ABSTRACT: High velocity motor vehicle accidents are associated with an increase in mortality rates and a significant number of facial injuries. Accidental deployment of airbags and the associated release of hot gases can result in both thermal and mechanical injuries. The more commonly reported

maxillofacial injuries include temporomandibular joint fractures and dislocations, (1) dental trauma, (2) facial nerve paralysis, (3) and other orofacial pain complaints. The following case report describes a patient with facial trauma from the accidental deployment of an airbag resulting in complaints consistent with a neurological injury for which quantitative sensory testing was used in confirming the diagnosis.

The introduction of automotive seatbelts and airbags has resulted in a substantial reduction of facial injuries sustained during motor vehicle accidents from one in 40 to only one in 449. (4) However, as with any modern technology, there are inherent disadvantages, accidental deployment of airbags being one.

Accidental deployment of airbags often results in injuries. The more commonly reported injuries include upper extremity injuries, (5) cervical extension-flexion injuries, (6) carotid artery dissection, (7) venous injuries, injuries to the larynx, esophagus, eye, cranium, temporomandibular joint (TMJ), (8) and maxillofacial injuries. Isolated cases of facial nerve paresis are hypothesized to occur due to traumatic neuropraxia. (9) Disarticulation of the ossicular chain of the inner ear as a result of traumatic fracture, shattering of the vestibule around the oval window, and facial nerve injury can also result in facial nerve paralysis, loss of vestibular function, and total hearing loss. (3)

Patients with facial trauma often present with orofacial pain complaints. Pain complaints are usually either of musculoskeletal or neuropathic origin. Musculoskeletal injuries include various myalgic complaints and/or injuries to the TMJ. In cases of trauma to the face, an additional etiology can include trauma to sensory nerves resulting in both neuropathic pain and a loss of sensation. Until recently, there was no accepted method to objectively assess the results of such injuries. However, in cases of neuropathic pain, use of Quantitative Sensory Testing (QST), which employs heat, electrical, or other stimuli, may help validate the patient's complaints by randomly administering null stimuli during the period of testing and monitoring patient response.

Assessment of sensory nerve damage presents as a diagnostic challenge to clinicians. Lack of a gold standard to assess nerve damage compounds the problem. Traditional clinical methods employed often lack sensitivity to detect subtle cases of nerve damage. QST is increasingly being used to detect and quantify all stages of neuropathy, ranging from hyperesthesia to hypoesthesia to anesthesia. Hyperesthesia is defined as increased sensitivity to stimulation, excluding the special senses. Hypoesthesia is defined as decreased sensitivity to stimulation, excluding the special senses.

QST is basically an extension of methods used for the traditional neurological examination of sensory function. These methods are derived from psychophysical principles. An assessment of somatosensory function is typically undertaken by testing the function of primary afferent nerves using various implements such as a brush or cotton swab for the sensation of touch, a warm object, a cold object, and a pin for testing pain. (9)

QST is used to assess the function of primary afferent nerve fibers (A[beta], A[delta], C fibers). A[delta] and C fibers mediate painful stimuli, and the A[beta] fibers mediate nonpainful stimuli. QST uses electrical, mechanical, and vibratory stimuli to test the thickly myelinated A[beta] fibers, cold detection thresholds to test the thinly myelinated A[delta] fibers, and heat detection threshold for assessment of unmyelinated C fibers. (10)

QST methods can generally be classified into two main schemes:

1. Reaction time inclusive methods, such as the method of limits in which a patient signals detection of increasing stimulus or when the decreasing signal is no longer felt. This method is less accurate and more variable partly due to variation in the subject's reaction time.

2. Constant stimuli methods, such as the method of levels, in which a specific stimulus intensity is delivered and the patient signals on detection of pain from the stimulus. (11)

In general, the method of levels is considered more reliable. In our testing, we used an adaptive method called the staircase method in which the response is used to modify future stimulation. (12)

Elevated electrical detection threshold is indicative of nerve damage, whereas decreased detection threshold is indicative of perineural inflammation. Elevated detection threshold to heat, mechanical, and electrical stimuli is seen in cases of mechanical nerve damage or total nerve transection.

