Arnold-Chiari malformation

Arnold-Chiari malformation, or simply Chiari malformation (CM), are structural defects in the cerebellum, the part of the brain that controls balance. Normally the cerebellum and parts of the brain stem sit in an indented space at the lower rear of the skull, above the foramen magnum (a funnel-like opening into the spinal canal). When part of the cerebellum is located below the foramen magnum, it is called a Chiari malformation. [source: NINDS].

Technically speaking, it is the downward displacement of the cerebellar tonsils through the foramen magnum, sometimes causing non-communicating hydrocephalus as a result of obstruction of cerebrospinal fluid (CSF) outflow [source: Wikipedia]. It occurs when part of the skull is abnormally small or misshapen, pressing on the brain and forcing it downward [source: Mayo Clinic]. Functions controlled by these areas of brain may be affected.

A T1-weighted sagittal MRI scan, from a patient with an Arnold-Chiari malformation,
demonstrating tonsillar herniation of 7mm (credit)

The cerebrospinal fluid outflow is caused by phase differences in outflow and influx of blood in the vasculature of the brain. Obstruction of the CSF can cause headaches, fatigue, muscle weakness in the head and face, difficulty swallowing, dizziness, nausea, impaired coordination, and, in severe cases, paralysis.[source: Wikipedia].

Causes

CM has several different causes. It can be caused by structural defects in the brain and spinal cord that occur during fetal development, whether caused by genetic mutations or lack of proper vitamins or nutrients in the maternal diet. This is called primary or congenital CM.

It can also be caused later in life if spinal fluid is drained excessively from the lumbar or thoracic areas of the spine either due to injury, exposure to harmful substances, or infection. This is called acquired or secondary CM.

Primary CM is much more common than secondary CM.

Risk factors

There is some evidence that CM runs in some families. However, research into a possible hereditary component is still in its early phase.

Classification

The Austrian pathologist Hans Chiari in the late 19th century described seemingly related anomalies of the hindbrain, the so-called Chiari malformations I, II and III. Later, other investigators added a fourth (Chiari IV) malformation. The scale of severity is rated I - IV, with IV being the most severe. Types III and IV are very rare.

ADDENDUM: Another form of CM, currently being debated by scientists, is Type 0, in which there is no protrusion of the cerebellum through the foramen magnum but headache and other symptoms of CM are present [source: NINDS].

Pathophysiology

The most widely accepted pathophysiological mechanism by which Chiari Type 1 Malformations occur is by a reduction or lack of development of the posterior fossa as a result of either congenital or acquired disorders.

Congenital causes include hydrocephalus, craniosynostosis (especially of the lambdoid suture), hyperostosis (ex. craniometaphyseal dysplasia, osteopetrosis, erythroid hyperplasia), X-linked vitamin D-resistant rickets, and neurofibromatosis type I. Acquired disorders include space occupying lesions due to one of several potential causes ranging from brain tumors to hematomas.

Clinical features

Many people with CMs have no symptoms and their malformations are discovered only during the course of diagnosis or treatment for another disorder. When present, symptoms may include:

• Headaches aggravated by Valsalva maneuvers, such as yawning, laughing, crying, coughing, sneezing or straining
• Tinnitus (ringing in the ears)
• Vertigo (dizziness)
• Nausea
• Nystagmus (irregular eye movements)
• Facial pain
• Muscle weakness
• Impaired gag reflex
• Restless Leg Syndrome
• Sleep Apnea
• Sleep disorders
• Dysphagia (difficulty swallowing)
• Impaired coordination
• Increased intracranial pressure
• Pupillary dilation
• Dysautonomia: tachycardia (rapid heart), syncope (fainting), polydipsia (extreme thirst), chronic fatigue

The blockage of 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.

Associations / Complications

In some people, Chiari malformation can become a progressive disorder and lead to serious complications. In others, there may be no associated symptoms, and no intervention is necessary. The complications associated with this condition include:

• Hydrocephalus. Excessive buildup of CSF in the brain (hydrocephalus) may require placement of a flexible tube (shunt) to divert and drain the fluid to another area of the body.
• Spina bifida. A condition in which the spinal cord or its covering is not fully developed, may occur in CM. Part of the spinal cord is exposed, which can cause serious conditions such as paralysis. People with Chiari malformation type II usually have a form of spina bifida called myelomeningocele.
• Syringomyelia, or hydromyelia. Some people may develop a CSF-filled tubular cavity or cyst (syrinx) within the spinal cord’s central canal.
• Tethered cord syndrome. In this condition, the spinal cord attaches to the spine and causes the spinal cord to stretch. This can cause serious nerve and muscle damage in the lower body.
• Spinal curvature. Common among individuals with syringomyelia or CM Type I.

CMs may also be associated with certain hereditary syndromes that affect neurological and skeletal abnormalities, other disorders that affect bone formation and growth, fusion of segments of the bones in the neck, and extra folds in the brain.

DIAGNOSIS

A physical examination to check the person’s memory, cognition, balance (a function controlled by the cerebellum), touch, reflexes, sensation, and motor skills (functions controlled by the spinal cord) is essential.

The following diagnostic tests may be ordered:

• X-ray of the head and neck. Cannot reveal a CM but can identify bone abnormalities that are often associated with CM. This safe and painless procedure can be done in a doctor’s office and takes only a few minutes.
• Computed tomography (also called a CT scan). This painless, noninvasive procedure is done at an imaging center or hospital on an outpatient basis and can identify hydrocephalus and bone abnormalities associated with CM.
• Magnetic resonance imaging (MRI). Is the imaging procedure of choice for CM. Like CT, it is painless and noninvasive. MRI uses radio waves and a powerful magnetic field to produce either a detailed three-dimensional picture or a two-dimensional “slice” of body structures, including tissues, organs, bones, and nerves. Depending on the part(s) of the body to be scanned, MRI can take up to an hour to complete.

TREATMENT

Treatment for CM depends on the severity and the characteristics of the patient's own condition.

If there are no symptoms, likely recommendation is no treatment other than monitoring with regular examinations and MRIs.

When headaches or other types of pain are the primary symptom, pain medications may be recommended.

Reducing pressure by surgery:
Doctors usually treat symptomatic CM with surgery. The goal is to stop the progression of changes in the structure of the brain and spinal canal, as well as ease or stabilize symptoms.

When successful, surgery can reduce pressure on the cerebellum and spinal cord, and restore the normal flow of spinal fluid (i.e. CSF).

In the most common surgery for CM, called posterior fossa decompression, surgeons remove a small section of bone in the back of the skull, relieving pressure by giving the brain more room.

In many cases, the covering of the brain, the dura mater, may be opened. Also, a patch may be sewn in place to enlarge the covering and provide more room for the brain. This patch may be an artificial material, or it could be tissue harvested from another part of the person's body.

A small portion of the spinal column may also be removed to relieve pressure on the spinal cord and allow more space for it.

The surgical technique may vary, depending on whether a fluid-filled cavity (syrinx) is present, or if there is fluid in the brain (hydrocephalus). If a syrinx or hydrocephalus is present, a tube (shunt) to drain the excess fluid may be needed.

Surgical risks and follow-up:
Surgery involves risks, including the possibility of infection, fluid in the brain, CSF leakage or problems with wound healing.

The surgery reduces symptoms in most people, but if nerve injury in the spinal canal has already occurred, this procedure won't reverse the damage.

After the surgery, regular follow-up examinations with your doctor is needed, including periodic imaging tests to assess the outcome of surgery and the flow of cerebrospinal fluid.