Chiari malformation and arachnoid cysts
Two conditions often found by chance on a scan. In one, the lower part of the brain sits too low in the skull; in the other, a pocket of fluid presses on the surface of the brain. Most need watching, not surgery.
Greenslopes Private Hospital, Brisbane
Watkins Medical Centre, Spring Hill, or telehealth
A current GP or specialist referral is needed
At a glance
The lowest part of the cerebellum extends below the base of the skull into the top of the spinal canal.
Type 1 is the common adult form.
A fluid-filled cavity within the spinal cord that develops in some people with a Chiari malformation.
A fluid pocket between the brain and its coverings, usually present from birth.
Most never cause symptoms.
Observation for most.
Decompression for Chiari; fenestration or a shunt for cysts that cause symptoms.
What these conditions are
Both are structural findings rather than diseases. The scan shows them clearly; whether they are responsible for a person’s symptoms is the harder question.
In a Chiari type 1 malformation, the cerebellar tonsils, the lowest part of the cerebellum at the back of the brain, sit a few millimetres or more below the opening at the base of the skull. This crowds the space where the brain meets the spinal cord and can interfere with the normal flow of cerebrospinal fluid. In some people that disturbed flow leads to a syrinx, a fluid-filled cavity in the spinal cord itself, which can damage the cord slowly over time.
Many people with a Chiari malformation on their scan have no symptoms from it, and headaches of other kinds are common, so the finding does not always explain the complaint that led to the scan. Separating the two is a large part of the consultation.
An arachnoid cyst is a sac of cerebrospinal fluid trapped within the arachnoid membrane, most often in the temple region or behind the cerebellum. Most are present from birth, stay the same size for life and cause nothing. A minority are large enough to press on the brain, or grow, or, rarely, bleed after a head injury.
Symptoms
The pattern of symptoms matters more than their presence, because it is the pattern that suggests the finding on the scan is the cause.
- Chiari: a pressure-like headache at the back of the head that is brought on by coughing, straining, laughing or bending
- Chiari: neck pain, dizziness, unsteadiness, difficulty swallowing, hoarseness or a disturbed sleep pattern
- Syrinx: numbness or weakness in the arms and hands, loss of pain and temperature sensation, or a curve developing in a child’s spine
- Arachnoid cyst: usually none; occasionally headache, seizures, or, in a child, an enlarging head or developmental delay
- Either: symptoms that are progressing rather than stable
Neither condition is an emergency in itself. New weakness, numbness or difficulty walking that is getting worse should be reported to the rooms or your GP promptly.
How it is diagnosed
MRI of the brain and, for Chiari, the whole spine, to measure the position of the tonsils and look for a syrinx. A CSF flow study, a special MRI sequence, shows whether fluid is moving normally across the base of the skull. Arachnoid cysts are measured on MRI and compared with earlier scans where they exist; a stable cyst over years is strong evidence that it can be left alone.
The scan is on the screen, the options are set out on equal terms, and nothing is booked until you have had time to think.
Treatment options
Surgery is offered when symptoms clearly come from the malformation or cyst and are affecting life, or when a syrinx is present or growing. Otherwise the plan is to watch.
- ObservationInterval MRI and review, for a Chiari malformation without a syrinx and without clear symptoms, and for nearly all arachnoid cysts. This is the plan for most people.
- Posterior fossa decompressionFor Chiari: a small piece of bone is removed from the base of the skull at the back, sometimes with the arch of the top vertebra, and the covering membrane is opened and patched to make more room. Done under general anaesthetic through an incision at the back of the neck.
- Treating a syrinxA syrinx is nearly always treated by decompressing the Chiari malformation that caused it, rather than by operating on the cord itself. It shrinks over months as fluid flow is restored.
- Cyst fenestrationFor a symptomatic arachnoid cyst: the cyst walls are opened, with an endoscope or through a small craniotomy, so that the fluid drains into the normal spaces around the brain.
- Cyst shuntA small tube with a valve drains the cyst to the abdomen, used where fenestration is not suitable or has not worked. It carries the long-term considerations of any shunt.
Not every option applies to every person. Which ones apply to you, and in what order, is explained at the consultation and confirmed in writing before anything is booked.
Recovery and follow-up
Recovery after decompression is measured in weeks; the effect on symptoms and on a syrinx is measured in months.
After a Chiari decompression, a hospital stay of two to four days is usual, followed by neck stiffness and headache that ease over a few weeks. Cough-related headache often improves early; other symptoms take longer, and some do not change. A follow-up MRI at a few months checks the space that has been created and, where there was a syrinx, whether it is shrinking.
After cyst fenestration the stay and recovery are similar. The cyst may not disappear on the scan even when symptoms have improved; what matters is the pressure on the brain, not the picture.
Children
Both conditions are seen in children, and the thresholds for surgery are the same: symptoms that are clearly caused by the finding, or a syrinx. Children are operated on at the Queensland Children’s Hospital and followed as they grow.
Dr Gert TollessonNeurosurgeon, Brisbane
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