Synovial Cyst in the Lumbar Spine

A synovial cyst is a fluid-filled sac that develops in the spine due to degeneration. 

The cyst itself is benign, but it can cause symptoms ranging from mild discomfort to severe neurological complications requiring immediate medical attention.

Many people with a synovial cyst develop all the symptoms of stenosis of the spine, which can include pain and/or neurological symptoms.   For others, the level of pain or discomfort may be minimal and remain stable for many years.

Key Characteristics of Synovial Cysts

• Typically found in the lumbar spine, most commonly at the L4-L5 level
• Often associated with degenerative changes in the spine
• Most common in adults older than 60 years
• May lead to symptoms such as lower back pain, leg pain (sciatica), and/or leg numbness, tingling, and muscle weakness
• May remain stable for years without worsening
• While rare, can cause severe neurological issues like cauda equina syndrome

How a Synovial Cyst in the Lower Back Develops

Synovial cysts develop as a result of degeneration in the facet joint in the lumbar spine. 

The facet joints are small stabilizing joints in the back of the spine that allow motion while providing support for the spine. 

These are synovial joints that are covered with:

• Cartilage, which provides a smooth, slippery surface that reduces friction during movement.
• Synovium, a thin membrane that produces lubricating fluid within the joint.

As the cartilage wears down due to disc degeneration, osteoarthritis, or repetitive stress, the body responds by producing excess synovial fluid. This excess fluid can escape through small tears in the joint capsule, forming a synovial cyst. Over time, the cyst may grow larger, occupying space in the spinal canal and potentially compressing or irritating the nearby spinal nerve root.

The pain is thought to stem from the venous blood around the nerves not being able to drain, which leads to pain and irritation of the nerves. Sitting down allows the blood to drain and relieves the pressure.

Because the fluid does not build up under a lot of pressure it does not usually cause severe problems. Because of this, serious problems such as neurological deficits or cauda equina syndrome (loss of bowel and bladder control) are rare even for very large cysts.

Symptoms of a Synovial Cyst

The patient's symptoms will depend largely on the cyst's size, position, and how it is impacting the surrounding neural structures.

Common symptoms include one or a combination of:

• Leg pain (sciatica), which may occur in one or both legs and may radiate down the back of the leg and into the foot.
• Pain worsening with standing or walking, as the spinal canal narrows in an upright position, increasing nerve compression.
• Neurological symptoms, such as numbness, tingling, muscle weakness, or difficulty lifting the foot (foot drop).
• Neurogenic claudication, a painful, cramping sensation in the legs that worsens with activity.
• Lower back pain, which may be mild or absent.
• Muscle spasm,  The lower back muscles may spasm in response to the stress, and low back muscle spasms can be surprisingly painful.

Typically, there is minimal or no pain if the patient is seated, because in the seated position the spinal canal opens up and there is not as much pressure on the spinal nerves. When standing up straight or walking, however, the spinal canal narrows and creates more pressure on the nerves.

A cyst only develops the lumbar spine, and it almost always develops at the L4-L5 level (rarely at L3-L4).

It is common for symptoms from a lumbar synovial cyst to remain stable and not progress for long periods of time.

Rare But Serious Complications

In very rare cases, a synovial cyst can lead to cauda equina syndrome, which causes severe nerve compression. Symptoms of this medical emergency include:

• Loss of bowel or bladder control (incontinence)
• Severe numbness in the groin area (saddle anesthesia)
• Progressive weakness in the legs

Immediate surgical intervention is required to prevent permanent nerve damage.

Diagnosis of Synovial Cysts

A diagnosis that a synovial cyst is the cause of a patient’s symptoms is confirmed through imaging tests.

An MRI scan is the gold standard to diagnose a synovial cyst, and the image shows a fluid-filled lesion with high signal intensity (similar to water).

X-rays are also important, including flexion/extension motion x-rays, to check for spinal instability.  Often facet joint degeneration is accompanied by degenerative spondylolisthesis, in which one vertebral body slips forward over the one below it.  This indicates that the joint is unstable and incompetent.

It is very important to identify any instability before surgery for the synovial cyst, because if the instability is not addressed at the time of surgery, a revision surgery will often be required at a later date.