Why perform medial branch blocks?

Medial branch blocks are used to diagnose facet joint pain. Facet joint pain typically presents with low back pain with, or without referred pain into the leg or neck pain on rotation and extension. Studies have shown that facet joints can be damaged in accidents (e.g. motor vehicle accidents), and that these changes in the facet joints may not show up on X-ray, CT scan or MRI scanning.

The only way to determine whether the facet joints are the cause of pain is to inject the joint or its nerve supply with local anaesthetic. If pain is substantially eliminated while the anaesthetic is acting, it is assumed that the facet joint is the source of pain. This injection process is known as a block (the aim is to block out the pain). The nerve supply to the facet joint is via the medial branches of the adjacent dorsal rami (branches of nerves), which originate from the spinal nerve. Thus, blocks of the nerve supply to the facet joints are called medial branch blocks.

What do the results mean?

Studies show that a successful block means that there is a 70 per cent chance that the diagnosis is facet joint pain. That means, false positive blocks (blocks that incorrectly diagnose the facet joints to be causing the patient’s symptoms) do occur. A second positive block using other anaesthetic agents increases the diagnostic confidence to 90 per cent.

A positive medial branch block suggests that treatment targeting the nerve supply to the facet joint can potentially provide long-term pain relief. This treatment is known as radiofrequency neurotomy (RFN), or radiofrequency denervation. This treatment applies heat lesions to the nerve.

Scoring your pain

The major reason for the injection is to determine whether pain is altered in the six hours after the injection. Therefore, it’s important to have a method for recording the pain score. A commonly used pain scale rates pain from 0 to 10. When represented in a linear fashion on a chart, it’s called a Visual Analog Scale (VAS). Zero (0/10) indicates no pain while 10/10 indicates pain that is almost unattainable. At 10/10 a person can’t think, move, or function at all. Each person needs to know what their VAS is and apply it to each area of pain. As the aim of the block is to see if pain is altered, it’s essential that pain measures at least a 4/10 on the day of the procedure. Thus, patients shouldn’t take pain medication on the day of the injection.

What does the procedure involve?

It’s normal to feel anxious about the injection. However, the injections are very safe, generally not too painful, and can be done quickly. The injection itself consists of placing a needle through the skin under X-ray or CT guidance. Generally, only one or two facet joint levels are investigated at a time to ensure the diagnosis is specific to a level in the lumbar spine. You should be driven home after the procedure.

If there is no relief following the first session then it may be necessary to investigate other levels, which may require further injections. Your doctor will determine if other injections should be done. If there is still no relief then the conclusion is the facet joints are not the cause of the pain.

You may need to describe separately different pains that you experience. For example, you may have low back pain and leg pain, and these two pains should be described separately. For instance, the injection may totally relieve the low back pain, but may not help the leg pain. This is important information for your doctor to assess when making recommendations regarding further treatment.