The information gained by QST thus provides a tool for systematic analysis of sensory function. These objective quantitative and sensory tests aid in the diagnosis and localization of damaged areas and in clinical decision-making (treatment planning, monitoring treatment efficacy, and medico-legal evaluation). In this case, QST was used to confirm the diagnosis.

Case Report

A 43-year-old female presented to the Orofacial Pain Clinic at the University of Medicine and Dentistry of New Jersey-New Jersey Dental School with chief complaints of numbness and a tingling sensation in her left chin. The anesthesia and paresthesia extended from the angle of the left side of her mouth to the midline of the lip and chin, consistent with the distribution of the mental nerve. Paresthesia was reported at the outer borders of the central area of total anesthesia (Figure 1). Paresthesia is defined as an abnormal sensation, whether spontaneous or evoked.

[FIGURE 1 OMITTED]

The patient reported that these complaints had been present for approximately two months. She stated they were constant and exaggerated by cold air. She reported that she was not aware of these symptoms during sleep. She explained that this problem began after air bags spontaneously deployed while she was sitting in her parked car. The deploying air bags struck her with a diagonal blow across the left side of her face with a major portion of the impact concentrated on the left side of her chin. She was taken to the emergency room at a local hospital where she was treated for first and second degree burns on her face (Figures 2 and 3), probably caused by the hot gases associated with the deployment of the exploding airbag. After two weeks, the burns began to heal, but the patient stated she was left with a numb and tingling sensation. The numbness subsequently worsened over the following two months.

[FIGURES 2-3 OMITTED]

The patient's medical history was positive for Diabetes Type 1. Although she has had diabetes since childhood, there is no history of diabetic neuropathies or any similar complaints prior to this accident. Her diabetes is well controlled by Lantus (Sanofi-aventis US, Bridgewater, NJ) and Novalog (Novo Nordisk US, Princeton, NJ).

Objective findings included loss of sensation to both light touch and pin prick in the affected area. This altered sensation was located in a small circular region below the left angle of the mouth on the left chin. Masticatory musculature and TMJ palpation were negative. An intraoral examination found no abnormalities in the teeth and soft tissues. A full mouth series of radiographs was negative for dental pathology.

Ancillary Testing

Technique and Instrumentation for QST

In order to better understand and objectively measure the patient's subjective complaints of altered sensation, electrical and heat detection thresholds were performed.

Electrical stimuli were delivered with Neurometer CPT (Neurotron, Inc., Baltimore, MD), a transcutaneous electrical stimulator. This neurosensory testing device delivers sinusoidal electrical stimuli via surface electrodes at frequencies of 5Hz, 250Hz, 2000Hz, and at a current intensity range of 0.01 to 9.99 mili-amperes. Two facial sites were selected for testing, each in the traumatized and contralateral sides. They were in the territory of the infraorbital nerve (the skin overlying maxillary sinus and the lateral aspect of nose [ION]) and in the mental nerve territory on the skin overlying the chin [MNT]. The higher frequency currents (250Hz, 2000Hz) activate myelinated nerve fibers, while the low frequency (5 Hz) activates thin unmyelinated nerve fibers.

The patient was asked to identify the presence or absence of stimuli using the following protocol. After an initial tentative threshold was determined for detection of a stimulus, subsequent stimuli were presented that varied around the presumed detection threshold to confirm threshold stability and replicability. To prevent anticipation on the part of the patient, results were verified with placebo stimulation. The placebo stimulation was given by turning off all current without informing the patient and presenting absent stimuli. Three detection thresholds were evaluated for each location and the mean calculated. Results were compared to known normal standards.

Heat detection thresholds were assessed with the Medoc TSA (Medoc Ltd., Advanced Medical Systems, Ramat Yishai, IS) using a 15x15-mm water-cooled peltier probe (Figures 4 and 5). Detection thresholds were assessed in the ION and MNT territory using the staircase method, an adaptive method, based on the method of levels where a stimulus of predetermined intensity is given and the response is used to modify intensity of future stimulation.

