A syrinx is a fluid-filled cyst that forms inside the spinal cord. It contains cerebrospinal fluid (CSF), the same watery liquid that normally surrounds and cushions the brain and spinal cord. When that fluid accumulates inside the cord tissue itself, it creates a cavity that can expand over time and damage the surrounding nerve fibers. The condition of having a syrinx in the spinal cord is called syringomyelia, and it affects roughly 5 to 8 people per 100,000.
Where a Syrinx Forms
Most syrinxes develop in the cervical spine (the neck region), though they can extend downward through the thoracic spine and beyond. The cyst expands within or near the central canal, a narrow channel that runs through the middle of the spinal cord. Because nerve fibers carrying pain and temperature signals cross right through this central area, those sensations are often the first to be disrupted.
In rare cases, a similar cavity forms in the brainstem rather than the spinal cord. This is called syringobulbia. Because the brainstem controls vital functions like swallowing and facial sensation, syringobulbia can cause a different set of problems, though the underlying process is similar.
What Causes a Syrinx to Develop
The most common cause is a Chiari malformation, a structural condition where the lower part of the brain (the cerebellar tonsils) extends down through the opening at the base of the skull. About 65% of people diagnosed with a Type 1 Chiari malformation also have a syrinx. The herniated brain tissue acts like a plug, blocking the normal flow of cerebrospinal fluid between the skull and the spinal canal.
With each heartbeat, the brain expands slightly as it fills with blood, and CSF is normally pushed downward into the spinal canal to accommodate that expansion. When the cerebellar tonsils block this pathway, the fluid has nowhere to go. The piston-like movement of the tonsils creates pressure waves that force CSF into the spinal cord’s central canal, gradually dilating it and forming the syrinx. Some researchers describe this as a “slosh” effect: fluid is essentially sucked into the cord during each cardiac cycle, and over time, the cavity grows.
Spinal cord injuries are another significant cause. MRI studies suggest that up to 22% of people with spinal cord injuries develop a syrinx, sometimes appearing years or even decades after the original trauma. Other causes include spinal tumors, tethered spinal cord, infections like meningitis, and spinal dysraphism (a group of birth defects affecting spinal development). In some cases, no clear cause is found, and the syrinx is labeled idiopathic.
Symptoms and How They Progress
A syrinx often develops slowly, and symptoms may not appear for months or years. The earliest and most common symptom is pain, typically a deep, aching, or burning sensation in the neck, shoulders, arms, or upper back. Because the expanding cyst first disrupts the nerve fibers that carry pain and temperature signals, many people notice they can no longer feel hot or cold in their hands or arms while their sense of touch remains intact. This “dissociated” sensory loss is a hallmark of the condition.
The pattern of numbness often follows a “cape-like” distribution across the shoulders, upper back, and arms, corresponding to the spinal levels where the syrinx sits. As the cyst enlarges, it can damage motor nerve fibers too, leading to muscle weakness, stiffness, and loss of coordination in the hands and legs. Some people develop progressive scoliosis (curvature of the spine), particularly when the syrinx forms during childhood.
In post-traumatic cases, symptoms may include loss of a previously present voiding reflex, changes in bowel function, or loss of erectile function. Motor weakness tends to be a later finding, appearing after sensory changes are already well established.
How a Syrinx Is Diagnosed
MRI is the primary tool for diagnosing a syrinx. On imaging, the cyst appears dark on one type of scan (T1-weighted) and bright on another (T2-weighted), matching the signal of fluid. To be considered a true syrinx rather than a mildly dilated central canal (which is a common, harmless finding), the cavity generally needs to measure at least 3 mm across on axial imaging.
Size and location provide clues about the cause. Syrinxes linked to Chiari malformations tend to be wide (averaging about 8 mm) and long, spanning roughly 8 spinal levels, with their upper end in the cervical spine. A syrinx wider than 5 mm that starts in the cervical spine has a 99% specificity for being Chiari-related. Idiopathic syrinxes, by contrast, are typically narrower (around 4 mm) and more likely to start lower in the spine. Post-traumatic syrinxes tend to be long, averaging about 10 spinal levels, and form near the site of the original injury.
Treatment Options
Not all syrinxes require surgery. Small, stable cysts that aren’t causing symptoms are often monitored with periodic MRI scans. Treatment becomes necessary when the syrinx is growing, symptoms are worsening, or neurological function is declining.
When a Chiari malformation is the underlying cause, the primary surgery is posterior fossa decompression. This involves removing a small piece of bone at the base of the skull (and sometimes the upper vertebra) to create more room for the cerebellar tonsils and restore normal CSF flow. By fixing the root obstruction, the syrinx often shrinks on its own over the following months. Long-term studies show surgical success rates around 84%, with most patients experiencing meaningful improvement in symptoms.
When decompression alone isn’t sufficient, or when the syrinx has a different cause, surgeons may place a shunt, a small tube that drains excess fluid from the syrinx cavity into another body space. Shunting directly reduces the cyst’s size, but shunts carry a higher risk of complications over time, including clogging and the need for revision surgery.
What Happens Without Treatment
An untreated, expanding syrinx can cause progressive and potentially permanent neurological damage. The nerve fibers destroyed by the growing cyst do not regenerate well, so deficits that develop before treatment tend to be harder to reverse than those caught early. Over time, complications can include significant motor weakness, chronic neuropathic pain, loss of bowel and bladder control, joint damage from loss of protective sensation (known as Charcot joints), autonomic instability (problems regulating blood pressure and heart rate), and an increased risk of falls and burns to areas that have lost sensation.
If the cyst extends upward into the brainstem, it can affect swallowing, facial sensation, and breathing. Bleeding into the syrinx, while uncommon, can cause a sudden worsening of symptoms. These risks underscore why regular monitoring matters even for syrinxes that appear stable, since progression can be gradual and difficult to notice without imaging.