Of the 83 patients, 36 could not recall anything unusual about the injection and were not aware of the "electric shock sensation" that often accompanies nerve involvement. The other 47 patients either received a very painful injection or felt the electric shock sensation. From a symptomatic standpoint, 28 patients reported dysesthesia as their most troublesome symptom, while, in the other 55 patients, paresthesia or anesthesia as predominant.

To date, five patients have undergone surgical exploration of their injuries. The senior author (M.A.P.) operated on three of these patients. The other two were treated by different surgeons, and we obtained reports on their operations and spoke to the surgeons. In all five cases, there was no obvious damage to the nerve seen at surgery even when the fascicles were dissected, apart from a slight increase in adhesions around the nerve and a slightly whiter appearance of the nerve in the area of the injections compared with a creamier appearance elsewhere. Patients had not received nerve grafts or other nerve surgery, and none of the five patients benefited symptomatically from the surgery. In fact, two patients stated that their pain was worse after the surgery. In five patients, the symptoms of dysesthesia have taken on the characteristics of causalgia or sympathetically mediated pain with a deep, boring, burning pain and occasional flushing over the associated cheek, symptoms that have been reported previously.⁴ In six patients, the dysesthesia appeared to have spread proximally to involve other trigeminal nerve trunks, particularly the maxillary branch (Figure 1).

View larger version (134K): [in this window] [in a new window]   Figure 1. The area of altered sensation (anesthesia and dysesthesia) resulting from an inferior alveolar nerve block in two patients. In each case, the area increased over a one-year period after commencing on the lower lip.

Residual nerve involvement ranged from mild to complete anesthesia (in two cases) and, in all cases, the area outlined covered virtually the whole distribution of the affected nerve.

	DISCUSSION

TOP ABSTRACT SUBJECTS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
The incidence of nerve involvement from inferior alveolar nerve block is unknown, but it can be informally calculated in the following manner. First, current sales of local anesthetics for dentistry in the United States are approximately 161 million carpules per year (Richard D. Finkelman, D.D.S., Ph.D., Astra-Zeneca Corp., oral communication, 1999). In this study, 75 patients living in northern California (population 10.2 million) who had such nerve involvement were seen over a 48-month period beginning in August 1995 (an average of 18.75 patients per year). Northern California contains about 3.74 percent of the U.S. population (approximately 272,878,000 people). Using these figures for the basis of an informal estimate, and calculating that around half of all the local anesthetic carpules administered during that 48-month period were for inferior alveolar nerve blocks, this gives an estimated incidence of permanent nerve involvement of 1:160,571 inferior alveolar nerve blocks. This is around four times greater than previous studies have suggested.^2,3,7,8 Even so, it probably represents an underestimate, as it is doubtful that we in the University of California San Francisco Department of Oral and Maxillofacial Surgery are seeing all cases of this condition.

In terms of anecdotal evidence, the senior author kept a telephone log of calls received since our previous article on this subject was published.³ For each patient actually seen and examined who have this problem, telephone information has been received for five other patients who apparently have the same problem. If all these other patients were examined and found to be similarly affected, this would mean the incidence would be on the order of 1:26,762 inferior alveolar nerve blocks, which is within the range of blocks that the full-time practicing dentist might administer during a lifetime of work. Thus, perhaps every full-time practitioner will find that he or she has one patient during his or her career who has permanent nerve involvement resulting from an inferior alveolar nerve block. However, it is still doubtful that we are receiving information about all the patients with the condition, so the true incidence remains unknown.

In any patient who has had surgery and suffered nerve damage, we have to assume—given the present state of knowledge—that the damage was caused by the surgery. It is possible that in some of these cases, the nerve involvement was caused by the inferior alveolar nerve block and not the surgery, but there is no way of determining that.

