Leg pain in a patient with back pain is often assumed to be “sciatica”. Distinguishing somatic referred pain from radicular pain and radiculopathy changes the likely source, the role of imaging, and which interventions may be appropriate.
Definitions
- Somatic referred pain
- Pain perceived in a region remote from its source, arising from stimulation of nerve endings in a somatic structure such as a zygapophysial (facet) joint, intervertebral disc or sacroiliac joint. It does not involve stimulation of a nerve root.
- Radicular pain
- Pain evoked by ectopic discharges from an affected dorsal root or its ganglion, most commonly associated with disc herniation and inflammation of the nerve root.
- Radiculopathy
- Objective loss of neurological function (sensory loss, weakness or reflex change) due to conduction block in a spinal nerve or its roots. It may occur with or without radicular pain.
Clinical features
| Somatic referred pain | Radicular pain | |
|---|---|---|
| Quality | Deep, dull, aching; hard to localise | Shooting, lancinating, electric |
| Distribution | Diffuse; buttock and thigh most common; not dermatomal | Narrow band travelling along the limb, commonly below the knee |
| Neurological signs | Absent | May be present if there is coexisting radiculopathy |
| Common sources | Facet joints, disc, sacroiliac joint | Disc herniation; foraminal or lateral recess stenosis |
Pain distribution alone is an imperfect guide, and referral patterns overlap. Distal extent of pain is not a reliable discriminator on its own; the quality of pain and the presence of neurological signs add useful information.
Why the distinction matters
- Investigation: imaging is generally reserved for suspected serious pathology, progressive or significant neurological deficit, or when a procedure or surgery is being considered. Imaging findings such as disc bulges and facet degeneration are common in asymptomatic people and must be correlated clinically.
- Interventions: epidural steroid injections are directed at radicular pain; the evidence does not support their use for non-radicular back pain. Diagnostic medial branch blocks and radiofrequency neurotomy are considered for suspected facet-mediated (somatic) pain in carefully selected patients.
- Medicines: gabapentinoids are frequently prescribed for leg pain, but a large randomised trial (PRECISE) found pregabalin no more effective than placebo for sciatica, with more adverse effects. Australian and UK guidance advise against routine use of anticonvulsants for low back pain and sciatica.
Red flags
Assess for features suggesting serious pathology, including cauda equina syndrome (urinary retention or incontinence, saddle anaesthesia, bilateral or progressive neurological deficit), malignancy, infection, fracture and inflammatory disease. These warrant urgent investigation and referral according to their nature.
Referral considerations
- Persistent radicular pain despite conservative care, where an epidural injection is being considered.
- Suspected facet- or sacroiliac-mediated pain with significant impact on function, after an adequate trial of active management.
- Diagnostic uncertainty, complex medication use (including opioids), or psychosocial complexity.
- Progressive neurological deficit or suspected surgical pathology should be directed to an urgent surgical pathway.
Helpful referral information: what to include in a referral.