Unilateral masseter muscle hypertrophy: A case report

Roberto Trujillo, JrVFlávia Noemy Gasparini Kiatake Fontâo, MSVSimone Maria Galvào de Sousa, MS,

Masseteric hypertrophy is a benign increase in the size of the masseter muscle, secondary to muscle tiy-pertroptiy. It produces facial asymmetry and is important in the ditferential diagnosis of other entities. Theaim ot this articie is to report a case of uniiaterai masseteric muscie hypertrophy. The characteristics anddiagnostic features of such alterations and options for treatment wiii be presented. (QuintessencG Int2002:33:776-779)

Key words: asymmetry, hypertrophy, masseter muscle

M asseteric hypertrophy is a benign increase in thesize of the masseter muscle that may affect one

or both sides of the face. This relatively rare alterationoften affects young people. Although the etiology iscontroversial, it has been associated with genetic pre-disposition, bruxism, clenching, temporomandihulardisorders (TMD), and psychological reactions.Zachariades et al' speculated that a vascular lesionmay gradually subside to a residual muscular hypertro-phy. They reported two cases in which this occurred.

Sensitivity has been reported, but the change in ap-pearance is the most frequent complaint of patients withmasseteric hypertrophy. The panoramic radiograph isthe most practical examination to complement the clini-cal diagnosis. When the physical examinafion suggestsmasseteric hypertrophy, an increased angle of the jawregion may be observed. ' The secondary enlargementof the mandibular angle is a result of the functional re-modeling that occurs at the muscle insertion sites.'*

Computed tomography (CT), magnetic resonanceimaging (MRI), and sonography also have been usedto evaluate masses in the huccomasseteric

'Graduate Student, Scfiool of Dentistry, Sacred iHeart University, Bauru,Sao Paulo, Brazil,

^Professor. Department of Oral Radiology, Sohooi oí Dentistry. SacreflHeart tJniversity, Bauru. Sao Paulo. Brazil

'Professor, Depaitment of Oral Pathology, School of Dentistry, SacredHeart Uniuersity. Bauru, Sao Pauio, Brazil,

Reprint requests: Profa Dra Simone Maria Galvào de Sousa, Disci-plina de Patología Bucal. Facjidade de Odontologia, Universidade doSagrado Coraçâo, Rua iimâ Arminda, 10-50, 17044-160 Bauru, Sao Paulo,Brazil, E-niail:srrisousa@usc.t!r

Because of their high resolution and good delineation,CT and MRI are useful in demonstrating the extentand location of buccomasseteric masses. Yonetsu et alreviewed the diagnostic images of pathologic condi-tions in this area. In their series of 66 cases, therewere 60 benign lesions; of these 28 were myopathies.The most common pathologic changes of the bucco-masseteric region were masseteric hypertrophy (22),which was followed by atrophy of the masseter muscle(6). Although the images obtained from those exami-nation modalities are important to establish the diag-nosis, clinical correlation is essential.

The treatment of masseteric hypertrophy rangesfrom conservative to invasive therapies. Occlusal ad-justment, relation therapy, spasmolytics, tranquilizers,and antidepressant therapy are some examples of con-servative treatment. In some cases, mainly when stressis involved, psychological followup may be required inassociation with other treatment.

However, conservative therapy is often not effec-tive. Smyth' and Moore and Wood"* suggested an alter-native therapy using the intramuscular injection ofhotulinum toxin type A. When applied in small doses,the toxin will decrease muscle activity. These injec-tions produce localized paralysis by blocking the re-lease of acetylcholine at the neuromuscular junctionwithout producing undesirable systemic effects.

Capra et al evaluated the effects of intramuscular in-jections of botulinum toxin type A on the ultrastructuralchanges in the masseter muscle in monlîeys to provide abetter understanding of the morphologic response ofthis tissue to such treatment. Despite the major changesin masseter muscle fibers after botulinum injections, the

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Fig la (ieft) Clinicai appearance of a 16-year-old boy with pro-gressive sweiling ol the right side of the tace.

