What is a diagnostic local anesthetic block?

A local anaesthetic is a drug that causes a temporary loss of pain sensation to an area of the body (numbness). In the case of a diagnostic local anaesthetic, it is injected under the skin, around a nerve, or into a joint, to help a clinician make (or confirm) a diagnosis.

A diagnostic local anaesthetic can be given to help a clinician reach a diagnosis when the clinical examination and / or imaging (e.g. x-ray, MRI or ultrasound) is inconclusive. It may also be given if you have had a treatment that has failed to alleviate your symptoms and the diagnosis remains uncertain. It can help distinguish between different structures in the foot/ankle that may be causing your pain.    

What can I expect from the injection?

As with any injection, there will be a sharp scratch and some stinging as the drug is injected. This is usually over quickly and eases as soon as the injection is over and the drug starts to take effect. An injection like this should only take a few seconds or so to perform

The area that has been injected will begin to go numb after several minutes (or sometimes up to 30 minutes later depending on which drug is used). The length of time the numbness lasts is dependent on the type of anaesthetic that has been injected. In general, this can vary from around an hour or two, up to eight hours or more. The local anaesthetic is always temporary and will wear off.

You will still be able to walk and move your foot. You may still feel sensations such as touch, pressure and temperature.

What happens once the local anaesthetic starts to work?

You should not drive after the injection, a member of the Foot and Ankle team will advise how long you should not drive for. Besides not driving, it is important that you put your foot to the test once the local anaesthetic starts to work. You should carry on with your usual daily activities or try and do activities that would usually bring the pain on. The aim is to see if the pain goes away with the local anaesthetic in place.

It can be useful to keep a written diary of how things feel during the day. Write down things such as:

• How long did the pain go away for, if at all?
• If pain was still present, was it slightly better, the same or worse than usual?
• Once the local anaesthetic wore off, how did things feel? Better, worse or the same as before?

What are the risks of a diagnostic local anaesthetic block?

• An injection may cause some very minor bleeding and/or bruising. This may be more likely to happen if you have a bleeding disorder or take drugs that thin the blood. There may be some localized swelling where the injection is placed.
• Some people can be prone to fainting during an injection.
• Having an injection poses a very small risk of infection due to the needle entering the skin. Your skin will be cleaned before the injection and a small plaster applied afterwards to minimize this risk.
• There is a risk that the injection doesn’t give the clinician any further answers as to the diagnosis. In which case, further imaging or tests may be undertaken or a second opinion sort from a colleague.
• There is a small risk that the injection may exacerbate (make worse) your symptoms. This would usually be temporary and may be due to bruising and irritation caused by the injection itself.
• Some people have an allergic reaction (anaphylaxis) to local anaesthetic. This is rare but can be very serious.
• Some people have a toxic reaction to local anaesthetic. This is when too much anaesthetic is injected or it is injected into a blood vessel. This is unlikely to happen with the volumes used for a diagnostic local anaesthetic in the foot or ankle and steps are taken by the clinician to ensure they do not inject into a blood vessel.

Important points to remember

If you get pain relief from the injection, it will almost certainly be temporary. The aim is to use local anaesthetic to diagnose and not treat your problem at this stage.

Contact details

If you have any issue after your injection please contact the foot and ankle secretary on:

0121 685 4357
0121 685 4169