February 12, 2018
Have you ever had a patient wake up blind after anesthesia? Although it’s rare, anyone who’s been in practice for a number of years knows of a case either directly or indirectly. Many possible causes have been implicated in causing postoperative blindness, including anesthetic agents and hypoxia. In recent years, another possible explanation has been proposed.
A retrospective study from 2012 showed a possible link between blindness and mouth props. This study retrospectively looked at possible correlations among 20 cases of postanesthetic cortical blindness in cats.1 Three cats underwent cardiac arrest during the procedure, therefore hypoxia secondary to arrest was suspected in these cases. In the other 17 cats, no specific cause was pinpointed. Thirteen of 20 cats underwent dental procedures (in which mouth props are commonly used), four cats had endoscopy procedures (also common; mouth props help avoid trauma to the endoscope), two cats were neutered, and one cat was treated for urethral obstruction. Sixteen cats were documented to have mouth props used. Fourteen of 20 cats had documented recovery of vision, four cats remained blind, and two cats were lost to follow-up while still blind. The authors concluded that mouth props were a possible risk factor for cerebral ischemia and blindness in cats.
The plot thickened in 2013 and 2014 when prospective studies were published that looked at six healthy adult cats that had electroretinography (ERG),2,3 dynamic computed tomography (CT),2 brainstem auditory evoked response test (BAER),2 and magnetic resonance angiography (MRA).3 In the 2014 study, tests were performed before and after placement of mouth props of various submaximal (20, 30, and 42 millimeters) and maximal degrees of mouth opening. Maximal mouth opening resulted in alterations in ERG waveforms in one of six cats and reduction of MRA signal intensity in four of six cats. No changes were noted in any of the submaximal settings except for decreased MRA signal in one cat at 42 millimeters.3
Another study in 2014 looked at structural and functional changes related to maxillary artery flow in cats with the mouth opened and closed utilizing computed tomography (CT) and digital subtraction angiography. CT showed that the maxillary artery coursed between the angular process of the mandible and the tympanic bulla, and the distance between these structures was less with the mouth opened. Digital subtraction radiography showed reduced opacification of the maxillary artery, and the cerebrum in cats with the mouth opened.4 It was concluded that compression of the maxillary artery results when the mouth is maximally opened.
I have never been a fan of mouth props during dental procedures, mainly due to my own experiences in the dental chair. I find that my TMJ joints hurt if I keep my mouth open for an hour’s worth of dental work, even in the absence of any spring-loaded mechanism. It is possible to gain access to all areas of the mouth without use of mouth props. I use a laparotomy sponge to keep the mouth open just enough to access all teeth. I prefer a laparotomy sponge rather than a simple gauze sponge, as gauze sponges can be forgotten in the back of the mouth at the end of the procedure. Also, using a dental mirror minimizes the need to prop the mouth open, as the mirror makes it easier to visualize the lingual and palatal surfaces of the teeth without maximally opening the mouth. If you feel the need to use something more than a laparotomy sponge, consider a needle cap that has been cut to 20 or 30 millimeters rather than spring-loaded mouth gags and full-length needle caps.5
1 Stiles J, Weil AB, Packer RA, Lantz GC. Post-anesthetic cortical blindness in cats: twenty cases. Vet J. 2012; 193(2): 367-373.
2 Barton-Lamb AL, Martin-Flores M, Scrivani PV, Bezuidenhout AJ, Loew E, Erb HN, Ludders JW. Evaluation of maxillary arterial blood flow in anesthetized cats with the mouth closed and open. Vet J. 2013 Jun; 196(3): 325-31.
3 Martin-Flores M, Scrivani PV, Loew E, Gleed CA, Ludders JW. Maximal and submaximal mouth opening with mouth gags in cats: implications for maxillary artery blood flow. Vet J. 2014; 200(1): 60-64.
4 Scrivani PV, Martin-Flores M, van Hatten R, Bezuidenhout AJ. Structural and functional changes relevant to maxillary arterial flow observed during computed tomography and nonselective digital subtraction angiography in cats with the mouth closed and opened. Vet Radiol Ultrasound. 2014; 55(3): 263-71.
5 Reiter AM. Open wide: blindness in cats after the use of mouth gags. Vet J. 2014; 201(1): 5-6.
Dr. John Lewis practices veterinary dentistry and oral surgery at NorthStar Vets in Robbinsville, N.J.
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