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Chiari Malformation


Chiari malformation is estimated to affect less than one in 1000 in the general population.  This congenital condition is described as abnormalities of the brain at the juncture of the skull with the spine. In some cases, condition can be acquired and may be associated with syringomyelia and hydromyelia, two closely related conditions. Chiari malformations are often asymptomatic and do not require medical treatment.


Types of Malformations

Chiari malformations are categorized as Type I, Type II, Type III or Type VI.  The malformation can affect the cerebellum which controls the coordination of motion and is normally located inside the base of the skull. The cerebellum is composed of two lateral halves, or hemispheres, and a narrow central portion between these hemispheres, known as the vermis. Along the under surface of the hemispheres, there are two small protrusions called the tonsils. The fourth ventricle is a space filled with cerebrospinal fluid (CSF) located in front of the cerebellum (and behind the brainstem). All of these structures are located just above the foramen magnum, the largest opening at the base of the skull through which the spinal cord enters and connects to the brainstem.


Type I Chiari malformation occurs during fetal development.  Type I is characterized by the downward displacement of the cerebellar tonsils by more than four millimeters beneath the foramen magnum into the cervical spinal canal.  This abnormality may block the normal pulsations of cerebrospinal fluid between the spinal canal and the intracranial space.  Type I diagnosis is more common in adolescents or adults and may be associated with syringomyelia or hydromyelia.  30 to 50% of patients with Type 1 Chiari malformation may have the following anomalies:


  • Compression of the brainstem due to compression of the upper part of the spine into the base of the skull
  • Bony union of the first level of the spine to the base of the skull
  • Partial bony union of the first and second levels of the spine
  • A congenital union or fusion of levels of the spine within the neck with possible associated mal developments of the cervical spine known as Klippel-Feil deformity
  • Defect in the posterior part of the spine known as Cervical Spina Bifida
  • Scoliosis, especially in children with immature spines, may be present in 16 to 80% of hydromyelia patients.


Symptoms of Type I Chiari malformation may include:

  • Severe head and neck pain
  • An occipital headache felt at the base of the skull that is made worse by coughing, sneezing or straining
  • Loss of pain and temperature sensation of the upper torso and arms
  • Loss of muscle strength in the hands and arms
  • Collapsing to the ground due to muscle weakness
  • Spasticity, dizziness, balance problems
  • Double or blurred vision
  • Hypersentivity to bright lights
  • Sleep apnea
  • Children and infants may display non-specific symptoms such as hypotonia, gross motor delay, swallowing/choking difficulties and failure to thrive and opisthotonus


Type II Chiari malformation is usually found in patients with myelomeningocele or an open spinal cord defect at birth.  This type of malformation is characterized by the downward displacement of the medulla, fourth ventricle and cerebellum into the cervical spinal cancel, as well as elongation of the pons and fourth ventricle.  Myelomeningocele is often accompanied by other abnormalities such as hydrocephalus, cardiovascular, imperforate anus, gastrointestinal and genitourinary. 


Symptoms of Type II Chiari malformation may also be associated with abnormalities related to myelomeningocele and hydrocephalus, including:

  • Breathing apnea or the alteration in the pattern of breathing
  • Depressed gas reflex
  • Involuntary, rapid, downward eye movements
  • Loss of arm strength


Type III Chiari malformation is rare and associated with a high mortality rate or severe neurological deficits in patients that survive. The malformation is characterized by a form of dysraphism with a portion of the cerebellum and/or brainstem pushing out through a defect in the back of the head or neck.  Early operative closure of the defect is necessary as well as shunting when hydrocephalus is present. Life-threatening complications often exist in infants with Chiari III Malformation.


Type IV Chiari malformation is rare and the most severe. The cerebellum failed to develop normally and there may be other malformations of the brain and brainstem. Survival of infancy for most babies with this type of malformation is not expected.  



Chiari malformation and associated abnormalities such as syringomyelia may be diagnosed through:

  • Magnetic resonance imaging (MRI) to provide three-dimensional images
  • Computed tomography scan (CT or CAT Scan)to assist at defining the size of the cerebral ventricles and show any blockages
  • Sleep study to monitor breathing, snoring, oxygenation and, if any, seizure activity to identify sleep apnea
  • Swallowing study to determine if there is an abnormality that may suggest a lower brainstem dysfunction
  • Brainstem auditory evoked potential (BAER) to determine if brainstem is working properly
  • Myelogram to determine pressure on spinal cord or nerves due to malformations
  • Somatosensory evoked potentials (SSEP) to assess sensation related to peripheral nerve, spinal cord and brain function.



Treatment is dependent on the type of Chiari malformation and may not require medical intervention. Chiari I malformation often does not display symptoms nor require treatment.  Chiari II malformations are only treated if symptoms are present, and there are no complications present from hydrocephalus. Symptomatic Chiari II infants may require urgent or emergency treatment. 



The objective of surgery for Chiari malformation is to relieve symptoms and is dependent on the type of malformations or other abnormalities. Outcomes sought through Chiari surgery are optimal decompression of nerve tissue and reconstruction of normal cerebrospinal fluid flow around and behind the cerebellum. Surgery for Chiari I malformations normally occurs to either decompress overlying bones; the membrane covering the brain and spinal cord; or, the bone and dura and some degree of cerebellar tissue resection. Some patients also may require cervical spinal fusion. Decompression procedures are generally performed under general anesthesia. 


Chiari II malformations may have a similar surgical treatment plan as Type I. Chiari II surgery procedures are usually focused on decompressing the tissues in the spinal canal. 



The benefits of surgery of Chiari malformations may not outweigh the associated risk and should be carefully considered.  While some patients experience relief of symptoms, it is not guaranteed for every individual.  Further, Chiari malformations are often accompanied by other types of nerve damage which cannot be reversed.


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