Cauda Equina Mass
A cauda equina mass is an abnormal growth or lesion involving the cauda equina, the bundle of nerve roots that extends from the lower end of the spinal cord through the lumbar and sacral spinal canal. Because these nerves control sensation, muscle function, and bowel and bladder control in the lower body, a mass in this region can cause significant neurological symptoms. Cauda equina masses can include tumors (benign or malignant), cysts, infections, and other lesions. MRI is the primary imaging test used to identify and characterize these findings.
What is it?
The spinal cord ends in most adults at about the level of the first or second lumbar vertebra. Below this point, the spinal canal contains the cauda equina—Latin for “horse’s tail”—a bundle of nerve roots that travel downward to supply the lower abdomen, pelvis, and legs. These nerves are responsible for movement and sensation in the lower body, as well as bowel, bladder, and sexual function. A cauda equina mass is any abnormal lesion that develops within or compresses this bundle of nerves and can include a wide variety of underlying causes.
Cauda equina masses can be primary (arising from the nerves or surrounding tissues) or secondary (spread from elsewhere). The most common primary tumors in this region are schwannomas (arising from the nerve sheath), neurofibromas, ependymomas (especially the myxopapillary type, which has a particular affinity for the lower cauda equina), and meningiomas. Secondary causes include metastatic spread from cancers elsewhere in the body, lymphoma or leukemia infiltrating the nerve roots, and so-called “drop metastases” from brain or upper spinal tumors. Non-tumor causes that can produce a mass-like appearance include arachnoiditis, large nerve root cysts (Tarlov or perineural cysts), infections and abscesses, and, occasionally, large herniated discs or hematomas. Identifying the specific type of mass is important because management differs substantially between benign tumors, malignant tumors, and non-tumor conditions.
Symptoms vary depending on the size, location, growth rate, and type of mass. Common features include lower back pain that may be persistent and is sometimes worse at night, sciatica-like radiating leg pain, numbness or tingling in the legs, weakness, and gait or balance difficulty. As a mass enlarges, it may produce features of cauda equina syndrome: bilateral leg symptoms, “saddle anesthesia” (numbness in the inner thighs, perineum, or buttocks), urinary retention or incontinence, fecal incontinence, and sexual dysfunction. Cauda equina syndrome is a neurosurgical emergency requiring rapid evaluation and treatment to reduce the risk of permanent neurological injury.
MRI of the lumbar spine, generally with contrast, is the preferred imaging test for evaluating a suspected cauda equina mass. MRI provides detailed views of the nerve roots, surrounding cerebrospinal fluid, and any associated mass, and contrast enhancement helps characterize the lesion and detect smaller or multiple findings. CT and CT myelography may be used when MRI is contraindicated. Once a mass is identified, the imaging features—combined with the patient’s age, medical history, and other findings—help narrow the diagnosis. In some cases, additional MRI of the brain and entire spine, laboratory testing, or biopsy is needed to determine the specific cause.
Important to Know
Treatment depends entirely on the type of mass and the symptoms it is causing. Benign tumors such as schwannomas, neurofibromas, and meningiomas are often managed with surgical removal, particularly when they are causing symptoms; many patients experience significant improvement after surgery. Myxopapillary ependymomas are typically treated surgically as well, sometimes followed by radiation therapy. Metastatic disease and lymphoma involving the cauda equina are usually managed with a combination of systemic therapy (such as chemotherapy or targeted treatment), radiation, and, in select cases, surgery for decompression. Non-tumor causes such as infection, large cysts, or large disc herniations are managed according to their specific diagnoses.
When cauda equina syndrome develops, urgent surgical decompression is generally needed to relieve pressure on the nerves and reduce the risk of permanent loss of leg, bladder, bowel, or sexual function. Care is typically coordinated by a multidisciplinary team that may include neurosurgery, spine surgery, oncology, radiation oncology, and rehabilitation.
Red flag symptoms include sudden or rapidly worsening back or leg pain, severe new leg weakness or numbness, gait or balance difficulty, saddle-area numbness, new urinary retention or incontinence, fecal incontinence, sexual dysfunction, or unexplained weight loss. These symptoms require urgent medical evaluation, as they may indicate cauda equina syndrome or another serious underlying condition.