Back pain is a common, self-limited condition in many people, often without an identified cause. Cauda equina syndrome, discitis and cancer are also causes of back pain which are not so innocent and self-limiting. Describe these conditions and suggest some questions a physician might ask to help detect the diagnoses?

Cauda Equina Syndrome


The cauda equina consists of the nerve roots that branch off of the lower end of the spinal cord (the so-called conus). The cauda equina includes the nerve roots from L1 through S5. These roots, although branched off the cord itself at roughly the L1 vertebral level, remain within the central canal until they exit at their appropriate neural foramen.


The nerve roots branching off the spinal canal beneath the termination of the spinal cord are said to resemble a horse's tail, hence the name cauda equina (Figure 1).


Figure 1: the cauda equina. The neural elements are shown in yellow. The region of the spinal cord (SC), ending at L2 or so, is identified by the green bracked, and that of the cauda equina (CE) in blue.

Cauda equina syndrome occurs when the nerves of the cauda equina are compressed by a space-occupying lesion. The most common such lesions are a massive herniated disc, tumor, epidural abscess, or bony protrusion.


Signs and symptoms

The compression of multiple nerve roots often leads to weakness of voluntary muscles, anal/urethral sphincter weakness, and bladder muscle dysfunction. This presents as lower extremity weakness, bowel/bladder dysfunction including incontinence, and urinary retention with overflow incontinence and increased post-void residual.


Patients may also present with sexual dysfunction, saddle anesthesia, bilateral leg pain, and bilateral absence of ankle reflexes. While back pain is often the symptom that brings patients in for evaluation, not all instances of cauda equina syndrome are necessarily associated with pain.


Diagnosis and treatment

Diagnosis of cauda equina syndrome is usually confirmed by an MRI (Figure 2) or CT scan to identify the compressive lesion. Treatment typically involves urgent surgical decompression.

Figure 2: MRI of lumbar spine with epidural abscess causing cauda equina syndrome. (Credit:,_causing_cauda_equina_syndrome.jpg)


Discitis is an infection in the intervertebral disc space. (The “-itis” suffix suggests an inflammatory, not infectious condition, but infectious it is: “pyogenic spondylitis” might be a better term but nobody seems to use that.)


Bacteria can reach the spine by hematogenous (blood-borne) spread from distant sites of infection (e.g. endocarditis), contiguous spread from adjacent infections (e.g. vertebral osteomyelitis), or direct inoculation from trauma, surgery or injection (e.g. epidural).


Staphylococcus aureus accounts for more than half of the cases of discitis in the United States.


Discitis is often seen in people who inject street drugs and those who are immunocompromised.


Signs and symptoms

Symptoms from discitis include severe back pain that is exacerbated by physical activity and is often reproduced on palpation of the spinous process. This pain can often lead to difficulty with movement or complete immobility. Fevers and chills may be present in some patients as well.


Diagnosis and treatment

The diagnosis of discitis is typically confirmed by MRI (Figure 3). Treatment includes antibiotics tailored to the underlying infectious agent and utilization of a back brace to restrict mobility. A bone biopsy of the affected tissue, with subsequent culture analysis, can inform the appropriate antibiotic regimen needed for treatment. Surgical debridement or removal of hardware in the affected area may be indicated if antibiotic treatment is ineffective.

Figure 3: Discitis of the lumbar spine on MRI. (Case courtesy of Radswiki,, rID: 11366

Common Malignancies Found in the Spine


Primary spinal cancers are rare. A spinal tumor is likely a metastasis. The spine is the third most common site for cancer metastases after the lung and the liver.


The most common primary cancers that spread to the spine are breast, lung, thyroid, kidney, and prostate cancer. A helpful mnemonic to remember these common sites of spinal metastasis is “a BLT with a Kosher Pickle (Breast, Lung, Thyroid, Kidney, and Prostate).”


Concern for spinal metastases should be raised in individuals that have a history of cancer or risk factors thereof (e.g. an extensive smoking history). That is, the primary cancer may not be known; a metastasis may be the initial presenting sign.


Signs and symptoms

Patients typically present with generalized back pain. Unlike the pain from degenerative conditions or a sprain/strain injury, this pain is often worse at night. Weight loss, chills, and night sweats are other associated findings.


Diagnosis and treatment

Detection of malignancy in the spine warrants prompt workup to identify the primary source, given the rarity of primary spinal cancer itself. MRIs and bone scans (Figure 4) are often utilized to discover primary tumor sites and the overall burden of metastatic bone disease, respectively.

Figure 4: Bone scan showing evidence of metastases (Case courtesy of Dr Bruno Di Muzio,, rID: 53413)

There are likely many questions an experienced physician might ask to help identify worrisome causes of back pain.  In general, one asks about the character of the patient’s presenting symptoms, the timing of symptoms, inciting and palliating factors, and the presence of “red flag” symptoms, such as unintended weight loss, fevers, night sweats, pain at night, and weakness.


For students, other good questions to recall include the following:

  • For cauda equina: “When did you last urinate?” Bowel function varies a lot between individuals; questions about sexual function might be awkward or irrelevant. But everybody must urinate at least 30 cc/hr and most people will empty their bladders when 240 ccs are in them, hence most people urinate at least once every 8 hours. A lack of urination in the past 8 hours is thus a troubling but informative history.
  • For discitis: “Are you at risk for discitis?”--though this is not the exact wording. Rather, ask “do you abuse IV drugs? Have you been diagnosed with any disease? (You will be looking for those which could cause compromise of the immune system).
  • For cancer: “Have you ever been diagnosed with cancer?” (And the related question: “Do you smoke?)