Can selective nerve root blocks help diagnose patients who need lower back surgery? A systematic review

It can be difficult to diagnose the exact cause of lower back and leg pain. Finding out whether a patient is experiencing lumbar radiculopathy – pain from a nerve that connects the back to the leg, also known as a ‘pinched nerve’, – or a different type of back pain is crucial when planning treatment.

Nerve roots exit the spinal cord and divide into nerves that travel to the arms and legs. Some patients with radiculopathy may benefit from spinal surgery to relieve pain and reduce disability. In most patients, nerve root pain is diagnosed through symptoms and scans. But these tests aren’t always reliable, so diagnosis can be difficult.

When patients have an uncertain diagnosis, doctors often use an additional test, called a selective nerve root block (SNRB), to help decide whether surgery will be effective. An SNRB involves injecting a local anaesthetic around one or more nerves in the lower back. The patient’s responses can help confirm if one of the numbed nerves is the source of the symptoms and whether they are likely to benefit from surgery.

What we did

We conducted a systematic review of all the studies on this topic to summarise the evidence on how accurate SNRB is in identifying which patients with radiculopathy are likely to benefit from spinal surgery.

There is no ‘gold standard’ method to diagnose the exact cause of lower back and leg pain. This makes it difficult to determine whether SNRB gives the correct diagnosis, because there is no test to compare it to. Instead, a range of alternatives have been used by researchers to decide whether the SNRB test results were correct. For example, you can compare the SNRB test results to what the surgeon finds during back surgery or to the success rate of the surgery. We included all studies using any of these approaches. We only combined the data of studies that compared the same things.

What we found and what this means

We found six studies to include in our review. In two studies, patients were followed up for several months after surgery, to see if it resolved their back and leg pain.

One study compared SNRB with observations made during surgery and another used both the observations during surgery and in the months after.

The remaining two studies compared the pain-killing effect of SNRB at the nerve root that was suspected of causing the pain, with the pain-killing effect in an unaffected ‘healthy’ nerve root. If SNRB is accurate, you would not expect a patient to experience a reduction in pain when injecting a healthy nerve root.

The results from each study were quite varied. Some concluded that SNRB was an accurate method, whereas others found the opposite. When pooling the results of the studies that used findings from during surgery, we found that SNRB correctly identified nine in 10 patients with a pinched nerve. However, SNRB was not accurate in excluding patients without pinched nerves: only half of the patients without pinched nerves were correctly identified.

When we pooled the results of the studies that followed up their patients to see if surgery had been successful, we found similar results. Although SNRB could correctly identify nine in 10 patients with a pinched nerve, the test was even less reliable for excluding patients without pinched nerves: only one in five patients was correctly identified.

We assessed the quality of the studies and rated all studies as low, which means that the results should be interpreted with caution.

What next?

Even though diagnostic SNRB is already used in clinics, there is only a handful of studies – all low scientific quality – that investigate the accuracy of this test. The evidence available shows that SNRB is not an accurate or reliable way to identify the patients who will benefit from back surgery. We are working to ensure our findings will inform clinical practice in this area.