Chiari Malformation

Patologie Malformative o della Dinamica Liquorale

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Definition and Etiology

Chiari Malformation (CM) is a pathology of the posterior cranial fossa, in which the cerebellum and the brainstem are normally contained. CM is characterised by an “encumbrance” of the neural structures in the area of the passage between cranium and spine and by a reduced size of the posterior cranial fossa. The cerebellar tonsils and the brainstem are therefore displaced through a small opening, called the foramen magnum, in the spinal cavity. Chiari malformation may also be associated with many other diseases, including myelomeningocele, syringomyelia, spina bifida and hydrocephalus.

Two main types of CM have been identified, with different etiology, age of onset, and severity: Type I and Type II.

Type of Malformation	Chiari Type I	Chiari Type II
displaced neural structures	cerebellar tonsils	Cerebellar vermis, lower portion of the cerebellar hemispheres, medulla oblungata and forth ventricle
typical age of onset	young adults	childhood
associated diseases	Hydrocephalus, abnormalities of the skull base, syringomyelia	Spina bifida, hydrocephalus, syringomyelia

Type of Malformation

Chiari Type I

Chiari Type II

displaced neural structures

cerebellar tonsils

Cerebellar vermis, lower portion of the cerebellar hemispheres, medulla oblungata and forth ventricle

typical age of onset

young adults

childhood

associated diseases

Hydrocephalus, abnormalities of the skull base, syringomyelia

Spina bifida, hydrocephalus, syringomyelia

It is believed that the malformation is related to an insufficient development of the posterior cranial fossa, causing the cerebellum to be pushed downwards, through the foramen magnum.

Onset of Type I usually occurs in young adults. Morphologically it is characterised by the displacement of the cerebellar tonsils in the upper portion of the cervical canal. 30-85% of all cases are associated with syringomyelia and 1/4 of all cases are affected by bone abnormalities in the skull base.

Onset of Type II usually occurs at birth, and is typically associated with spina bifida. Morphologically, also the cerebellar vermis, the lower portion of the cerebellar hemispheres, the medulla oblungata and the forth ventricle are displaced into the cervical canal.

Type II is more severe than Type I, and its treatment is more complex.

Symptoms

Patients affected by CM may be asymptomatic or develop symptoms throughout their lives, usually in their twenties or thirties. The symptomatology is extremely polymorphic and includes tension headache, visual disorders (blurred vision, diplopia), oto-neurological disorders (vertigo, hearing loss), lower cranial nerve disorders (alterations of the voice, sleep apnea, difficulty swallowing), disorders of the cerebellum (nystagmus, ataxia, balance disorders) or secondary disorders involving dysfunctions of the spinal cord (paresthesia, muscle weakness). Headache is often one of the initial symptoms; it typically occur during stressful situations and radiate from the lower back of the head.

Diagnosis

Magnetic Resonance Imaging (MRI) of the brain is the test of choice in the diagnosis of Chiari Malformation, as it allows to assess the position and the morphology of the cerebellar tonsils, and the possible presence of syringomyelia. In symptomatic patients, the cerebellar tonsils generally take on a triangular shape in the sagittal images and tend to entirely clog the passage of spinal fluid at the foramen magnum. The MRI also makes it possible to study the alterations of the spinal fluid circulation induced by Chiari Malformation (Cine-mode MRI). These abnormalities are the pathological basis for the development of syringomyelia.

The descent of the cerebellar tonsils is not always an expression of Chiari Malformation. A tonsillar herniation can result from a CSF (cerebrospinal fluid) hypotension syndrome, from the development of an expansive process in the posterior cranial fossa, from the presence of hydrocephalus, or from the existence of cranio-vertebral junction abnormality (basilar invagination).

Thus, it is recommended to extend the MRI study to the whole spine, in order to highlight any cavity associated to syringomyelia.

A further diagnostic examination is represented by the somato-sensory evoked potentials and motor responses in order to better define the presence and the degree of bone marrow involvement.

Treatment

The treatment is exclusively surgical, and is advisable when Chiari Malformation is associated with symptoms that significantly alter a patient’s daily life and when syringomyelia is present. It is acknowledged that once the symptoms of syringomyelia are established, they tend to be progressive and they only partially improve after being treated.

The surgical procedure consists in an incision on the lower back of the head of about 10 cm. After dissecting the muscles, you reach the atlanto-occipital joint. There, you proceed with an osteo-dural decompression (removal of a small bone portion at the base of the skull and at the back of the first cervical vertebra and opening of the dura mater) and a coarctation of the cerebellar tonsils is performed. Subsequently, the atlanto-occipital joint and the space of the posterior cranial fossa are expanded by means of an expansile duraplasty with autologous pericranium or heterologous material.

It is a safe procedure that involves 4-5 days of hospitalization and a subsequent recovery period of about 30 days.

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