Have you ever seen a feline or canine patient whose jaw chattered? I bet you have. Sometimes, chattering is the presenting complaint noted at home by pet owners. When you saw chattering in your patient, what disease manifestation was it associated with? This month’s column will discuss common causes of jaw chattering in dogs and cats. What is jaw chattering? Early papers on the subject referred to it as “reflex jaw opening” and “jaw-depression reflex,” but these days, it is most commonly called “jaw-opening reflex.” As you may recall from anatomy class, while there are multiple muscles of mastication that close the mouth, there is only one muscle responsible for opening the mouth (the digastricus). The jaw-opening reflex is a protective, involuntary response that causes the jaw to open when a force is applied to the teeth or jaw, triggered by stimulation of sensory receptors, brain-stem centers, and motor neurons, primarily through the trigeminal nerve. One of the earlier descriptions of jaw-opening reflexes in cats was in a manuscript published in 1969 in the Archives of Oral Biology. This study noted the reflex was in response to stimulation of periodontal mechanoreceptors,1 but other studies showed elicitation of the jaw-opening reflex when nerves within the pulp were stimulated.2 What causes chattering? Clinically, the jaw-opening reflex is associated with pulp injury and diseases affecting the periodontium. However, it can also occur when tissues other than the pulp or periodontium are involved. Certain diseases of the oral and palatal mucosa can result in the jaw-opening reflex. Tooth resorption Perhaps the most common oral condition that results in jaw chattering is tooth resorption. This condition competes with periodontal disease as the most common dental problem of domestic cats. Of all the teeth in the mouth, the mandibular third premolar is the most commonly affected tooth in cats. Most cases of tooth resorption are a type of external root resorption that may eventually manifest as loss of crown structure in the cervical portion of the tooth. A student named Meredith, who went through the dentistry rotation when I was a professor at Penn, once stated, “It seems gingiva is the ‘omentum of the mouth,’” which I felt was a very thoughtful metaphor. Gingiva fills the resorption defect and covers a portion of the cervical crown. Even under anesthesia, lightly running an explorer along the gingival margin may detect an area of resorption, resulting in the tip of the explorer “catching” in the defect and a quiver or chatter of the lower jaw (Figure 1). When tooth resorption reaches the crown and results in exposed dentinal tubules, discomfort occurs when fluids, air, or a dental explorer passes across those exposed dentinal tubules, resulting in the jaw-opening reflex. Figure 1A. A very subtle area of redness and gingival enlargement at the buccal gingiva margin of tooth 104 in a cat that presented with jaw-opening reflex. Photo courtesy Dr. John R. Lewis Figure 1B. A dental radiograph shows the full extent of resorption. Photo courtesy Dr. John R. Lewis Tooth fracture Acute fracture of a tooth results in exposed nerves. An uncomplicated crown fracture results in indirect exposure of the pulp by exposing dentinal tubules, which contain odontoblastic processes and nerves. A complicated crown fracture directly exposes the pulp, which contains a bundle of nerves and vessels. When a complicated crown fracture occurs acutely, possible clinical signs include bleeding, ptyalism, and jaw-opening reflex. Interestingly, most dogs that incur complicated tooth fracture show no outward signs at all, and the fracture is noted days, months, or years later. Eosinophilic oral disease Eosinophilic oral disease has been documented in the literature in both dogs and cats for years, but to my knowledge, only recently has a connection been made between eosinophilic oral disease and chattering of the jaw.3 A retrospective study from 2019 described features of eosinophilic oral disease in dogs, identified possible risk factors or predispositions to the condition, and reported overall treatment response. Canine medical records from a veterinary teaching hospital and private referral practice over a 17-year period were reviewed for a diagnosis of eosinophilic oral disease. Twenty-four dogs with 26 lesions met the inclusion criteria. Patient mean age and body weight were 6.8 (3.8) years and 13.4 kg, respectively. The most commonly affected breeds were Cavalier King Charles spaniel (16.7 percent), Labrador retriever (12.5 percent), and West Highland white terrier (12.5 percent). Eosinophilic lesions were found in the palate (65.4 percent), tongue (26.9 percent), and other oral locations (7.7 percent). Palatal and tongue lesions were more likely in smaller dogs, whereas lesions in the other category (lip or mucosa) were more likely in larger dogs. There was a correlation in lesion location and resolution: all dogs with palatal lesions became asymptomatic at their last recheck. Seventy percent of asymptomatic dogs resolved without medication or with allergen therapy alone, suggesting that asymptomatic dogs may respond well to conservative management. Although Siberian huskies were not a commonly affected breed in the 2019 retrospective study, they have been documented in other studies to be prone to oral eosinophilic disease.4 Cats and dogs may deal with eosinophilic oral disease differently. Clinically, nearly every case I’ve seen of canine eosinophilic oral lesions has presented with a history of chattering, whereas cats affected by oral eosinophilic granuloma do not seem to show chattering as a consistent finding. Periodontal disease Severe periodontal disease can stimulate a jaw-opening reflex, likely due to sensory stimulation of mechanoreceptors in bone or exposed root surfaces. One breed with a prominent jaw-opening reflex in the face of periodontal disease is the greyhound, particularly when the periodontal disease has severely affected the incisor regions. Eosinophilic palatitis in an Italian greyhound. This dog exhibited decreased appetite, ptyalism, and jaw-opening reflex. Photo courtesy Dr. John R. Lewis A “telltail” sign It is important to distinguish between chattering and tooth grinding. Tooth grinding can be seen in both dogs and cats, but it is more common in cats. Grinding can be a manifestation of oral pain, but it can also be due to discomfort further down the gastrointestinal tract, such as gastric ulcers or a gastric tumor. Teeth grinding can also be unrelated to underlying discomfort, similar to people who grind their teeth in their sleep. Hopefully, now you know what all the chatter is about. Email me at info@siloacademy.com if you have seen a case of eosinophilic palatitis and whether the case showed evidence of a jaw-opening reflex. John R. Lewis, VMD, FAVD, DAVDC, practices at Veterinary Dentistry Specialists and teaches at Silo Academy Education Center, both in Chadds Ford, Pa. References Hannam AG, Matthews B. Reflex jaw opening in response to stimulation of periodontal mechanoreceptors in the cat. Arch Oral Biol. 1969;14(4):415-419. Clarke RW, Matthews B. The thresholds of the jaw-opening reflex and trigeminal brainstem neurons to tooth-pulp stimulation in acutely and chronically prepared cats. Neuroscience. 1990;36(1):105-114. Mendelsohn D, Lewis JR, Scott KI, Brown DC, Reiter AM. Clinicopathological features, risk factors and predispositions, and response to treatment of eosinophilic oral disease in 24 Dogs (2000-2016). J Vet Dent. 2019;36(1):25-31. Madewell BR, Stannard AA, Pulley LT, Nelson VG. Oral eosinophilic granuloma in Siberian husky dogs. J Am Vet Med Assoc. 1980;177(8):701-703.
