Chiari I Malformation

Chiari I malformation is a condition in which there is crowding of the cerebellar at the base of the skull and the upper spinal area are not formed properly. For most patients, treatment for Chiari I consists of surgery to repair the abnormality and relieve symptoms. However, some patients who do not have symptoms may not need surgery. Instead they are seen routinely by their doctors for follow-up examinations.

Chiari malformation is spectrum of abnormalities in which the brainstem, cerebellum, upper cervical cord, and skull base are affected. Chiari I, the most common and least severe subtype of Chiari malformation, often goes unnoticed until symptoms arise in the adolescent or adult years of life. Most instances of Chiari I are congenital, though some can be acquired after birth.

Chiari I occurs when the improper formation of the skull base and upper spine results in the cerebellar tonsils being pushed down through the foramen magnum and into the spinal canal. Cerebrospinal fluid normally passes through the foramen magnum, so when this opening becomes partially obstructed, cerebrospinal fluid flows with more force through the narrowed opening, in turn further pushing down the cerebellar tonsils and blocking the foramen magnum.

The result is that the herniated cerebellar tonsils compress the brainstem, leading to several symptoms reflecting brainstem and cerebellum dysfunction. Also, syringomyelia may develop in the spinal cord, a condition characterized by the development of a cyst in the spinal cord. Rarely hydrocephalus arises.


The following are the most common symptoms of a Chiari I malformation; however, each person may experience symptoms differently, and some never have symptoms. Symptoms tend to show during adolescence or adulthood.

  • Headache
  • Neck pain
  • Decreased sensation in the arms and legs
  • Ataxia
  • Dysequilibrium
  • Vertigo
  • Dysphagia
  • Feeling of hoarseness when talking
  • Vision problems, such as diplopia or blurred vision
  • Nystagmus
  • Scoliosis


Diagnosis is usually made after the onset of specific signs and symptoms and after diagnostic testing; however, sometimes, Chiari I is diagnosed incidentally during an imaging scan for a different condition.

Most often, diagnosis starts with the physician obtaining a complete medical history and conducting a neurological examination to identify any problems. A neurological examination consists of evaluating motor function, swallowing, eye movements, sensation, balance, and coordination.

The key component to diagnosis of Chiari I malformation is imaging. One or a combination of the following imaging tests may be performed to confirm a diagnosis:

  • Magnetic resonance imaging (MRI) – A diagnostic procedure that uses a combination of large magnets, radiofrequencies and a computer to produce detailed images of organs and structures within the body. MRI is used to image the head and spine.
  • Computed tomography (CT) scan – An imaging study used to view skull bones and soft tissue in the head. This study can be performed with or without contrast enhancement.

Risk Factors

Chiari I malformation occurs when the section of skull surrounding the cerebellum is too small, resulting in overcrowding of the brain and herniation of the cerebellar tonsils. Although the exact cause of congenital Chiari I is unknown, it is thought that a problem during fetal development may cause the abnormal skull formation.

Although a genetic component is unclear, Chiari I is more common among individuals diagnosed with with the hereditary disorder Ehlers-Danlos syndrome and has been seen to cluster in some families. Chiari I also occurs more frequently among those with scoliosis, a condition in which the spine is curved from side to side and syringomyelia. Researchers continue to study the genetic component of this condition.

Chiari I is more common among females than males. Most instances of Chiari I are congenital and symptoms develop later on during adolescence or adulthood, though most people with Chiari I live asymptomatically. Occasionally, symptoms can be seen in infants and toddlers.


At Columbia, our neurosurgeons use the most sophisticated surgical approaches to treat Chiari I malformation, providing the best possible outcomes for patients.

If a person is diagnosed with Chiari I but has mild or no symptoms, surgery may not be the first line of treatment. It may be reserved for later on if symptoms begin to show.

Surgical treatment for Chiari I is usually an open procedure called posterior fossa decompression surgery.

Posterior fossa decompression surgery aims to uncrowd the area at the base of the cerebellum where the cerebellar tonsils are pushing against the brainstem and spinal cord. This is done by performing a suboccipital craniectomy, a procedure in which the neurosurgeon removes a small portion of bone at the base of the skull, and often also a cervical laminectomy, a procedure in which the neurosurgeon removes a part of the bony arch of the first vertebra in the neck. Occasionally, the bony arches of additional vertebrae are removed too.

The operation may be modified if a syrinx is in the spinal cord or if the patient has hydrocephalus. Most patients who have the surgery do quite well and experience improvement of their symptoms.

One key aspect of surgery is the decision whether to open or leave intact the section of dura mater that envelops the cerebellar tonsils. This decision can greatly affect length of recovery and complication rate.

At the Pediatric Neurosurgery Center, we have been collecting information for decades to help determine whether opening of the dura mater is a necessary component of surgery for individuals with Chiari I malformation.

Using intraoperative electrophysiological monitoring, our neurosurgeons discovered that most of the improvement in nerve impulses through the brain and spinal cord occurs after removal of the bone. They did not see any further improvement after opening the dura mater. This suggests that individuals may not require this additional step of surgery.

Accordingly, for several years, we have been performing a less invasive operation in which the dura mater is not opened during surgery. At this time, we have seen excellent clinical and radiographic results without any significant operative complications after bony decompression without dural opening. The complication rate is very low–2.6 percent. This is important because the complication rate after opening the dura mater during surgery has been reported to be 15-25 percent.

A small percentage of patients treated with this less invasive option will require a second surgery to open the dura mater for further relief of symptoms. However, for most, the less invasive option is highly successful and patients recover faster with fewer complications.