In terms of incidence, there was no statistical difference between the right and left sides. However, the lingual nerve was affected 79 percent of the time; this may be because when the mouth is wide open, the lingual nerve is held tightly in the tissues and is unable to be deflected by the needle. Taste also was affected often, indicating involvement of the corda tympani. In the current study, when the type of local anesthetic was known and only a single agent was used, 48 percent of patients received lidocaine, 47 percent received prilocaine and 5 percent received mepivacaine. This corresponds with national sales figures that suggest that of all local anesthetics used by dentists in the United States, lidocaine is used 62 percent of the time, mepivacaine 23 percent of the time, prilocaine 13 percent of the time and a long-acting local anesthetic such as etidocaine or bupivacaine 2 percent of the time (Richard D. Finkelman, D.D.S., Ph.D., Astra-Zeneca Corp., oral communication, 1999).

Perhaps every full-time practitioner will find that he or she has one patient during his or her career who has permanent nerve involvement resulting from an inferior alveolar nerve block.

Although the numbers in this study are small, it does appear that prilocaine may be involved more frequently and mepivacaine may be involved less often than either agent’s rate of use nationwide (as gauged by sales) would indicate, though there may be regional differences in use of the different anesthetics. Other researchers also have noted a higher incidence of permanent nerve involvement with prilocaine.^8,9 All three manufacturers selling local anesthetics for dentistry in the United States (Astra-Zeneca Corp.; Cook-Waite, whose product is manufactured by Abbott Pharmaceuticals; and Novocal of Canada, which manufactures Septodont and its own name brands) were represented in this study. There were examples of nerve damage resulting from each relevant product; no products were exempt.

The difference in referral rates in women as opposed to men with nerve involvement caused by local anesthetic injections is hard to explain, when it would be assumed that the incidence of the condition would not have a gender bias. It is possible that some of the same etiologic factors are involved as in temporomandibular joint dysfunction, in which the incidence may be equal between the sexes but women more frequently seek care for the condition at a referral center.¹⁰ Alternatively, however, there have been some recent animal studies suggesting that nerves may respond differently to injury in female animals than in male animals.¹¹

The fact that several patients suffered from dysesthesia and even causalgia and sympathetically mediated pain is of obvious concern. The true incidence of this is unknown; furthermore, this study may well represent a self-selected group with a greater preponderance of such patients, as they are more likely to be referred for evaluation. Comparisons need to be made with the dysesthesia and causalgia rate associated with nerve involvement stemming from other sources, such as third-molar removal.^12,13 The fact that the area affected in all cases involved the whole area served by the nerve suggests that whatever process is involved affects the whole nerve and not just one or two fascicles.

Information gathered in a previous study¹³ showed that when nerve involvement resulted from tooth removal, eight of 95 patients (8 percent) complained primarily of dysesthesia, compared with 28 of 83 patients (34 percent) in the present study. The reasons for this difference are unclear. Haas and Lennon⁸ stated that 12 percent of their patients reported experiencing a burning feeling in addition to altered sensation.

The etiology of this condition remains uncertain. Direct trauma from the needle seems unlikely, because it is known that most cases of trauma resulting from needle contact resolve spontaneously.¹ It is difficult to envision how needle trauma can damage the whole nerve, which appeared to be the case with all patients.² Additionally, in the five patients who underwent surgery, no evidence of macrotrauma caused by a needle was observed.

Another prospective study by the senior author has been carried out in an attempt to evaluate the normal incidence of contact between a local anesthetic needle and the inferior alveolar or lingual nerve in the course of performing general dentistry. For the present study, the authors contacted local dentists via dental society meetings and personal contacts and gave them forms to complete prospectively, recording the number of inferior alveolar nerve blocks they gave each day and the number of presumed contacts between the local anesthetic needle and the inferior alveolar lingual nerve. A contact was presumed to have been made when the patient felt an electric shock sensation or stated that injection was particularly painful. They were to keep records for six months. Forms were returned by 42 dentists. The results are quite variable and to a certain extent reflect the workload of the dentists involved. However, it does appear that the average busy dentist administers around eight to 12 inferior alveolar nerve blocks per day and is made aware of a needle contact with a nerve somewhere between once every two weeks and once every eight weeks (anywhere between one in 80 and one in 320 inferior alveolar nerve blocks). This is a further indication that for the vast majority of inferior alveolar nerve blocks in which contact is made directly with the nerve, there are no subsequent long-term sequelae. Other studies have suggested that patients may feel the electric shock with between 3 percent¹ and 7 percent² of inferior alveolar nerve blocks.

