Chiari Type I Malformation

June, 2001


This information is published by the Pediatric Section of the American Association of Neurological Surgeons and Congress of Neurological Surgeons. It is one in a series covering pediatric neurosurgical topics. It has been written and edited by members of the society. This article is sponsored by the Section as a service to parents. Its purpose is to help you understand the complexities of the condition described. It is hoped that with a better knowledge of the condition, you will have a better understanding of what your child”s physician has explained to you. In doing so, you will be better able to ask any questions you may have. At the end of this article is a glossary of medical terms to refer to if a word, test, or procedure in the text is not understood. Understanding the terms will help you to understand the condition. In addition a list of support groups follows the glossary for further information and assistance.Chiari Diagram

This drawing to the right is of the Chiari Type I Malformation. This shows the cerebellar tonsils are abnormally positioned below the base of the skull. They have been pushed down into the spinal canal. This has caused spinal fluid to be pushed down into the middle of the spinal canal to form a syrinx.


In order to understand this problem we must start by defining its anatomy. The posterior fossa is the portion of the skull which houses the brain stem, cerebellum, and fourth ventricle. The upper spine is that portion of the spinal column that is formed by the bones of the neck. It is referred to as the cervical spine. Each bone is referred to as a vertebra (plural=vertebrae). The cervical vertebrae surround the spinal cord enclosing it in a bony tube called the spinal canal. It protects the spinal cord just as the skull protects the brain.

The cerebellum is the back part of the brain located in the posterior fossa. It is composed of the vermis located in the center and two hemispheres located on each side. The structures which hang from the lower end of the hemispheres are called the cerebellar tonsils. They are an important part of the Chiari Type I malformation.

The cerebellum functions as a coordination and balance center. It fine tunes movements of the entire body and helps the muscles to work in a smooth and coordinated manner. For example, it helps you to stand upright without falling and to walk in a straight line.

The fourth ventricle also lies in the posterior fossa and is a fluid filled cavity located between the brain stem and cerebellum. It is one of four fluid filled cavities within the brain, the two lateral and the third ventricles being the other three. Within these cavities spinal fluid is produced. Normally, all of these cavities communicate with each other. The spinal fluid passes from one to the other and ultimately exits the brain through the fourth ventricle.

Chiari Type I Malformation

This malformation is the extension of the cerebellar tonsils into the upper spinal canal. The brain stem and fourth ventricle remain within the skull in their normal position as does the remainder of the cerebellum. Because of the downward displacement of the tonsils there is too much tissue in the upper cervical spinal canal.


The cause of the Chiari Type I malformation is unknown. There are several theories or unproven ideas. It is not known for certain if this malformation develops before or after birth. If it is congenital, it probably forms between the third and sixth month of pregnancy. There are no specific factors during pregnancy that can be linked to the Type 1 malformation. It does not appear to be a genetic or inherited problem when it is found alone. When it is found in combination with certain other types of abnormalities, there can be a familial tendency for the condition.

Table 1 Signs and Symptoms Chiari I

  • Headache
  • Neck pain
  • Swallowing difficulties
  • Scoliosis
  • Weakness in hands and arms
  • Spasticity in legs
  • Tingling in arms, hands, legs
  • Walking difficulty (ataxia)
  • Eye movement abnormalities
  • Hoarse voice
  • Hiccups
  • Snoring
  • Speech difficulties
  • Drop attacks
  • Breathing abnormalities


The symptoms develop because of pressure in the upper cervical spinal canal by the herniation of the cerebellar tonsils down into the cervical canal. This displacement of the cerebellar tonsils results in an obstruction to the normal circulation of spinal fluid through the fourth ventricle. When spinal fluid encounters difficulty exiting the fourth ventricle two problems can occur. The spinal fluid flow may be blocked, causing the ventricles to become enlarged. This causes a condition called hydrocephalus. More commonly however the flow of spinal fluid within the fourth ventricle is directed into the central canal of the spinal cord. The central canal is a normal structure within the center of the spinal cord. If spinal fluid is forced into the central canal it dilates and causes a fluid cavity within the spinal cord. This “hydrocephalus” of the spinal cord is called syringomyelia, hydromyelia, or a syrinx. While these terms have slightly different meanings they are frequently used interchangeably. The syrinx may compress the surrounding spinal cord and prevent the spinal cord from working normally. This is frequently associated with scoliosis or curvature of the spine. In fact, the most common problem that leads to a test that shows a Chiari Type I malformation is scoliosis. Syringomyelia may also cause pain in the shoulder or weakness in the arms or trunk. There may be a change in the ability to “feel” in the upper extremities or trunk as well as tightness or spasticity of the lower extremities causing difficulty walking.

