Originally published in the January 2015 issue of Veterinary Practice News I’ve had a run of feline patients who presented with the inability to close their mouth. Coincidentally, a recently published review in the Journal of Veterinary Dentistry discussed diagnoses behind the inability to close the mouth.1 Let’s review some of the more common causes. Differential Diagnoses When a cat is suddenly unable to close its mouth, there are only a handful of differentials: Temporomandibular joint (TMJ) luxation; TMJ fracture or caudal mandible fracture; Open-mouth jaw locking; Acquired dental malocclusion; Mandibular neurapraxia (a.k.a. trigeminal neuropathy or trigemninal neuritis). The patient history and comprehensive physical examination provide clues for an accurate diagnosis. If no history of trauma exists and no signs of head trauma are seen, TMJ luxation and TMJ/caudal mandibular fractures are unlikely. Caudal mandibular fractures and TMJ fractures may result in a shift of the mandible and an inability to close the mouth. Symphyseal separation or parasymphyseal fracture will not typically cause an inability to close the mouth, therefore if a cat presents with symphyseal separation and is unable to close the mouth, this suggests additional trauma caudally, usually in the form of a TMJ luxation or caudal mandibular/TMJ fracture. An oral examination on the conscious patient may show significant swelling or bruising if the mouth is gently opened to examine the caudal mandibular area (Figure 1). John R. Lewis, University of Pennsylvania Figure 1: Head trauma of unknown origin, resulting in severe bruising of the soft palate and pharyngeal tissue, a puncture wound, and left-sided swelling in the area of a caudal mandibular fracture in a cat. A fracture of the caudal mandible usually results in a shift of the lower jaw to the side of the fracture (Figure 2), since the portions of the mandible rostral and caudal to the fracture site will overlap upon themselves. In contrast, luxation of the TMJ in a rostrodorsal direction (the most common direction) results in a shift of the mandible to the side opposite the luxation (Figure 3). John R. Lewis, University of Pennsylvania Figure 2: Presenting occlusion of the cat in Figure 1 with a left caudal mandibular fracture. Note the shift of the lower jaw to the left and the mandibular canine teeth are distal to the maxillary canine teeth. Open mouth jaw locking (OMJL), unlike TMJ luxation or mandibular fractures, causes an inability to close the mouth even though the maxillary and mandibular teeth don’t touch. In cases of OMJL, the mouth is wide open and the lower jaw often has a subtle shift to the side where the coronoid process is locking on the ventral aspect of the zygomatic arch (Figure 4). Mandibular neurapraxia may occur secondary to trauma, neoplasia or infectious causes. Unlike the other four differentials listed above, this disease presents with a “dropped jaw” appearance that can be manually closed by the clinician, but the patient cannot maintain the mouth in a closed position due to loss of motor innervation of the mandibular branch of the trigeminal, and the patient cannot keep its mouth closed without support (Figure 5). John R. Lewis, University of Pennsylvania Figure 3: Cat with a right TMJ luxation in a rostrodorsal direction, resulting in protrusion of the mandible and a shift of the lower jaw to the left. Acquired dental malocclusions occur either due to severe extrusion or severe periodontal disease, most commonly seen with the maxillary or mandibular canine teeth, resulting in tooth-to-tooth contact. Periodontal disease and the loss of tooth attachment structures can cause luxation of teeth from their sockets, resulting in inability to close the mouth (Figure 6). Treatment Treatment of TMJ luxations usually involves closed reduction with the use of a wooden dowel—a pencil works well—as a fulcrum to reposition the rostrodorsally displaced condylar process into the mandibular fossa of the temporal bone. TMJ fractures are often treated conservatively, with close monitoring for TMJ ankylosis. Treatment of caudal mandibular fractures varies widely depending on the location of the fracture, but regardless of the fracture repair technique, two equally important goals should be attained: fracture stablization and maintenance of normal occlusion as the fracture site heals. John R. Lewis, University of Pennsylvania Figure 4: Open-mouth jaw locking occurring due to laxity of the TMJ joint, resulting in ipsilateral locking of the coronoid process of the ramus on the ventral zygomatic arch. Figure 5: Mandibular neurapraxia results in a dropped jaw appearance in the absence of a malocclusion. This patient’s mandibular neurapraxia was due to the neurologic form of lymphoma, which also caused right-sided Horner’s syndrome. John R. Lewis, University of Pennsylvania Figure 6: Palatally deviated left maxillary fourth premolar tooth due to severe periodontal disease. Patient was unable to close its mouth due to the mandibular first molar contacting the luxated maxillary fourth premolar tooth. Mandibular neurapraxia is most commonly caused by trauma or stretching of the motor innervation provided by the mandibular branch of the trigeminal nerve. Supportive care is provided, as usually this condition resolves within three to eight weeks of onset. Extraction of affected teeth is necessary when acquired dental malocclusions cause an inability to close the mouth. The causes and treatment of open-mouth jaw locking will be discussed in the February 2015 issue of Veterinary Practice News. Hopefully this refresher will keep your jaw from dropping next time you see a cat that can’t close its mouth. References 1. Constantaras ME, Charlier CJ. Maxillofacial injuries and diseases that cause an open mouth in cats. J Vet Dent. 2014; 31: 168-175.
