The confusion around "diagnostic" nerve blocks for chronic pain

Diagnostic blocks	Where does the pain come from?
Prognostic blocks	Will a subsequent treatment be effective?
Test blocks	Is the nerve involved in the patient's pain?

"Where does my pain come from?"

When patients present with pain at the hip, neck, low back, or other body parts, they would like to know where the pain comes from. Does it stem from a joint, a ligament, a nerve, or any other structure? Similarly, healthy care professionals are interested in this information to establish a treatment plan.

However, it is frequently challenging to identify the primary injury based on the location of the pain. For instance, "referred pain" can confuse the diagnosis (learn more here). Clinical examination and imaging can contribute to the identification of a lesion that causes the pain, but they are frequently insufficient to establish a valid diagnosis. For instance, tenderness to pressure can mislead the diagnosis (learn more here). Imaging can be of little help (learn more here).

Nerve blocks are commonly performed for the diagnosis of chronic pain conditions. Are they helpful?

Let's consider shoulder pain

Pain at the shoulder can originate from different structures, such as the cavity of the joint ("glenohumeral joint"), tendons ("rotator cuff"), or muscles, among others (see figure below).

Shoulder joint. Source: NIH - NIAMS.

The suprascapular nerve is one of the nerves that supply the shoulder joint (picture below, showing an ultrasound image). If anesthetizing the nerve produces pain relief, we would assume that the pain originates from the tissues supplied by the nerve. However, the suprascapular nerve supplies not only the glenohumeral joint, but also the acromioclavicular joint, the supraspinatus and infraspinatus muscles. Therefore, when a patient becomes pain-free after the injection, we will not be sure about the precise origin of the pain. Strictly speaking, this block cannot be considered "diagnostic".

These considerations apply to most blocks commonly performed. Because most nerves supply multiple tissues, the anesthetic block of the nerves lack "face validity" to be considered diagnostic.

Ultrasound image of the suprascapular nerve at the neck, as it departs from the 5th cervical spinal nerve (C5). Siegenthaler A, Moriggl B, Mlekusch S, Schliessbach J, Haug M, Curatolo M, Eichenberger U. Ultrasound-guided suprascapular nerve block, description of a novel supraclavicular approach. Reg Anesth Pain Med. 2012;37:325-8.

We should rather use the term "test block"

In a book chapter (1) and review article (2) on nerve blocks, Nikolai Bogduk and I distinguish between diagnostic, prognostic, and test blocks (see table at the top of this page). We call blocks that are not diagnostic "test blocks". A test block aims to evaluate whether a nerve is involved or not in the transmission of the patient's pain. Since the nerve supplies multiple tissues, the pain signal may arise from any tissues supplied by the nerve, or from the nerve itself ("neuropathic pain"). The latter can happen after a nerve injury or entrapment.

Are test blocks useful?

Test blocks are potentially useful if they guide treatment decisions. Not uncommonly, a test block is used to determine the indication of a subsequent interventional or surgical treatment. For instance, considering the case of suprascapular nerve blocks, positive responses may encourage physicians to offer the implant of a peripheral nerve stimulator to treat the pain, or "therapeutic nerve blocks", which anecdotally may lead to prolonged pain relief. However, in this case, the block would fall into the category of "prognostic blocks". For blocks to be considered "prognostic", a number of criteria have to be fulfilled. There are very few nerve blocks that have been shown to have prognostic value. This topic will be the objective of a future blog.

In certain instances, physicians may use test blocks to narrow down the number of possible diagnoses. In this case, the information gained by test blocks does not lead per se to a diagnosis, but is used in association with other elements, such as the history, the physical examination, and imaging, to identify potential sources of pain. This approach can be useful if the result of the block is expected to influence the management plan. Unfortunately, there is virtually no research on the contribution of test blocks to the diagnostic process and the outcome of treatments. Therefore their clinical usefulness remains unclear.

I recommend:

• Ask yourself how you are using the results of a block to determine a treatment plan, before offering it to patients. What would you do differently if you perform or not the block?

• If a patient has complete pain relief after the block, do not tell them that you know where the pain comes from. Instead, use the terms "likely" or "possibly".

• Patients who become pain-free after a test block develop hopes. For some of them, this is the first time they feel no pain at all. Support the hope, but stress that the pain will come back, and this is one step of a process that will "hopefully" lead to a better management of their pain.

• Pain is not merely the result of the transmission of an input from an injured tissue to the brain. Very few treatments on nerves can lead to substantial reduction of pain and improvement of function. Make sure you address cognitive and emotional components, and implement strategies to improve other relevant outcomes such as physical function and sleep. Explain patient that this will require their collaboration, and a multi-dimensional approach.

References

(1) M. Curatolo, N. Bogduk. Diagnostic and therapeutic nerve blocks. In: Bonica’s Management of Pain, 5th edition, Edited by J.C. Ballantyne, J.P. Rathmell and S.M. Fishman. Wolters Kluwer, Philadelphia, USA, 2019, pg. 1595-1610.

(2) M. Curatolo, N. Bogduk. Diagnostic blocks for chronic pain. Scandinavian Journal of Pain 2010; 1: 186-192.