Chiari Type I malformations can also be found on MRI scans done for other reasons such as a head injury or as part of the evaluation of headaches. This malformation is sometimes seen with less common bony anomalies at the base of the skull and upper spine such as basilar impression. When this happens spasticity and balance problems can occur.

Problems most commonly start in childhood or adolescence, although they can occur at any age. Headaches and neck pain are common symptoms.


When a patient comes to the doctor with symptoms suggesting a Chiari I malformation (see table 1 above) tests are performed to confirm the diagnosis.Chiari MRI

The test most frequently used is an MRI scan (refer to picture at right). An MRI scan does not involve radiation. It has no known adverse effects. It will give a clear picture of the anatomy of the brain and spinal cord and any other associated anomalies that may be present. The CAT scan and myelogram which were commonly used in the past, are not usually needed now because of the availability of the MRI scan. The CAT scan may still be indicated under specific circumstances. Plain x-rays may also be helpful. Bony changes in the skull and upper spine seen on x-ray are often diagnostic of the malformation. In Type I malformations a small posterior fossa, basilar impression, abnormalities of the bones of the upper spine, and scoliosis may be identified. Electrical studies such as somatosensory evoked response or potential (SER or SSEP), brain stem auditory evoked response (BAER), and electromyography (EMG) are other tests that may be performed. These tests are used to determine the degree of damage to the nervous system, and follow the progression of damage. The SER or SSEP is used to determine if the spinal cord is working normally. The BAER is used to determine if the brain stem is working properly. The EMG tells the doctor about spinal cord function as well as the function of sensory and motor nerves. There may be other tests that you have heard about, but they are used less commonly and are beyond the scope of this booklet. You are encouraged to discuss these with your doctor.


The majority of physicians would agree that an asymptomatic malformation should be left alone. It is usually when the malformation becomes symptomatic or progresses that treatment is required. A variety of approaches are used. The object of treatment is to stop the progression of symptoms, and hopefully to relieve symptoms. This may be accomplished by decompression of the the brain stem, cerebellum and spinal cord, as well as re-establishment of spinal fluid circulation. Decompression is achieved by removing the bony structures compressing the brain stem and cerebellum. Several different surgical procedures can accomplish this. These should be discussed with your doctor. A posterior fossa decompression is the removal of the bone from the back of the posterior fossa, called the sub-occipital bone. The outer covering of the brain, called the dura, may be opened and a graft inserted. This helps make more room for the brain and the spinal cord. A cervical laminectomy involves the removal of the bone forming the back of the vertebra or lamina. Both procedures are done at the same time, through the same surgical incision in the back of the head and upper neck. If the bony anomaly includes basilar impression, surgery may be necessary transorally (through the mouth). This route is used to gain access to and remove the bone which is pushed upward into the skull base. It cannot be removed by operating from the back of the neck.

Re-establishment of spinal fluid circulation may be accomplished in a variety of ways depending on the problem created by the abnormal spinal fluid circulation. Primarily two different problems-hydrocephalus and syringomyelia-can occur. Treatment of these disorders generally involves a posterior decompression, a shunt, or a combination of both. Shunts are surgically placed tubes which allow spinal fluid to by-pass a block. There are many types of shunts.

The type of surgery your surgeon chooses to use depends on the individual patient and the surgeon’s preferences. There are many correct ways to achieve the same result and we encourage you to discuss these details with your physician.


If the symptoms have become worse with time, the response to treatment is usually good if treatment is not delayed. Symptoms due to syringomyelia are likely to improve. Rarely do all of the problems resolve if they are longstanding. For this reason early treatment is important. Of course, there is always a risk that symptoms may come back after surgery. This results from a failure of the treatment to permanently redirect spinal fluid flow causing recurrence of the syringomyelia. Shunt malfunction and scar tissue formation are likely reasons for syringomyelia to return after surgery. Another operation may be required in these cases.


The Chiari Type I malformation is a complicated problem with many parts. If you are confused, don’t feel alone. Read this information over several times. Make sure you understand the words used throughout the article. Refer to the glossary below for problem words. If you need to, print out this page and underline the parts of the article that are confusing and bring it to your child’s physician. Ask her or him to review the parts you have underlined. With help you should begin to understand your child’s illness better. Also, take advantage of the resources listed after the glossar

Read More about the Chiari Team Center at the Pediatric Neuroscience Center of the Rocky Mountain Hospital for Children

What To Expect if Your Child Needs Surgery for a Chiari Malformation