Abstract
Low back pain is a common reason for patients to visit their GP. Following initial assessment, the GP is often presented with a dilemma of whether to get an x-ray or not. Imaging is often requested to reassure both the patient and the clinician that there is no serious underlying cause. The purpose of this article is to explain the merits of the common imaging modalities and enable the clinician to request imaging appropriately, with knowledge of the limitations of each technique. See curriculum statement below.
The GP curriculum and imaging of low back pain
GP
Describe when blood tests and imaging methods are required for diagnosis of musculoskeletal problems and how to interpret them and how they influence management
Avoid investigations or treatments that are unlikely to alter outcomes in musculoskeletal problems so that availability of these resources is increased (e.g. imaging methods).
The problem with low back pain is that there are myriad causes. These range from poor posture and osteoarthritis to more serious pathology such as osteoporotic fracture, infection, ankylosing spondylitis and metastatic malignancy. The primary method for discriminating between these causes is history and examination. Consideration of history and examination lies outside the scope of this article but an article in the June 2008 issue of this journal covers this in more detail (Davies, 2008).
The first decision to make regarding imaging is whether it is required at all. This decision should be made knowing what the investigation will and will not be able to demonstrate. X-ray and magnetic resonance imaging (MRI) are the most commonly performed imaging studies of the lumbar spine. National Institute for Health and Clinical Excellence (NICE) guidelines state that patients with non-specific back pain should not routinely be imaged (NICE, 2009). However, these patients do require proactive management including advice and possibly drug treatments. Other treatments, such as exercise and manual therapy should be considered. Symptoms of non-specific back pain persisting for more than a year, despite these measures, may warrant referral to spinal surgeons for consideration of spinal fusion.
If the patient has sciatica or other radicular symptoms, i.e. pain, numbness or weakness relating to a dermatome or myotome, this may be due to compression or irritation of a nerve root by disc material or osteophyte. If these symptoms do not improve with conservative treatment, then imaging should be considered. Patients with signs of more serious spinal pathology, indicated by red flag symptoms (see Box 1), should be referred urgently to secondary care or directly for urgent imaging.
Red flags for possible serious spinal pathology
Presentation less than age 20 or onset over age 55 years
Constant, progressive non-mechanical pain
Thoracic pain
History of carcinoma
Systemic steroids
Drug abuse and human immunodeficiency virus (HIV)
Systemically unwell
Weight loss
Persisting severe restriction of lumbar flexion
Cauda equina syndrome/widespread neurological disorder
Lumbar spine x-ray
A typical lumbar spine x-ray series usually involves an antero posterior (AP) view and a lateral view. The ionizing radiation dose for these views (combined dose of 3.70 mSv) is typically equivalent to 60 chest x-rays or almost 18 months of background radiation.
Performing an x-ray in patients with non-specific back pain has not been shown to improve outcome. This is because they will often appear normal. Moreover, even if the x-ray shows that the patient has, for example, changes of osteoarthritis, it will not affect patient management. If the patient has radicular symptoms, a definite cause for the symptoms will be missed. The patient could have a large disc prolapse and the plain film would be normal. Equally, it is not possible to say whether an osteophyte on the plain film is causing nerve root compression or not.
Osteoporotic fracture and metastases will usually be apparent on plain films and so it may be appropriate to perform plain x-ray if you suspect these pathologies. However, particularly with suspected malignancy, a normal x-ray should be treated with scepticism and diagnosis should be kept under review. A nuclear medicine bone scan may be more appropriate in this situation.
Another pathology that can be diagnosed on plain x-ray is ankylosing spondylitis. However, MRI of the sacroiliac joints and lumbar spine will also show this pathology without the radiation burden. MRI is also more sensitive for early inflammatory changes associated with ankylosing spondylitis.
Magnetic resonance imaging
Many radiology departments allow direct referral for MRI by GPs. MRI does not involve any ionizing radiation. However, the patient needs to be able to lie very still and fat for at least 20 minutes.
Contraindications to MRI include claustrophobia, a pacemaker, some mechanical heart valves, intracranial clips and intra-orbital metallic splinters. Joint prostheses are not a contraindication but, if in doubt, check with your local radiology department.
MRI is excellent for assessing the bone and soft tissues. Unlike plain x-ray, disc pathology and any resultant compression of nerve roots or spinal cord can be seen. Metastases are also more readily visible on MRI than on plain film. When filling in a request for MRI, it is important to document the site of pain and, particularly if there are radicular symptoms, to state the side and dermatomes involved. This will enable the radiologist to tailor the report so that it addresses the clinical symptoms. If you are uncertain as to whether MRI is indicated, it is worth speaking to a radiologist.
Key points
If there are clinical signs (red flags) of a serious spinal pathology, then the patient should be referred urgently to secondary care for further investigation or, if appropriate, directly for MRI
Patients with non-specific back pain do not need any imaging unless they are being referred for spinal fusion
Plain x-ray is suitable for suspected osteoporotic fracture and spinal metastases
If the patient has associated radicular symptoms, then plain film will not help and MRI is more appropriate.