Chiari malformation

Definition

Chiari malformation (Arnold-Chiari anomaly) is a condition in which brain structures in the posterior cranial fossa descend caudally and protrude through the greater occipital foramen. Depending on the type, Chiari anomaly may present with occipital headache, cervical pain, dizziness, nystagmus, fainting spells, dysarthria, cerebellar ataxia, laryngeal paresis, hearing loss and ear noise, visual impairment, dysphagia, respiratory apnea, stridor, sensory disturbances, muscle hypotrophy, and tetraparesis. Chiari anomaly is diagnosed by MRI of the brain, cervical, and thoracic spine. Chiari anomaly accompanied by persistent pain syndrome or neurological deficits should be treated surgically (decompression of the posterior cranial fossa or shunt surgeries).

General information

Chiari anomaly is associated with the protrusion of part of the anatomical structures of the posterior cranial fossa into the lumen of the greater occipital foramen. It compresses the structures of the medulla oblongata and spinal cord located in this area and impairs the outflow of cerebrospinal fluid from the brain, leading to hydrocephalus. 

According to various data, Chiari anomaly occurs in 3-8 people per 100,000 population. Depending on the type, Chiari anomaly may be diagnosed in the first days after birth or may be an unexpected finding in an adult patient. In 80% of cases, Chiari anomaly is combined with syringomyelia.

Causes

There is still no consensus on the etiology of this disease. Some authors believe that Chiari anomaly is associated with a reduced size of the posterior cranial fossa, leading to the fact that as its structures grow, they begin to protrude through the occipital opening. Other researchers suggest that the Chiari anomaly develops due to an enlarged brain that pushes the contents of the posterior fossa through the occipital opening.

Hydrocephalus, in which the brain’s total volume increases due to enlargement of the ventricles, can trigger the transition of a mildly expressed anomaly into a severe clinical form. Since Chiari anomaly, along with dysplasia of the bony structures of the craniovertebral junction, is accompanied by underdevelopment of the ligamentous apparatus of this area, any head trauma can lead to aggravation of the cerebellar tonsils wedging into the occipital foramen with the manifestation of the clinical picture of the disease.

Classification

Chiari anomaly is categorized into four types:

• Chiari I anomaly is characterized by a prolapse of the cerebellar tonsils below the greater occipital foramen. It usually occurs in adolescents or adults and is often accompanied by hydromyelia, an accumulation of cerebrospinal fluid in the central canal of the spinal cord.
• Chiari II anomaly manifests itself in the first days after birth. In addition to the cerebellar tonsils, the cerebellar vermis, medulla oblongata, and IV ventricle also protrude through the foramen magnum. Chiari anomaly type II is much more often combined with hydromyelia than the first type and is, in the vast majority of cases, associated with myelomeningocele—a congenital spinal hernia.
• Chiari III anomaly is characterized by the cerebellum and medulla oblongata descending through the greater occipital foramen, located in the meningocele of the cervical-occipital region.
• Chiari IV anomaly consists of hypoplasia (underdevelopment) of the cerebellum and is not accompanied by caudal displacement. Some authors refer to this anomaly as Dandy-Walker syndrome, in which cerebellar hypoplasia is combined with congenital cysts of the posterior cranial fossa and hydrocephalus.

Chiari II and Chiari III anomalies are often seen in combination with other dysplasias of the nervous system: cortical heterotopia, polymicrogyria, abnormalities of the corpus callosum, cysts of the foramen of Magendie, kinking of theSylvian aqueduct, hypoplasia of subcortical structures, cerebellar nucleus, and cerebellar sulcus.

Symptoms of Chiari anomaly

Chiari anomaly type I is the most common type in clinical practice. It is manifested by liquor-hypertensive, cerebellar-bulbar, and syringomyelic syndromes, as well as lesions of the cranial nerves. Chiari I anomaly usually manifests during puberty or in adulthood.

The liquor-hypertension syndrome associated with Chiari I anomaly is characterized by headache in the back of the head and cervical region, which increases during sneezing, coughing, pushing, or straining the neck muscles. Vomiting may be observed, independent of food intake and its nature. Examination of patients with Chiari anomaly reveals increased neck muscle tone. Among cerebellar disorders, speech disorders (dysarthria), nystagmus, and cerebellar ataxia are observed.

