What is Arnold-Chiari Malformation (Chiari malformation)?
Arnold-Chiari, or Chiari malformation, is a term used to describe a group of deformities in the back part of the brain (cerebellum, pons, and medulla oblongata). These issues can vary greatly, from the herniation (or protrusion) of a part of the cerebellum through a large natural opening at the base of the skull, to the absence of the cerebellum, and may occur alongside other brain or physical defects such as hydrocephalus (water on the brain), syrinx (fluid filled cyst in the spinal cord), encephalocele (brain tissue protrusion), or spinal dysraphism (incomplete formation of the spine).
Chiari malformations are grouped based on their features and severity, typically detected through scanning or post-mortem examination. Chiari I, which is the least severe, often shows no symptoms and is discovered accidentally. It’s characterized by one or both parts of the cerebellum extending 5 mm below the large opening at the base of the skull. This is measured by a line drawn from the front to the back of this opening (McRae Line).
Chiari II involves the downward displacement of the brainstem and an enlarged cerebellum in addition to the herniated cerebellum and medulla due to an unsealed lower part of the spine (myelomeningocele). Chiari III involves the downward displacement of the cerebellum, with or without the brainstem, into an outpouching at the back of the head or top part of the neck.
Chiari IV is no longer used as a classification. Before it was discontinued, it referred to a severe underdevelopment of the cerebellum, similar to the absence of the cerebellum from birth. Some used to believe it could include the presence of a myelomeningocele, while others believed that should be classified as a Chiari II with a disappearing cerebellum.
There are other disputed classifications, including Chiari 0, Chiari 1.5, and Chiari V. Chiari 0 is characterized by the formation of a fluid filled cyst in the spinal cord without the downward displacement of the back part of the brain, while Chiari 1.5 seems to be a progression of Chiari I with further downward displacement of a part of the cerebellum and some involvement of the brainstem. Chiari V, the most severe form, represents the absence of the cerebellum with the descent and herniation of the occipital lobe (back part of the brain) through the large opening at the base of the skull.
What Causes Arnold-Chiari Malformation (Chiari malformation)?
There are several theories about why Chiari malformation, where the lower part of the brain sinks into the spinal canal, might occur. This can happen due to different factors affecting the brain and the fluid around it.
In the case of Chiari I malformations, it is most commonly due to the back part of the brain space being too small. This may be a congenital condition, meaning a person is born with it, or it could happen due to changes over time, like premature closure of the areas between the skull bones, changes in the shape of the skull bones, or due to specific genetic syndromes. Research has identified mutations on chromosomes 1 and 22 as possible reasons for hereditary cases of this condition.
For Chiari II malformations, it is often linked to an open spinal cord defect known as myelomeningocele. This defect causes cerebrospinal fluid, the fluid that surrounds and protects your brain and spinal cord, to leak or be misdirected, causing a key brain cavity to collapse and leading to the brain sinking lower in the skull. The circumstances would be similar for Chiari III malformations, but in this case, the defect would be higher up in the neck or brain area. Folates deficiency or certain genetic changes could increase the risk of these spinal cord defects, and hence, of Chiari II or III malformations.
The causes for other types of Chiari conditions remain debated and unclear. It’s possible that an injury could lead to the brain sinking, but if the back part of the brain space is normal, it would not typically be categorized as Chiari.
Risk Factors and Frequency for Arnold-Chiari Malformation (Chiari malformation)
The Chiari I malformation is the most prevalent type, occurring in roughly 0.5 to 3.5% of people. It is slightly more common in females with a ratio of 1.3 females for every male.
Chiari II is less common, occurring in 0.44 out of every 1000 births. This version does not favor either gender, and its frequency can be reduced if the mother uses folate supplements during pregnancy.
The other types of Chiari malformations are very rare. Out of these, Chiari III is the most common, making up 1 to 4.5% of all Chiari malformations.
Signs and Symptoms of Arnold-Chiari Malformation (Chiari malformation)
Chiari I malformation is a disorder in which the lower part of the brain pushes down into the spinal canal. The most common symptom is pain in the base of the skull or neck (identified in 80% of cases) that worsens when performing the Valsalva maneuver, a breathing technique that increases pressure in the chest. Other common symptoms are visual disturbances, dizziness, hearing loss, imbalance while walking, and constant tiredness.
In some rare cases, people may present with just limb pain or weakness, with one report pointing to a case of one-sided shoulder pain and distinct muscle weakness in a sports medicine clinic.
Issues with the spinal cord (myelopathy) usually result in a unique type of sensory loss where pain and temperature sensations are lost, but fine touch and body-awareness sensations are maintained. Weakness in movements can also occur.
