In neurology, the Chiari malformation (/kiˈɑːri/ kee-AR-ee; CM) is a structural defect in the cerebellum, characterized by a downward displacement of one or both cerebellar tonsils through the foramen magnum (the opening at the base of the skull).

[7] Less often, people may experience ringing or buzzing in the ears, weakness, slow heart rhythm, or fast heart rhythm, curvature of the spine (scoliosis) related to spinal cord impairment, abnormal breathing, such as central sleep apnea, characterized by periods of breathing cessation during sleep, and, in severe cases, paralysis.

[citation needed] Headache is the most common symptom in those with Chiari malformation type 1 (in which only the cerebellar tonsils descend below the foramen magnum).

This headache is usually occipital or sub-occipital in location (but may also present in other cranial areas), is usually dull or throbbing in character and is characteristically associated with Valsalva maneuvers (such as bearing down, coughing, sneezing, bending over or forcefully exhaling against a closed airway).

[10] Symptoms that may be due to Chiari malformations include: The blockage of cerebrospinal fluid (CSF) flow may also cause a syrinx to form, eventually leading to syringomyelia.

Central cord symptoms such as hand weakness, dissociated sensory loss, and, in severe cases, paralysis may occur.

Syringomyelia symptoms include pain, weakness, numbness, and stiffness in the back, shoulders, arms or legs.

Other symptoms include headaches, the inability to feel changes in the temperature, sweating, sexual dysfunction, and loss of bowel and bladder control.

The exact development of syringomyelia is unknown but many theories suggest that the herniated tonsils in type I Chiari malformations cause a "plug" to form, which does not allow an outlet of CSF from the brain to the spinal canal.

[18] The most common pathophysiological mechanism by which Chiari type I malformations occurs is due to a congenitally small posterior fossa.

Such causes include hydrocephalus (an accumulation of cerebrospinal fluid [CSF] around the brain), space occupying lesions in the brain such as tumors, subdural hematomas or other subdural fluid collections, arachnoid cysts, craniosynostosis (early closure of the cranial sutures)(especially of the lambdoid suture), hyperostosis (an excessive growth of bone) (such as craniometaphyseal dysplasia, osteopetrosis).

[27] The selected cutoff distance for abnormal tonsil position is somewhat arbitrary, as not every person will be symptomatic at a certain amount of tonsil displacement, and the probability of symptoms and syrinx increases with greater displacement; however, greater than 5 mm is the most frequently cited cutoff number, though some consider 3–5 mm to be "borderline"; pathological signs and syrinx may occur beyond that distance.

Neuroradiological diagnostics evaluate the severity of crowding of the neural structures within the posterior cranial fossa and their pressure against the foramen magnum.

[4][38][39] Tonsillar ectopia below the foramen magnum, with greater than 5 mm below as the most commonly cited cutoff value for abnormal position (although this is considered somewhat controversial).

The presence of a syrinx is known to give specific signs and symptoms that vary from dysesthetic sensations to algothermal dissociation to spasticity and paresis.

[55][56] Decompressive surgery[5] involves removing the lamina of the first and sometimes the second or third cervical vertebrae and part of the occipital bone of the skull to relieve pressure.

[10] Other complications that are possible in surgical repair of type I Chiari malformations include an aseptic meningitis due to irritation from the dural grafts which is seen in 32% of cases.

believe that detethering the spinal cord as an alternate approach relieves the compression of the brain against the skull opening (foramen magnum), obviating the need for decompression surgery and associated trauma.

They include bleeding, damage to structures in the brain and spinal canal, meningitis, CSF fistulas, occipito-cervical instability, and pseudomeningocele.

[55] In certain cases, irreducible compression of the brainstem occurs from in front (anteriorly or ventral) resulting in a smaller posterior fossa and associated Chiari malformation.

[58] A study using upright MRI found cerebellar tonsillar ectopia in 23% of adults with headache from motor-vehicle-accident head trauma.

The evolutionary changes included increased size and shape of the skull, decreased basal angle and basicranial length.

[62] The history of Chiari malformation: The condition was brought to the mainstream on the series CSI: Crime Scene Investigation in the tenth-season episode "Internal Combustion" on February 4, 2010.