[FIGURES 4-5 OMITTED]

The test results, however, could be influenced by some factors such as the patient's reaction time, the rate of change of stimuli and electrode size. As suggested by the American Academy of Neurology, the lab environment, instructions to subjects, motivation of subjects, and physical characteristics of subjects may all influence testing results. In this case, variations in results were minimized by using fixed electrode size, a consistently quiet lab environment, maintaining the same method of testing, follow-up by the same investigator, and a clear set of instructions to the patient to try to maintain reproducible results.

Interpretation of QST Results

The manufacturer suggests the following as the normal values for the electrical:

2000Hz--minimum 40, maximum 138, mean 73

250Hz--minimum 11, maximum 53, mean 10

5Hz--minimum 5, maximum 40, mean 8.9

However, studies (13) show that there are conflicting and inconsistent results with electrical stimulation. Therefore, inter-patient variability can be reduced by using the healthy side as a control for the pathology side.

First Visit: Objective findings of this test confirmed the patient's complaints of altered sensation. Sensory testing done (Table 1) in this manner revealed hyposensitivity with respect to the large diameter myelinated fibers, thinly myelinated fibers, and the unmyelinated fibers in the left mental and left infraorbital nerve region. Based on the history, clinical findings, and sensory testing, it was concluded that the patient developed complaints of diminished sensation due to mental nerve trauma.

Follow-up: The patient continued to report no improvement over one year after the accident. A review of the literature finds that in cases where regeneration does not occur after one year, 70-90% of the nerve undergoes degeneration. (14) In cases of complete nerve transection, if regeneration does not occur in twelve weeks, there is a minimal possibility for regeneration. (15) The sensory testing one year after the incident showed bilateral hyposensitivity in the mental nerve territory (Table 2). Based on the results of further quantitative sensory testing, which was followed for a period of over one year, it was concluded that the nerve damage is permanent.

Discussion

Airbag induced injuries can be classified into five general types of injury, (16-18) abrasions (68.6%), contusions (37.8%), laceration (18.2%), burns (7.8%), and fractures (3.2%). Burn injuries secondary to air bag deployment have been reported in the literature. (19-25) Burn injuries are further classified into thermal (direct/indirect), chemical (alkaline corrosive/particulate matter) and friction. (18)

Thermal burns can result in direct burns from high temperature gases or may result in indirect burns from melting clothing.

Chemical burns result from a highly corrosive alkaline aerosol formed from the by-products of combustion containing sodium hydroxide, sodium carbonate and other metallic oxides. Some of the gases generated by the deployment of airbags are found to be toxic to certain organs and probably neural tissue.

Friction burns are primarily the result of high velocity fabric impact from the force of inflation. Other forms of blunt facial trauma resultant from motor vehicle accidents are also common.

The patient presented with complaints of altered sensation following both a thermal burn and blunt trauma to the face. We hypothesize that a combination of both the burn and blunt trauma resulted in nerve damage causing her symptoms. Her medical history, which is positive for Diabetes Type 1, may have also increased her predisposition towards developing neuropathy. Her subjective complaints were confirmed by quantitative sensory testing. The use of null stimulus in this instance insured the avoidance of purposeful manipulation of stimuli by the patient. QST can thus be used in conjunction with psychophysical methods to validate and quantify neuropathic pain complaints in patients involved in medico-legal cases.

However, owing to the complex multidimensional aspect of pain, currently QST is adjunctive to the diagnosis of various pain syndromes and can add a further dimension to pain evaluation.

Conclusion

Until recently, there was no reliable technique to clinically measure or objectively assess a patient's complaints of loss or distortion of sensory perception. This case represents a common form of orofacial neuropathic pain seen following motor vehicle accidents. In this case, the use of quantitative sensory testing provided a reliable and reproducible adjunctive diagnostic test that can objectively assess the region affected and the extent of injury.

In cases of loss of, or distorted sensation, the use of quantitative sensory testing represents a rational approach to quantifying orofacial neuropathic injuries.

Acknowledgements

The authors would like to acknowledge Riva E. Touger-Decker, Ph.D., Program Director, Graduate Programs in Clinical Nutrition, Director, Institute for Nutrition Interventions, Clinical Associate Professor, Director Division of Nutrition, New Jersey Dental School for her help and support.