The brainstem, cranial nerve nuclei, and their roots are affected and manifest as decreased visual acuity, diplopia, swallowing disorders, cochlear neuritis-type hearing loss, systemic vertigo with the illusion of rotation of surrounding objects, ear noise, sleep apnea syndrome, recurrent short-term loss of consciousness, orthostatic collapse. When turning their heads, patients with Chiari anomaly noted increased dizziness and ear noise. Turning the head in such patients can provoke fainting. There may be atrophic changes in half of the tongue and laryngeal paresis, accompanied by hoarseness of voice and difficulty breathing. There may be tetraparesis with a more significant decrease in muscle strength in the upper limbs than in the lower limbs.

In cases when Chiari I anomaly is combined with syringomyelia, syringomyelic syndrome is observed: dissociated sensory disorders, numbness, muscle hypotrophy, pelvic disorders, neuroarthropathy, and disappearance of abdominal reflexes. Some authors point out the discrepancy between the size and location of the syringomyelic cyst and the prevalence of sensory disorders, the severity of paresis, and muscle hypotrophy.

Chiari II and Chiari III anomalies have similar clinical manifestations, which become noticeable from the first minutes of the child’s life. Chiari II anomaly is accompanied by noisy breathing (congenital stridor), periods of short-term respiratory arrest, bilateral neuropathic paresis of the larynx, and impaired swallowing by throwing liquid food into the nose. In newborns, Chiari II anomaly is also manifested by nystagmus, increased muscle tone in the upper extremities, and skin cyanosis occurring during feeding. Motor disorders can be expressed in varying degrees and progress up to tetraplegia. Chiari III anomaly has a more severe course and is often a fetal developmental disorder incompatible with life.

Diagnosis

Neurologic examinations and the standard list of neurologic examinations (EEG, echo-EEG) do not provide specific data to diagnose Chiari anomalies. They generally reveal only signs of a significant increase in intracranial pressure, i.e., hydrocephalus. Skull radiography reveals only bony abnormalities that may accompany Chiari anomalies. 

It should be noted that CT scans of the brain with good visualization of the bony structures of the craniovertebral junction do not allow a sufficiently accurate judgment of the soft tissue formations of the posterior cranial fossa. Therefore, magnetic resonance imaging is the only reliable method of diagnosing Chiari anomaly today. In addition to an MRI of the brain to detect meningocele and syringomyelic cysts, an MRI of the spine, especially its cervical and thoracic sections, is also necessary. In this case, MRI studies should be aimed not only at diagnosing Chiari anomaly but also at searching for other anomalies of the nervous system, which are often combined with it.

Treatment of Chiari anomaly

Asymptomatic Chiari anomaly does not require treatment. In cases where Chiari anomaly is manifested only by pain in the neck and occipital region, conservative therapy, including analgesic, anti-inflammatory, and myorelaxant drugs, is performed. If Chiari anomaly is accompanied by neurological disorders (paresis, sensory and muscle tone disorders, cranial nerve disorders, etc.) or pain syndrome that does not respond to conservative therapy, surgical treatment is indicated.

The most common treatment for Chiari anomaly is craniovertebral decompression. The surgery includes widening the occipital foramen by removing part of the occipital bone; elimination of compression of the brain stem and spinal cord by resection of the cerebellar tonsils and the posterior halves of the first two cervical vertebrae; normalization of cerebrospinal fluid circulation by suturing a patch of artificial materials or allograft to the dura mater. In some cases, Chiari anomaly is treated with shunt surgeries aimed at draining cerebrospinal fluid from the dilated central canal of the spinal cord. Cerebrospinal fluid can be drained into the thoracic or abdominal cavity (lumboperitoneal drainage).

All these treatment options are available in 62 hospitals worldwide (https://doctor.global/results/diseases/chiari-malformation). For example, Chiari decompression surgery can be done in 9 clinics across Turkey for an approximate price of $6.0 K (https://doctor.global/results/asia/turkey/all-cities/all-specializations/procedures/chiari-decompression-surgery). 

Prognosis

The type to which the Chiari anomaly belongs has a significant prognostic value. In some cases, Chiari I anomaly may remain asymptomatic throughout the patient’s life. Chiari III anomaly is fatal in most cases. When neurological symptoms of Chiari I anomaly and Chiari II anomaly appear, timely surgical treatment is of great importance since the resulting neurological deficit is poorly restored even after successful surgery. According to various data, the effectiveness of surgical craniovertebral decompression is 50-85%.