What symptoms associated with acute low back pain could potentially warrant emergent management?

When presented with acute low back pain, one of the chief concerns is neurologic involvement. It is important to ask questions related to leg strength and bowel/bladder function to assess for damage of the spinal cord or the cauda equina. Patients may note leg weakness or difficulty walking. Damage to the sacral segments can lead to saddle paresthesia, urinary retention, or bowel/bladder incontinence. Diagnostic imaging, ideally with magnetic resonance imaging (MRI), can help identify the cause and determine whether urgent neurosurgical consultation is necessary for immediate interventional management. An acute spinal cord injury or cauda equina syndrome may be the result of severe spinal stenosis, severe herniated vertebral disc, cyst, abscess, or neoplasm. In the event one of these other causes is identified, appropriate management should be initiated.

What other red flags should be elicited on the history?

After a cauda equina syndrome has been excluded, acute low back pain can be divided into three groups: nonspecific low back pain, back pain related to radiculopathy or spinal stenosis, and an alternative cause. The third group may present with a pattern of symptoms consistent with a specific diagnosis, which are commonly referred to as red flags because the pathology can be particularly concerning. Possibilities include an infectious, neoplastic, or inflammatory cause. Questions regarding the presence of fevers, history of infection with human immunodeficiency virus (HIV), use of immunosuppressant medications, or intravenous (IV) drug abuse suggest the possibility of an infection. Recent unintentional changes in weight, increased fatigue, and pain worse at night can be suggestive of a spinal tumor. A younger individual with increased pain and stiffness in the morning that improves through the day could be presenting with a spondyloarthropathy. Recent trauma or fall, especially in an older individual with a history of osteoporosis, can result in a new vertebral fracture, associated back pain, and possible neurologic damage. Because no single symptom can definitively confirm the presence of a diagnosis, it is important to perform a thorough history and exam to form a complete picture of the possible diagnosis.

What specific symptom description can help differentiate between a referred pain and radiculopathy?

Pain that occurs outside of the original location is considered referred pain. Although it typically emanates from the original source, it can also occur as a new discomfort in a separate location that begins about the same time. A common example is a myocardial infarction with pain that extends into the left shoulder, down the left arm, or into the back. When evaluating low back pain with associated symptoms, it can be difficult separating referred pain from a radiculopathy, which is a separate injury caused by damage to a nerve root as it exits the vertebral column.

One way to delineate the two is by asking questions regarding the specific course of the radiation. Pain that travels past the knee and into the foot is typically associated with a radiculopathy. If it extends down into the medial aspect of the lower leg, there is suspicion for L4 nerve root involvement. Pain that goes down the lateral aspect of the thigh and leg and ends along the outside of the foot suggests L5 involvement. If the pain runs along the posterior thigh and extends down to the heel, it may be due to an S1 nerve root.

However, pain that does not extend beyond the knee does not automatically rule out a radiculopathy. Damage to the L2, L3, and L4 nerve roots can cause pain to radiate from the low back anteriorly into the groin, anterior thigh, or knee. Involvement of the S2, S3, and S4 roots may remain proximal by extending into the sacral or gluteal regions and potentially end in the perineal area.

How can sensation and muscle function contribute to the diagnosis?

Dermatomes and myotomes refer to associations between a specific area of skin or muscle groups innervated by a single nerve root or set of nerve roots. Numerous maps have been created, though due to variations in development from one person to the next there are variations between different versions. Although not always an exact representation of nerve root involvement, deficits in sensation and muscle strength elicited on a physical exam can yield clues to the involved nerve root of a suspected radiculopathy. Decreased sensation over the medial knee can suggest involvement of the L4 nerve root, the lateral malleolus with L5, and the heel with S1.

Common myotomes of the lower limbs are L2 with hip flexion, L3 with knee extension, L4 with ankle dorsiflexion, L5 with extension of the great toe, and S1 with ankle plantar flexion. Despite these generalities, each action is typically the result of multiple muscles. For example, hip flexion can occur via contraction of the iliopsoas or rectus femoris portion of the quadriceps and both are innervated by L2, L3, and L4. The tensor fascia latae (TFL) also contributes to hip flexion and is innervated by levels L4, L5, and S1. Weakness in hip flexion can be a result of any one or multitude of levels. Damage to one level may not cause detectable weakness on exam because the remaining muscles can make up for the loss. Thus, weakness in an action can prompt suspicion of nerve injury, but intact strength does not necessarily rule it out.

What special exam maneuvers are good for diagnosing acute radiculopathy?

When suspecting the presence of an acute radiculopathy, there are several special exam maneuvers that aid in diagnosis. The straight leg raise (SLR or Lasègue’s sign) is most common. To perform it, the patient lies supine and the examiner passively raises the affected leg while the knee remains fully extended. When the pain is first reproduced, the angle of the leg compared to the horizontal is noted. Although the angle range is debatable, a positive finding is generally between 30 and 70 degrees. Outside of this range, the pain can be attributed to another cause such as hamstring or gluteal muscle tightness. This maneuver can be repeated on the unaffected leg, which is referred to as the contralateral SLR. If it causes pain in the affected side, the test is considered positive. Unlike most exam findings that have little associated research or low reliability, studies have resulted in general acceptance of the SLR and contralateral SLR. A Cochrane review found the former to have 90% sensitivity and the latter 90% specificity when diagnosing a radiculopathy.

The purpose of these tests is to cause increased tension or stretch on the damaged nerve and reproduction of the symptom. Other tests have been developed with similar intent, and any positive findings have been collectively referred to as neural root tension signs. Examples include the sitting SLR where the knee is extended while the patient is sitting at the edge of an exam table. Dorsiflexion of the ankle while the leg is extended during the SLR adds further tension. The slump sign is the observation of a patient slumping forward during a sitting SLR in an attempt to decrease tension on the nerve root. The flip test is when the patient changes posture from a forward leaning position to a rearward position when the leg is raised during a sitting SLR. Unlike the supine SLR and contralateral SLR, these and other tests like them have little evidence to support their reliability. Thus, an isolated positive sign may not strongly indicate the cause of the symptoms, whereas a collection of positive signs that correlate with this patient’s chief complaint can help narrow down diagnosis.