Complications in Local Anesthesia

Q: Are local anesthetics safe?

A: Local anesthetics are the safest, most effective drugs in all of medicine for the prevention and management of pain. In dentistry, in order to achieve pain control, local anesthetics are injected into the patient’s tissue using a needle attached to a syringe. Anytime a needle enters the patient’s body, injury is possible. Specifically in dentistry, since the local anesthetic needs to be deposited as close as possible to a nerve without actually touching it, the risk is elevated. Usually, the biggest problem in dentistry is when a nerve cannot be found, such as missing an inferior alveolar (IA) nerve block. On occasion, the needle may come into contact with the nerve, which can lead to complications following the injection.

Paresthesia is an unfortunate yet sometimes unavoidable complication associated with local anesthesia administration. As a result, paresthesia is a common cause of legal actions against dental professionals.

Definition

Q: What is paresthesia?

A: Paresthesia (or persistent anesthesia) is any alteration in sensation following a local anesthetic injection that persists more than 24 hours after the injection was administered. For example, when a patient leaves the dental office 1 hour after receiving an injection, the patient is still numb because the anesthetic is continuing to work in a normal manner. After 6 or 7 hours, the patient will most likely report that the numb sensation is gone. However, as people respond differently, some will experience anesthesia that lasts longer. I try not to label a patient as having a paresthesia in less than 24 hours after the initial injection. If after 24 hours, the patient continues to experience residual loss of sensation, I would diagnose this as a paresthesia.

Q: Is paresthesia ever permanent?

A: It is extremely rare for a patient receiving local anesthesia in a dental office to never lose it, meaning that the area remains as numb as it was after the injection was given. More commonly patients remark: “A little area on the tip of my tongue on the right side is numb” or “The lateral border of my tongue on the right side still feels funny.” Or perhaps their sense of taste has diminished. One of the reasons for reduced sense of taste is that the chorda tympani nerve, which supplies the sensation of taste, is located within in the lingual nerve.

Most paresthesias resolve within 10-14 days, but I always advise patients that it may take up to 6 months. Virtually all paresthesias that are transient resolve within 6 months. On rare occasion, a paresthesia may be permanent.

Q: What causes paresthesia?

A: The most likely cause of paresthesia is trauma from the needle, ie, direct needle contact with the nerve. The most commonly reported area where paresthesia occurs is in the mandible. Within mandibular paresthesia, the majority involve the lingual nerve and a smaller number involve the IA nerve.

With IA nerve involvement, a patient will complain of some residual loss of sensation in his or her lower lip, perhaps on the buccal gingiva, and also potentially on the chin. With lingual nerve paresthesia or injury, a patient will complain of a loss of sensation on part of his or her tongue.

The mandible is the most common site of reported paresthesia because the IA nerve block is the most used injection technique. The lingual nerve is situated in proximity to the needle insertion site, located 5 mm or 6 mm below the surface mucosa. As we cannot see where our needle is going, it is impossible to prevent needle-nerve contact 100% of the time.

Most dental professionals who give a lot of injections experience the occasional patient who jumps as the needle is advanced through the soft tissue during an IA nerve block. The typical response is to immediately pull the needle out because it’s unclear what has happened. When asked what happened, the patient’s response is usually, “I felt an electric shock on my tongue.” Essentially, the needle contacted the nerve. This contact does not always cause paresthesia but if contact occurs and the patient does experience paresthesia, then the cause is clear: direct contact trauma from the needle.

Other complications

Another possible cause of paresthesia is a hematoma. When a needle goes through soft tissue, there are small blood vessels in the area. Small blood vessels also exist within the nerve. A needle contacting a blood vessel ruptures it and blood flows into the area. Blood in contact with the nerve irritates it and can produce paresthesia. The area of the IA nerve block is quite vascular so this is a possible cause of paresthesia.

A hematoma usually resolves within 10-14 days as the blood is resorbed. What normally happens when the hematoma is the cause of paresthesia is that the patient will report a gradual return of sensation over about 10-14 days.

Edema is another possible cause of paresthesia. This happens more often following surgical extraction of third molars but any edema or tissue injury can produce paresthesia. A paresthesia that arises following a surgical procedure or a traumatic dental procedure, resolving within 4-7 days is likely to have been caused by edema.

Currently, there is a buzz in dental circles that 4% local anesthetics may be associated with a higher incidence of paresthesia than 3% or 2% drugs. I personally remain unconvinced that this is the case. In my opinion, at the present time, there is no substantial scientific evidence that supports the statement that 4% anesthetics present a higher risk of paresthesia than other drugs.

Q: What is trismus?

A: Trismus is a spasm of the muscles of mastication where opening of the mouth becomes restricted. It is most commonly seen following an IA nerve block. Since an IA nerve block will almost always cause soreness because the needle is inserted through through muscle, causing an injury to the tissue by the needle. Since an IA nerve block injures the muscles of mastication (specifically the internal pterygoid muscle), the patient constantly irritates the tissue when talking and eating.

Trismus is more common when dental professionals have difficulty getting an IA nerve block and require multiple cartridges of local anesthesia to succeed. The tissues have been injured multiple times and larger volumes of fluid have been deposited each time, stretching the tissue. The patient does not notice it while in the dental chair due to numbness but when the patient wakes up the following morning after 6-8 hours of not using the mastication muscles, the muscles go into spasm. Gradually over the next day or so, the patient becomes able to open his or mouth a little more and eventually maybe within 3 or 4 days, trismus is gone. Management includes using a warm, moist compress on the area, but it just takes time to go away.

Q: Can these types of complications be prevented?

A: It is very difficult to prevent all local anesthesia-associated complications because the dental professional must deliver the injection without being able to see where the needle is going. Many nerve fibers are found in what is called a neurovascular bundle. For example, a nerve, an artery, and a vein commonly are found together. When trying to deposit local anesthetic as close to the nerve as possible, this could be right where the vein or the artery is located. This can happen with any intraoral injection but the only hematoma that is visible extraorally occurs following the posterior superior alveolar (PSA) nerve block. In a PSA nerve block, over-insertion of the needle might place it within the pterygoid plexus of veins, which may be punctured, resulting in bleeding into the area right behind the back of the maxilla and the pterigomaxillary space. Initially, the blood is located quite deep in the tissue spaces. As it’s not superficial, you don’t see it, but the cheek of the patient will become swollen and puffy as the blood forces itself laterally. If this occurs, apply pressure right away on the patient’s face by the temporomandibular joint, preferably with ice since it is a vasoconstrictor. Inform the patient that the blood is still in the tissue and that a bruise will appear on his or her cheek. Over the next 7-10 days, gravity will shift the bruise down toward the chin. This complication is not always preventable, but the use of a short needle during a PSA injection can greatly reduce its incidence. The recommendation for the PSA nerve block is a 27 gauge short needle. The most common reason for a hematoma following a PSA is overinsertion of the needle, which is much more common when a long needle is used for the injection.

From Dimensions of Dental Hygiene. October 2006;4(10): 28-33.