Symptoms involving the cerebellum (the part of the brain responsible for coordination and balance) and lower cranial nerves can emerge due to direct pressure on certain brain areas or a condition known as syringomyelia (a fluid-filled cyst in the spinal cord) or syringobulbia (a fluid-filled cyst in the brainstem). These symptoms include unsteady movements, lack of coordination, involuntary eye movements, and problems with the nerves that control the back of the throat and neck.
Sleep apnea, a sleep disorder where breathing stops and starts, can be observed in patients with Chiari malformation. It happens when the muscles in the throat, controlled by the brainstem or lower cranial nerves, cannot function properly due to compression.
However, it is noteworthy to mention that not all patients with Chiari malformation show these symptoms. In some cases, imaging tests may reveal Chiari malformation in people who do not display any symptoms at all. So, general symptoms like constant fatigue or migraines do not necessarily indicate a Chiari malformation. On the other hand, other variants of Chiari malformation (apart from 0 and 1.5) are often diagnosed during pregnancy or immediately after birth.
Testing for Arnold-Chiari Malformation (Chiari malformation)
When diagnosing Chiari malformations, different imaging methods can be used depending on the specific type of malformation. Chiari I, commonly found in children or adults, is usually evaluated using a magnetic resonance imaging (MRI) scan. In contrast, Chiari II to IV are first checked with ultrasound when the baby is in the womb, and if necessary, a follow-up fetal MRI study is performed for a more detailed view.
An MRI of the head and the part of the spine in the neck area is the best way to examine Chiari I. This scan can demonstrate whether the lower part of the brain (cerebellar tonsils) hangs more than 5 mm below a point in the skull called the foramen magnum. It can also show if the space at the back of the skull is unusually small or if there’s a fluid-filled cyst (called a syrinx) present. If this cyst extends into other areas, an additional MRI of the upper or lower back could be required. If the brain’s ventricles appear enlarged, further MRI sequences might be carried out to check the flow of the cerebrospinal fluid (CSF) and see if there’s any blockage at the foramen magnum.
In the management of Chiari I, there are other tests that can be beneficial:
* Myelography: A valuable alternative if an MRI cannot be taken for some reason.
* CT or x-rays of the neck and head: These can reveal bone defects that are often associated with Chiari I malformation, especially at the point where the skull and neck meet, providing crucial information for surgical planning.
A prenatal ultrasound scan usually done in the second trimester can demonstrate the typical imaging features of Chiari II and III malformations. Certain tell-tale sign can be seen, like a “lemon sign” which shows a flattening on the top front part of the skull, and a “banana sign”, where a part of the brain demonstrates abnormal shape and curvature. Chiari III might be indicated by an outpouching of the brain (meningoencephalocele) seen at the back of the skull.
A fetal MRI can provide more detail on Chiari II and III, as well as showing underdeveloped or absent parts of the brain (cerebellar hypoplasia/aplasia) in Chiari IV and V. It also better highlights a protuberance called tectal beaking that can occur in Chiari II.
While laboratory studies aren’t useful in diagnosing Chiari malformations, they are necessary for planning any surgical treatment. Typical tests include checking the complete blood count, clotting profile, electrolyte levels, performing a chest X-ray, and an electrocardiogram (ECG).
Treatment Options for Arnold-Chiari Malformation (Chiari malformation)
In simpler terms, Chiari malformation is a condition that doesn’t always require medical intervention. In cases where patients with Chiari malformation are not showing any symptoms, they can be managed with medications. These include muscle relaxants and NSAIDs (non-steroidal anti-inflammatory drugs), as well as temporary use of a neck brace. However, studies show this type of management may not improve walking difficulties in many symptomatic patients, even though it can help relieve headaches and nausea. It’s worth noting that about 90% of individuals with Chiari type I malformation, even those with a complication called syringomyelia, may never show any symptoms.
When it comes to surgical treatment of Chiari malformation, this is typically the main approach and aims to restore normal cerebrospinal fluid flow around the base of the brain and relieve pressure on the cerebellum and sections of the brain in the back of the skull. This is done by making more space in what’s known as the posterior fossa. Surgery is usually recommended for patients who are showing symptoms and who have an abnormality where parts of the brain are extending into the spinal canal. If no symptoms are present in this case, or if there is a fluid-filled cyst in the spinal cord, the doctors will usually prefer to wait and watch unless symptoms develop. Reports indicate that the surgery is more successful when performed within two years of the appearance of symptoms.
The common surgical procedure for Chiari I malformation involves creating more space in the posterior fossa by removing parts of the skull near the base and possibly the back parts of the first and second cervical vertebrae. The thick membrane covering the brain and spinal cord may or may not be opened. If the surgery doesn’t alleviate symptoms, a shunt can sometimes be placed to help drainage. In some cases, the tonsils of the cerebellum – the part of the brain that has descended out of place – may be reduced in size with heat (cauterization).