Fig Tb Close-up view of the mandible

neuromuscular junctions seemed to maintain tbeirstructural integrity. Moreover, the eftect in tbe massetermuscle may ultimately be completely reversible.

Studying five patients. To et aP used ultrasound andelectromyograpby to prospecfively evaluate the eiïectof botulinum toxin A on masseteric muscle hypertro-phy. All five patients (nine hypertrophied muscles)showed a good response; the maximal eftect of a 31%reduction in muscle bulk was observed 3 montbs aftertreatment. One year after injection, tbe effect re-mained stable for six of the hypertrophied muscles.Three muscles required a second injection to maintainthe reduction.

A more radical approach to treating masseter hy-pertrophy consists of partial muscle resection, usuallyin the lower portion. The surgical access can take ei-ther an intraoral or an exfraoral approach. Some au-thors-^'" have recommended the concomitant resec-tion or reduction of the mandibular angle when it isprominent. Nishida and Iizuka^ described an intraoralmethod for removal of the enlarged mandibular angleassociated with masseteric hypertrophy. The disadvan-tages of surgical reduction include the risks of generalanesthesia, postoperative hemorrhage, edema,hematoma, infection, scarring, and facial nerve dam-age.

Microscopic examination of the removed muscletissue ustially shows normal muscle fibers withoutchanges in length, thickness, or nuclear structure.-"'Zachariades et al' reported two cases in which phle-boliths were associated with masseteric hypertrophy.

Despite the benign aspect of masseteric hypertro-phy, attention must be paid to otber conditions thafcan produce facial swelling. Parotid inftammation,cysts, benign neoplasms sucb as hemangioma or

lipoma, and malignant diseases must he considered inthe differential diagnosis.'

CASE REPORT

A 16-year-oId white hoy was referred to the OraîDiagnosis Clinic of Sacred Heart University with achief complaint of progressive swelling af the rightlower side of the face (Figs fa and lh). His grand-mother reported that this swelling had become in-creasingly prominent since his infancy {Figs 2a to 2c).The patient revealed that the region was intermittentlypainful to the touch, ie, occurring some days but ab-sent for as long as 3 to 4 months, Tbe familial medicalhistory included the existence of allergy (grand-mother] and diabetes (grandfather).

Clinical examination did not reveal any abnormalityof tbe lips, tongue, floor of tbe mouth, soft paiate, hardpalate, throat, alveolar mucosa, gingiva, occlusion, ortemporomandibular joint. The patient had no history oftrauma, paresthesia, xerostomia, trismus, dysphagia, orany difficulty in mastication. There was no pain on pal-pation, and no thrills or bruits were noted. Bimanualpalpation during tbe muscle contraction revealed a rela-tionsbip between the volumetric increase in the lowerright side and the masseteric muscle. There was no re-striction of mouth opening and no associated lymphnode enlargement. The differential diagnosis includedalteration of the parotid gland. However, glandular sali-vation appeared normal, and no masses were detectedon palpation.

fn the panoramic and posteroanterior radiographs,no significant alteration was observed. The CT exami-nation did not reveal any abnormality either (Fig 3).

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Fig 3 Ccronal computed tomogíam

Fig 5 Coronal magnetic resonance T2-weighted ¡magerevealing higti signals in the maxillary and ettimoidal si-nuses, suggesting sinusitis.

Fig 4 Axial magnetic reaonanoe T1-weighted imagerevealing right masseierio hypertrophy.

The axial MRI revealed a discreet enlargement of themasseter muscle, which was more evident on the rightside (Fig 4). Coincidentally, other findings on the MRIexamination were diseases of the maxillary and eth-moidal sinuses (Fig 5).

All clinical information and images were correlated,and the diagnosis of unilateral masseter muscle hyper-trophy was made. The patient was referred for relaxationtherapy, and no surgical intervention was indicated.

