. Lack of bladder and bowel control can occur as well.

Affected individuals can also have hydrocephalus, the accumulation of excessive CSF in the brain ventricles which cause an increase pressure on the brain tissues. This leads to macrocephaly, an abnormally enlarged head. Symptoms can include nausea/ vomiting, irritability, seizures, and developmental delays.

Additional symptoms include, breathing issues or apnea, gag reflex, involuntary rapid eye movements, and weakness in muscles. Other neurological issues can occur as well.

Type III CM appears at birth and is very rare and serious; a part of the cerebellum and the brain stem sticks out in the back of the skull or neck. This type causes life-threatening complications and infants have a high early mortality rate. Affected individuals have symptoms similar to type II CM but can be more severe and debilitating, such as neurological defects, mental and physical developmental delays, and seizures. Hydrocephalus is commonly present.

Type IV CM is the rarest and severest form; the cerebellum does not develop fully or normally. The cerebellum is located in the normal position, but is missing parts, exposing parts of the skull and spinal cord. It can also include malformations of the brain and brainstem. Most infants with severe malformations do not survive infancy.  

CM is diagnosed through patient history, symptom severity, clinical assessment, and diagnostic tests. If an infant is born with spina bifida, additional tests are performed to detect malformations.

An examination of the individual’s memory, cognitive function, balance, touch sensitivity, reflexes, and motor skills can help assess one’s symptom severity.

Many affected individuals do not have symptoms. Their malformations are often accidently discovered during a diagnostic test for another condition.

There is no test available to determine if an infant will be born with CM. However, some malformations can be seen with ultrasound images before birth.

MRI of the brain and spine is the most common diagnostic test for CM, and also shows the presence of syringomyelia. The MRI gives an accurate view of the brain, the cerebellum, and the spinal cord, defining the extent of malformations and CM progression. This test can confirm type I CM because it shows the tonsils extending into the spinal canal. A special MRI test called cine MRI CSF flow study evaluates the flow of CSF and determines the impact of CM.

X-rays identify bone abnormalities associated with CM, such as spine curvature and skull malformations. X-rays cannot confirm CM.

CT scans produce images of the bone and blood vessels. It can identify the size of ventricles in hydrocephalus and bone abnormalities.

Other tests include, sleep study, swallowing study, brainstem auditory study to detect hearing and brain connections, somatosensory study to detect sensations, and myelogram X-ray of the CSF to the determine the pressure on the spinal cord.

Most effective treatment for CM is surgery. Surgery is intended to return the CSF flow or circulation to as close to normal as possible and adjust or correct the impact of the tonsils on the spinal canal. Correction or adjustment of CM can help relieve and diminish symptoms.

Mild symptoms can be treated with pain medication, physical therapy, or lifestyle changes.

Type 0 CM individuals have improvement after decompression surgery.

Type I CM individuals may be treated with decompression surgery and/or cervical spinal fusion.

Type II requires surgical intervention during infancy or childhood.

Type III often necessitates an operative closure of the defect. Hydrocephalus must be treated through shunting.

The benefits of surgery always outweigh the risks. Many patients experience decrease in symptoms. Surgery is successful for many affected individuals. Some individuals need several surgeries. Some individuals do not experience symptom relief with surgery. Surgery cannot reverse nerve and muscle damage.

Some forms of CM are asymptomatic and do not interfere with an individual’s daily activities. Healthcare providers recommend routine MRI monitoring. Individuals should report signs and symptoms of pain or headaches.

Type III infants have high early mortality rate due to life-threatening complications.

Type IV infants with severe malformations rarely survive infancy.

Arnold Chiari Malformation. UCLA Health

http://neurosurgery.ucla.edu/arnold-chiari-malformation

About Chiari Malformation, Quick Facts. Chiari & Syringomyelia Foundation (CSF), April 2016.

https://csfinfo.org/files/8314/6170/0906/About_CM-web.pdf

Chiari Malformation: Signs, Diagnosis & Treatment Options. Chiari & Syringomyelia Foundation (CSF), August 2017.

https://csfinfo.org/education/patient-information/chiari-malformation/

Chiari Malformations. National Organization for Rare Disorders (NORD). https://rarediseases.org/rare-diseases/chiari-malformations/

Chiari Malformations. American Association of Neurological Surgeons.

https://www.aans.org/Patients/Neurosurgical-Conditions-and-Treatments/Chiari-Malformation

Chiari Malformation Fact Sheet.  National Institute of Neurological Disorders and Stroke.

https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Fact-Sheets/Chiari-Malformation-Fact-Sheet