Although potentially dangerous barbs can develop on the needle tip after contact with bone¹⁴ and therefore cause trauma on withdrawal or on second use, it appears that most of the nerves in the present study were involved before withdrawal of the needle and on first use of the needle.

Intraneural hematoma caused by the needle’s striking one of the smaller intraneural blood vessels remains a possible hypothesis, as it is known that intraneural hematomas are neurotoxic.¹⁵ Hematomas could explain the findings in the five patients who underwent surgery. There appeared to be some intraneural fibrosis among these patients, as well as extra-neural adhesions that could be the result of leakage of blood and blood products through the epineurium via the hole or holes created by the needle.

Chemical damage from the local anesthetic itself remains a possibility,^16–25 though technically it is difficult to envisage how local anesthetic could be injected directly into the nerve. Experimentally, it is extremely difficult to inject local anesthetic into a nerve because the epineurium will not stretch, and in this study, as in others, the lingual nerve is predominantly affected. Also, at the time the injection is given, the needle tip is well past the lingual nerve and is in approximation to the lingual plate of the mandible and closer to the inferior alveolar nerve (Figure 2). To inject local anesthetic directly into the lingual nerve (which is the nerve affected in more than 70 percent of cases), one would have to hold the needle 3 to 4 millimeters beneath the mucosa but well short of the mandible itself. This does not happen in practice. Nevertheless, it could be envisaged that were the needle to have transfixed the lingual nerve and then gone on to be injected normally adjacent to the mandible, a tiny amount of the local anesthetic could come in contact with the intraneural contents as the needle is withdrawn through the lingual nerve. This is because some local anesthetic would remain within the lumen of the needle and also may coat the outside of the needle. However, it appears that in most cases, the problem occurs before needle withdrawal.

View larger version (33K): [in this window] [in a new window]   Figure 2. Axial view of the mandible at the level of the lingula showing the relative positions of the inferior alveolar nerve, the lingual nerve and a needle administering an inferior alveolar nerve block. Note that the needle passes the lingual nerve some distance before it contacts bone.

Exploratory surgery has been unhelpful in patients with permanent nerve involvement and may even have exacerbated the symptoms.

Exploratory surgery has been unhelpful in these patients and may even have exacerbated the symptoms. If the cases explored in this study are representative, then no macrotrauma is seen, but possibly some intraneural fibrosis and slight increase in extraneural adhesions may occur. It is tempting to think that excision of the area of the nerve showing intraneural fibrosis and replacement with a graft of some kind may be helpful. However, in the senior author’s experience, it is extremely difficult to accurately delineate the area of nerve involved. In addition, the nerve’s position is deep to the medial pterygoid muscle (further proximal than a nerve that would be involved in a third-molar removal), and so it is virtually impossible to perform a satisfactory grafting procedure at the proximal anastomosis site without having to extensively mobilize the medial pterygoid muscle and even detach it. This is not justified in light of the relatively poor results that are obtained from nerve-grafting procedures and the morbidity that could result.

At the present time, the optimum treatment is unknown, but it probably should be medical. If dysesthesia is the prime concern, it should be managed by a pain management professional.

	CONCLUSION

TOP ABSTRACT SUBJECTS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
An inferior alveolar nerve block can cause occasional peripheral nerve damage. The exact mechanism is unknown and there is no known prevention or treatment.