DISCUSSION

Masseteric hypertrophy is an uncommon muscular al-teration in which the chief complaint is related toesthetics. Some authors suggest that the use of theterm hypertrophy in this condition may he misleading,because the enlargement of the muscle is caused by an

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increase in the number of fibers and not an increase incell size,-'"

Meaningful emphasis must be given to the differen-tial diagnosis, because several malignancies ean pro-duce similar clinical features. In masseteric hypertro-phy, a uniform mass that is characteristically of longduration is noted. Additionally, further inspectiondemonstrates that, when the patient forcibly bites,loeal muscle contraction is felt in the area of theswelling- Another characteristic of masseteric musclealteration is that visual examination and palpation re-veal a more uniform swelling that is different from theirregular and nodular growth that characterizes otherbenign and malignant neoplasms, Yonetsu et aP dis-cussed the eharacteristic images of many pathologicconditions of the buccomasseteric region, emphasizingthe differential diagnosis. The final diagnosis shouldbe based on the clinical features, medieal history, andfindings of imaging modalifies.

After diagnosis of masseter muscle hypertrophy, pa-tients are encouraged to diminish their excitability lev-els, thereby decreasing muscle tension. Although thepatient in this ease complained of swelling, the ehiefconcern of his grandmother was that the swellingcould be a malignant tumor. The esthetic factor wasnot as important to them. Therefore, conservativedierapy was ehosen because of the small enlargementof the masseter muscle. Moreover, it was also desirableto allow the continuation of faeial growth, throughwhich the asymmetry ean be naturally minimized.

Another conservative treatment is the use of loealinjections of neurotoxin made of anaerobie organisms{Clostndium botulinum), but more elinieal and seien-tific evaluation of this method is needed. In addifion,the action of this therapy may be temporary. The pos-sible reasons for this temporary response are that newneuromuseular synapses ean be resynthesized over aperiod of a few months and antibodies may developbecause of the repeated injections.

Early diagnosis of masseterie hypertrophy is impor-tant so that the patient and parents ean be informedabout the likely development of facial asymmetry.Careful followup is required because this conditioncan be recurrent.

CONCLUSION

The professional must be prepared to recognize mas-seteric muscle hypertrophy, which is the most frequentcondition that occurs in the buccomasseteric region.The main features are facial swelling, muscle contrac-tion when the patient occludes, normal glandularfunction, and absence of pain. The condition shouldbe confirmed with imaging modalities. The correcttreatment is still debatable, hut the first choice shouldbe conservative therapy. Diagnostic mistakes couldlead to more aggressive and unwarranted therapy.

REFERENCES

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2, Addantc RR, Masseter muscle hj^iertrophy: Report of caseand literature review. J Orai Maxiilofac Surg 1994;52:1199-1202.

3, Nishida M, lizuka T. intraoral removal of the enlargedmandihular angle associated with masseteric hypertrophy, JOral Ma iiliofac Surg 1995:53:1476-1479,

4, To EWH, Ahuja AT, Ho WS, et ai. A prospective study ofthe effect of bgtulinum toxin A on masseteric muscie hyper-trophy with ultrasonographie and clectromyographic mea-surement, Br J Plast Surg 2001:54:197-200.

5, Yonetsu K, Nakayama E, Yuasa K, Kanda S, Ozeiii S,Shinohara M, Imaging findings of some buccomassetericmasses, Orai Surg Orai Med Oral Pathol Oral Radioi Endod1998;86:755-759.

6, Fyfe EC, Kabala J, Guest PG. Magnetic resonance imagingin the diagnosis of asymmetrical bilateral masseteric hyper-trophy. Den to maxiilofac Radioi 1999;28:52-54,

7 Smyth AG Botulinum toxin treatment of bilaterai masse-teric hypertrophy. Br J Oral Maxillofac Surg 1994:32:29-33.

8, Moore AP, Wood GD, The medical management of masse-teric hypertrophy with botuiinum toxin A, Br J Oral Max-iliofac Surg 1994;32:26-28,

9, Capra NF, Bernanke JM, Porter ]D, Uitrastructural changesin the masseter muscle of Macaca fascicuiaris resuiting fromintramuscuiar injections of botuiinum toxin type A. ArchOraiBioi 1991:36:827-836.

10, Newton JP, Cowpe JG, McClure IJ, Delday MI, Maitin CA.Masseteric hypertrophy?: Preliminary report, Br J OralMaxillofac Surg 1999:37:405-408,

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