A brain MRI ordered for headaches or a fall comes back, and one line of the report mentions an arachnoid cyst. Reading that phrase on a phone, before anyone has called to explain it, can be unsettling. Sorting out arachnoid cyst symptoms comes first, since the cyst and the reason for the scan may be unrelated. Most of these fluid sacs stay small for life, so doctors start by checking whether this cyst presses on anything.
Where Arachnoid Cysts Form
The arachnoid is one of three thin layers that wrap the brain and spinal cord. According to the Cleveland Clinic, a cyst forms when this layer splits or tears and fills with fluid. The fluid is the same cerebrospinal fluid that cushions the brain. A cyst near the fluid spaces deep in the brain can block normal drainage and lead to hydrocephalus that needs its own treatment. Plenty of arachnoid cysts sit in a space beside the temporal lobe, though, where they may never cause trouble.
Doctors call cysts present from birth primary cysts. The National Institute of Neurological Disorders and Stroke (NINDS) lists arachnoid cysts as the most common type of brain cyst. Secondary cysts form later, after a head injury, an infection such as meningitis, or brain surgery. Neither type is cancer.
Arachnoid Cyst Symptoms Around the Brain
Arachnoid cyst symptoms appear when a cyst grows large enough to press on the brain or block fluid flow. Because an adult skull cannot expand, pressure from a growing cyst shows up as symptoms while the head looks normal. Headache is one of the most common complaints, sometimes with nausea or vomiting. Other symptoms depend on where a cyst lies. A cyst at the back of the skull, near the brain’s balance centers, can cause dizziness and an unsteady walk.
Vision can blur when a cyst grows near the optic nerves. Seizures, sometimes the first sign anyone notices, can come from a cyst beside the temporal lobe. Symptoms can come and go and build over months, so a person may chalk them up to stress or poor sleep. Family members sometimes spot changes in memory or mood before the person does.
Arachnoid Cyst Symptoms Along the Spine
A smaller group of cysts forms inside the spinal canal. Arachnoid cyst symptoms there feel more like a pinched nerve than a brain problem. Back pain, leg pain, and tingling or numbness in the arms or legs are the usual signs. Weakness in the legs can also build slowly as a cyst presses on the spinal cord.
Because spinal cyst symptoms overlap with a herniated disc or spinal stenosis, a doctor may look for those problems first. A spine scan with contrast dye can outline a cyst’s edges. For a spinal cyst, the Cleveland Clinic describes surgery to open the area around the spine and remove the cyst. Long-lasting pressure on the spinal cord can leave permanent weakness, so surgeons move faster on a cyst pressing there.
When Arachnoid Cyst Symptoms Come From Something Else
Headaches and small arachnoid cysts are both common, so the two can show up in the same person by chance. A headache that started years before a scan, or feels the same as always, points away from a cyst as the cause. Someone whose migraines began in their teens, and whose MRI at 45 shows a small cyst far from the balance centers, is likely looking at two separate problems.
Radiology reports use a few terms that point toward or away from a problem. “Mass effect” means the cyst is pushing on nearby brain tissue. “Effacement” means the cyst is flattening the grooves or fluid spaces next to it. A report that uses neither term and calls the finding incidental usually describes a cyst pressing on nothing.
Repeat scans show whether a cyst is growing. According to NINDS, a CT or MRI scan also helps tell arachnoid cysts apart from other kinds of cysts. A doctor may order a follow-up MRI in six to twelve months, then space later scans out if nothing changes.
Watching the Cyst or Treating It
Most arachnoid cysts need nothing more than monitoring. Some doctors advise people with a large cyst to skip contact sports, since a hard hit during a game can tear the cyst and cause bleeding. For cysts that do need treatment, NINDS reports that symptoms usually improve or go away afterward. Surgeons start weighing an operation when a cyst grows, blocks fluid flow, or causes symptoms that make sense for its location.
Endoscopic fenestration is one option for a brain cyst. A surgeon passes a thin camera through a small opening and cuts windows in the cyst wall so fluid drains away. When a camera cannot reach the cyst, a thin tube called a shunt can drain the fluid into the belly. Shunts can clog, though, and some need another operation years later.
When two doctors disagree about surgery, a person can seek a second opinion on the same scans before booking anything. The reviewing surgeon compares the cyst’s size across scans and reads the images without the first doctor’s conclusion attached. Cysts come back after surgery in rare cases, so follow-up imaging continues once recovery is over.
A surgeon with skull base or cranial fellowship training is likely to see these cysts regularly. Someone comparing offices for the best neurosurgeon in San Diego can ask which method each surgeon would use on the cyst in question, and why. Asking who reads the follow-up scans, the surgeon or someone else on the team, saves confusion later.
Questions About Arachnoid Cyst Symptoms
Can an Arachnoid Cyst Go Away on Its Own?
Rarely. A few cysts shrink without treatment, mostly in young children. In adults, a cyst that looks the same on two or three scans usually stays that size for years. No medicine can dissolve an arachnoid cyst.
When Does a Cyst Need Emergency Care?
A sudden, severe headache, a first seizure, or new weakness on one side calls for emergency care right away. Bleeding inside or around a cyst is rare, though it can follow even a minor head injury. A headache that wakes someone at night or worsens when lying down should prompt a phone call to the doctor that week. Anyone with a known cyst who hits their head and then feels worse should get checked the same day.
What Should Someone Bring to a Neurosurgery Visit?
A week before the visit is the time to request older scans, since a surgeon needs them to measure growth. Every image should come along on a disc or online link, with the radiology reports and a dated symptom log. Asking each surgeon for a 12-month case count makes comparing neurosurgeons easier when two offices suggest different plans. A family member taking notes during the visit lets the person with the cyst listen.
Next Steps After a Cyst Shows Up on an MRI
Getting the cyst’s size and location in writing from the ordering doctor gives later scans a fixed starting point. A follow-up scan on that doctor’s schedule shows whether anything has changed. New headaches, balance trouble, or vision changes are a reason to call before that date. If the ordering doctor cannot answer questions about the cyst, a referral to a neurosurgeon is a reasonable next step.
Sources
National Institute of Neurological Disorders and Stroke. Arachnoid Cysts.
Cleveland Clinic. Arachnoid Cysts.