Recently, minimally invasive techniques have been developed that enable less damage to the tissues, require smaller incisions, and reduce the manipulation of the covering of the brain. These newer methods often result in faster recovery times, shorter hospital stays, and fewer complications.
For Chiari II malformation, the initial surgical correction often involves the treatment of a birth defect known as a myelomeningocele, usually within the first 48 hours after birth. This procedure can sometimes even be performed while the baby is still in the womb. Most of these children will eventually need a shunt to drain excess cerebrospinal fluid, as they usually develop a condition known as hydrocephalus. Further decompression procedures can be performed later if required.
Chiari III malformation is generally treated similarly to Chiari II. First, the fluid-filled sac that may be found at the back of the head or neck is corrected, followed by a procedure to repair the covering of the brain and a skull reconstruction. If the herniated tissue is too large in proportion to the brain, the patient might not be considered a good candidate for surgery. Again, a shunt might need to be placed if the patient also has hydrocephalus.
Certain situations require caution and might not be suitable for performing surgery to make more space in the posterior fossa. For instance, if the abnormal descent of the cerebellum into the spinal canal is caused by a disease other than Chiari malformation, such as low brain fluid pressure or a mass in the back of the brain, operating might not be appropriate.
What else can Arnold-Chiari Malformation (Chiari malformation) be?
If a person’s brain appears to be sinking, it can sometimes look like certain parts of the brain are herniating, or bulging out of place, when in reality this is due to low pressure inside the skull. This condition is called intracranial hypotension.
Occasionally, a normal part of the brain, called the cerebellar tonsils, can appear to be protruding more than normal, but this is just a harmless natural variation. This doesn’t fit the definition of a condition called Chiari malformation and it’s usually found by accident in someone who has no symptoms.
In contrast, tonsillar herniation, where the cerebellar tonsils actually start to press out of the skull, can be caused by increased pressure inside the skull. This high pressure can be caused by factors like abnormal growths (neoplasms), a build-up of fluid on the brain (hydrocephalus), physical damage (trauma), or bleeding.
What to expect with Arnold-Chiari Malformation (Chiari malformation)
The prognosis for Chiari I, a medical condition, is generally positive, although it can vary depending on any existing neurological issues. Most patients without any neurological problems usually have a very positive outcome.
However, Chiari II has a risk of 3% neonatal (newborn) in-hospital death, and a 15% mortality rate within three years. Surviving patients may experience worsening motor skills over time. It is recommended for these patients to return for consistent check-ups for evaluating the need for a shunt (a device to relieve pressure on the brain) or the potential failure of a shunt.
For the more severe forms of Chiari, the prognosis is usually quite poor and can often lead to early death. Moreover, those who experience chronic weakness or problems with walking usually don’t see improvement, marking their prognosis as uncertain.
Possible Complications When Diagnosed with Arnold-Chiari Malformation (Chiari malformation)
These are some possible complications:
- Pseudomeningocele (a collection of spinal fluid under the skin)
- CSF leak (a leakage of spinal fluid)
- Meningitis (an inflammation of the protective membranes covering the brain and spinal cord)
- Wound infection
- Lower brainstem malfunction (problems with the function of the lower part of the brain)
- Epidural hematoma (a condition where blood accumulates between the dura mater and the skull)
- Apnea (temporary cessation of breathing)
- Vertebral artery injury (damage to the arteries carrying blood to the brain)
Recovery from Arnold-Chiari Malformation (Chiari malformation)
After surgery, it’s important to watch for a leak of cerebrospinal fluid (CSF). Some people might have a false buildup of CSF, also known as a pseudomeningocele, which may need to be drained.
For a more personalized assessment of their recovery after surgery, patients can be rated using the Chicago Chiari Outcome Scale. This measurement system considers pain, non-pain symptoms, functionality, and any surgical complications, rating each on a scale of 1-4. This results in a total score from 4-16, with 4 indicating a debilitating outcome, and 16 indicating an outstanding recovery.
Patients should avoid exercise and heavy lifting for at least 3 to 4 weeks following the surgery.
Typically, full recovery from the surgery and a complete reversal of any major neurological issue might take 6 to 8 weeks. Even once the surgery site has fully healed, patients should think about avoiding contact sports.
A follow-up MRI scan will be needed to confirm that the syrinx, a fluid-filled cyst in the spine, has reacted well to the treatment.
Preventing Arnold-Chiari Malformation (Chiari malformation)
If the Chiari I condition is being treated without surgery, the patient should be advised to return to the doctor if they start to experience a cough, headaches, or a gradual weakening in their limbs. After surgery, the patient should be aware of the possible risk of developing certain conditions, such as pseudomeningocele (a fluid-filled swelling), a CSF leak (where the fluid that surrounds and cushions the brain and spinal cord leaks), and meningitis (an infection of the protective membranes around the brain and spinal cord).
