Some of the main complications that can occur are listed below:
- Failure to produce a block - missing the nerve can happen if the block is done "blind". Using a nerve stimulator improves the chances of success, and reduces the risk of damaging the nerve with the needle. Obesity is a definite cause of failure as sometimes we don't have needles long enough to reach the necessary location. Sometimes unusual anatomy can defeat even the most experienced operator.
- Infection - can be prevented by taking normal sterile precautions. Nerve blocks shouldn't be done in the presence of local skin / tissue infection at the site of the injection. Although an infection may be far away from the site of the proposed nerve block, bacteria can still spread via the blood stream to the site of the block and cause an infection. My personal practice therefore is not to perform nerve blocks at all in the presence of infection anywhere in the body. There may however be some medical professionals who disagree with this opinion.
- Bleeding - the block needle may have to pass through vascular tissues like muscles, or close to major blood vessels, to get to the necessary nerve. If the coagulation status is abnormal, this may produce local bruising. Severe bleeding around a nerve after the procedure can cause nerve compression with permanent nerve injury. If you are taking blood thinners (aspirin, Comadin, heparin, Plavix etc), or have been found to have a bleeding tendency, then your doctor should be informed prior to performing the block.
- Intravascular injection can occur when the nerve is anatomically close to a vein or an artery. Large doses of local anaesthetic injected quickly into these vessels causes grand mal seizures, brain stem depression affecting the respiratory and cardiovascular centres, and cardiac arrest. It is important therefore that all injections are performed slowly, and that the upper dose limit is adhered to, to help reduce this risk. The safety profile of the local anaesthetics is (safest first) Lignocaine > Prilocaine> Levobupivacaine > Bupivacaine. Levobupivacaine is fast replacing bupivacaine in hospital practice due to it's improved safety profile. Bupivacaine intravenously can cause DC shock resistant ventricular fibrillation requiring the use of intravenous mexiletine in doses up to 500 mg.
- Unexpected spread to other nerves can occur following a nerve block. Mostly this is an irritating side effect that wears off with time without any further action required. Occasionally it may have more serious consequences, and therefore the operator has to be prepared for such an occurrence and take appropriate action. The reasons why local anaesthetic spreads to other nerves are outlined below:-
- Needle slightly out of position during injection - e.g. a femoral nerve block may develop after performing an iliohypogastric block in the groin, causing unexpected leg weakness and a longer stay in hospital whilst it wears off. This happens because the local anaesthetic is injected too deeply under the transversus abdominis muscle.
- High volumes of local anaesthetic may track along tissue planes to reach other nerves close by - e.g. phrenic and recurrent laryngeal block may develop after an interscalene brachial plexus block in the neck causing paralysis of one side of the diaphragm and hoarseness respectively. Diaphragmatic paralysis can be a problem in those with critical chest disease, but not in normal adults. Recurrent laryngeal block can cause food and liquids to be inhaled into the trachea (windpipe), and therefore the patient must fast afterwards until it wears off.
- Close anatomical relationship - if one or more nerves is anatomically close to the one being blocked, whatever you do both nerves end up being blocked - e.g. the recurrent laryngeal nerve is always blocked at the same time as the cervical sympathetic chain during a stellate ganglion block.
- Prolonged block - sometimes a block lasts longer than expected. This can be due to:-
- Too high a dose - sitting it out for a few hours usually sees everything returning to normal again.
- Pre-existing nerve injury - in this situation the effects of the block may last days or weeks with persisting numbness and weakness. I would suggest that blocks should be avoided in the presence of pre-existing nerve injury.
- Excessive vasoconstrictor - this causes severe vasoconstriction with oxygen starvation to the nerve and subsequent permanent damage. Vasoconstrictors should not be used near end arteries (e.g. fingers, penis) due to the risk of producing gangrene.
- Faulty positioning after the block can cause excessive pressure on a nerve whilst the area is numb. Prolonged pressure causes oxygen starvation and permanent nerve injury. The part to be blocked should always be protected / padded / positioned properly afterwards to prevent this happening.
Intra-neuronal injection - injecting the local anaesthetic directly into the nerve, rather than around it, causes injury to the nerve directly proportional to the volume of local injected. This complication can be avoided by the proper use of a nerve stimulator which finds the nerve electrically using a small current passed directly through the needle tip.
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