Manuscript received December 27, 2005; revised manuscript received June 20, 2006; accepted June 26, 2006

References

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Mythili Kalladka, B.D.S.; Archana Viswanath, B.D.S.; Julyana Gomes, D.D.S.; Eli Eliav, D.M.D., M.Sc., Ph.D.; Richard Pertes, D.D.S.; Gary Heir, D.M.D.

Dr. Mythili Kalladka received her dental degree from Government Dental College, Bangalore, India in December 2002. She has completed a Fellowship in Orofacial Pain and is currently pursuing her Masters in Dental Science in orofacial pain at the University of Medicine and Dentistry of New Jersey. She is currently the chief resident of the Orofacial Pain Center.

Dr. Archana Viswanath graduated in 2000 with a B.D.S. degree from K.L.E. Dental College, Bangalore, India. She has completed a Fellowship in Orofacial Pain and is currently pursuing her Masters in Dental Science in orofacial pain at the University of Medicine and Dentistry of New Jersey.

Dr, Julyana G. Gomes graduated in 2001 with a D.D.S. degree from the Universidade Gama Filho Dental School, Rio de Janeiro, Brazil. She is currently pursuing her Masters in Dental Science in orofacial pain at the University of Medicine and Dentistry of New Jersey. She is currently the first year chief resident of the Orofacial Pain Center.

Dr. Eli Eliav is the Carmel Endowed Chair in Algesiology in UMDNJ-New Jersey Dental School. He also serves as a reviewer for "Pain", "Brain Research", "Journal of the American Dental Association" and is the 'Oral Medicine" section editor for Quintessence International. He has published numerous manuscripts in prestigious journals and lectures extensively at conferences and universities around the globe.

Dr. Richard A. Pertes received his dental degree from New York University College of Dentistry and earned a Certificate of Training in Orthodontics from Columbia University School of Dental and Oral Surgery. Dr. Pertes is a Clinical Professor in the Division of Oral Medicine, Department of Diagnostic Sciences at UMDNJ-New Jersey Dental School. He is also Director of Fellowship Program in Orofacial Pain. For many years, he served as Clinical Director and then Director of the TMD/Orofacial Pain Center at New Jersey Dental School. He is a Diplomate of the American Board of Orofacial Pain and has published several articles and textbook chapters.

Dr. Gary M. Heir is a clinical professor at the Division of Diagnostic Sciences and Orofacial Pain, UMDNJ-New Jersey Dental School. Dr. Heir is the past president of the American Academy of Orofacial Pain, diplomate and president of the American Board of Orofacial Pain and past chair of its examination board. Dr. Heir serves as a reviewer for several professional journals and has published numerous articles and authored several book chapters.

Address for correspondence: Dr. Gary M. Heir Room D-881 Dept. of Diagnostic Sciences UMDNJ-New Jersey Dental School Newark, NJ 07101 E-mail: heirgm@umdnj.edu

Table 1

Sensory Testing Three Months Following the Airbag Injury

Electrical
Detection      5 Hz      5 Hz        250 Hz
threshold    C fiber   C fber    A[delta] fiber

Site          Right     Left         Right
ION            5-15     45-55        10-20
Mental        15-25     35-45        40-50

Electrical
Detection      250 Hz          2000 Hz         2000 Hz
threshold    A[delta] fiber   A[beta] fiber   A[beta] fiber

Site              Left            Right           Left
ION              85-95           155-165         195-205
Mental           85-95           235-245         235-245

Table 2

Sensory Testing One Year Following the Airbag Injury

Electrical
detection     5 Hz      5 Hz         250 Hz
threshold    C fiber   C fiber   A[delta] fiber

Site          Right     Left         Right
ION           15-25     65-75        40-50
Mental       105-115    95-105      130-140

Electrical
detection        250 Hz          2000 Hz         2000 Hz
threshold    A[delta] fiber   A[beta] fiber   A[beta] fiber

Site              Left            Right           Left
ION              70-80           155-165         175-185
Mental          175-185          235-245         295-305

Heat
detection
threshold    Heat (C fibers)   Heat (C fibers)

Site              Right             Left
Mental            32.60             35.17
ION               32.20             32.50