The MRI [see Figure 6] also allows one to see the posterior fossa in a side view (sagittal),
frontal view (coronal) and cross-sectional view (axial) as seen in these scans. In this case, they show herniation of the tonsils and crowding of the foramen magnum. No cisterna magna can be seen.
Twenty-five to 50% of people with the Chiari malformation also have signs that the bones of the skull or spine did not develop properly. The C1 vertebra, which is shaped like a ring, may not be completely formed, a condition called a bifid C1. In some cases, C1 is congenitally fused to the bottom of the skull, a condition called assimilation of C1. Other cases may have the upper part of the cervical spine indenting to some degree into the foramen magnum, a condition called basilar invagination. Another possibility is the fusion of two or more vertebrae of the spine, a condition called Klipple-Feil syndrome.
Pathophysiology
Many researchers believe that the Chiari malformation results from underdevelopment of the lower part of the skull. The posterior fossa is too small and does not provide the room needed for the cerebellum and the lower brain stem. Thus, the lower part of the cerebellum, and sometimes the lower part of the brain stem, develops below the foramen magnum. Why most people dont show symptoms in infancy is not well understood. What is known is that the brain moves with every heartbeat. Special MRI studies, called CINE MRI, have shown that the brain expands quickly with each heartbeat as blood is pumped into the brain. The briefly expanding brain, restricted by the fixed skull, moves downward toward the foramen magnum, which is already crowded in patients with the Chiari I malformation. This continuous downward pulsation of the brain can result in malfunction of the neurological tissue in the area. (On average, the heart beats 72 times each minute. Multiplying this by 60 minutes each hour and by 24 hours per day for 103,680 brain pulses per day!) Even worse, when a person with the Chiari I malformation coughs, sneezes or strains, there is additional downward pressure which causes the tonsils to impact into the foramen and cause an increase in the headache.
For most patients the Chiari malformation does not run in their family. However, in some cases it can pass down from the mother or father to son or daughter. Further research in this area may lead to a better understanding of the causes of the Chiari malformation.
Obstruction at the foramen magnum means that the spinal fluid flow into the spinal canal will not be normal. In some patients with Chiari, this abnormal flow will cause fluid to build up inside the spinal cord, the condition called syringomyelia. (The cavity itself is often referred to as a syrinx.) This is the same condition identified by Dr. Ollivier and found to be associated with tonsillar herniation by Dr. Chiari over 100 years ago. The number of patients that develop a syrinx from the Chiari I malformation varies from 20 to 70% depending on the patients studied and the test used to determine its presence.
Although the leading cause of syringomyelia is the Chiari malformation, it can also be caused by trauma, tumor, or infection. In some cases, the cause is unknown (idiopathic).
Figure 7 shows a large syrinx in the upper part of the spinal cord. It extends from just below the C1 level to C4
Symptoms of the Chiari I Malformation
Symptoms from the Chiari I malformation often develop in a slow and insidious manner. The symptoms are often vague and progress slowly over a number of years. Many patients have symptoms for years before they are diagnosed with the Chiari I malformation. About 1 out of 4 patients develop symptoms following trauma. The trauma is often mild or moderate in nature. Patients have reported accidents such as a fall from a trampoline, fall off a concrete porch, being hit on the back of head when a gas hose broke, or falling after hitting dog while riding a bicycle.
The most common symptom of the Chiari I malformation is headache. The headache usually begins at the back of the head and radiates behind one or both eyes, the top of the head or temples. The headache is often described as a pressure sensation but can be heavy, sharp, or stabbing. For many, the pain varies between these and other sensations. The headache is often made worse by straining activities (known as valsalva maneuvers) such as coughing, sneezing, or even laughing or singing. Some patients report feeling as though my head will explode or like my head will pop off. Bending forward, or looking up can worsen the headache. When the headache is intense, some patients will note dizziness, blurring of vision, nausea, or a feeling of a mental fog. A variety of other visual symptoms can occur with the Chiari I malformation including double vision, decreased peripheral vision, fireflies, visual loss, blind spots, photophobia, spasm of the eyelids (blepharospasm), and jerking of the eyes (nystagmus). Symptoms of the ears and the balance system include dizziness, ringing in the ears (tinnitus), poor balance (disequilibrium), decrease or loss of hearing, and vertigo. Other symptoms related to the cranial nerves and their nuclei include hoarseness, problems swallowing (dysphagia), slurring of words (dysarthria), and numbness of the face. Many patients complain of weakness, numbness or tingling, or pain in the arms or legs. Often, only one side is involved.
General symptoms include depression, poor sleep (insomnia) and fatigue. Some note problems with memory, thinking and speech. People report the feeling of a brain fog with difficulty in thinking and concentration. Difficulty in finding the right word is not uncommon.
Effects of the Chiari malformation on the respiratory and heart centers can cause shortness of breath, chest pain, episodes of rapid heart rate (tachycardia), black out spells, and hypertension. Abdominal symptoms may include nausea, abdominal pain, or vomiting.
With so many possible symptoms, most which can be caused by other conditions, a physician may not suspect the Chiari malformation as a possible cause of the symptoms. However, many patients seem to show a common pattern of symptoms as noted below.
Common pattern of symptoms:
- Occipital pressure headache, radiating forward, worse with valsolva
- Dizziness, often worse with headache or changing positions
- Transient blurring of vision, worse with HA
- Weakness of all or some extremities
- Pain or numbness in face or extremities
- Nausea
- Swallowing problems
- Chronic fatigue syndrome
- Fibromyalgia
- Sleep apnea
- Spinal CSF leak
- Arachnoid cyst
- Degenerative disc disease
- Psychiatric disease
- Migraine
- Cluster headache
- Rebound headache
- Pseudotumor cerebri
- Occipital neuralgia
- Post-concussion syndrome
- Hydrocephalus
- Multiple sclerosis
