These findings are suggestive of a left cortical lesion and cortical blindness. Blindness and cerebral ischemia in cats following oral and transoral procedures is uncommon; the literature on this topic consists largely of isolated case reports and case series, leaving the true prevalence of postanesthestic blindness in the feline population unknown.1,2 Recovery from postanesthetic blindness is variable. Approximately 70% of cats will have partial or complete recovery of vision within 1 day to 6 weeks, but some will remain blind.2 Eighty-five percent of affected cats will have concurrent neurologic deficits (eg, abnormal mentation, ataxia, circling, head tilt) that may or may not improve with time.2
Postanesthetic blindness is often attributed to hypoxemia or hypotension during anesthesia1; however, because mouth gags apply continual force to keep the mouth open, a hypothesis has suggested that mouth gags are to be attributed.3 Opening the mouth to such an abnormal degree has been thought to disrupt maxillary arterial blood flow, potentially causing cerebral ischemia and blindness.2,4 In one study, mouth gags were used in 16 of 20 cats with postanesthetic cortical blindness and were identified as a possible risk factor.2
A knowledge of feline anatomy and the relationship of the maxillary artery to surrounding structures is essential to understanding why cats are at increased risk for ischemia as compared with other species. Feline anatomy is unique in that the internal carotid artery is functionally absent5; thus, the blood supply to the brain, retina, and inner ear is largely supplied by the maxillary artery.2,3,5,6 The maxillary artery is located at the caudal aspect of the mandible and between the tympanic bulla and angular process of the mandible.4,5 Surrounding soft tissues, specifically the pterygoid and temporal muscles, are hypothesized to bulge and compress the maxillary artery when the mouth is held in a fully extended position.2,3,5
In addition, cadaver dissection has revealed that full jaw extension causes stretching of the vasculature.2,4 Blood flow through the maxillary artery is compromised due to compressive and stretching forces. Spring-loaded mouth gags, especially when fully opened, may be particularly dangerous because the forces holding the mouth open are continuous and strong. Blood supply to critical brain and eye structures are at risk for compromise when the jaw is fully opened for a period of time, which increases the risk for ischemia.
Cats can be affected with blindness unilaterally or bilaterally. Vascular angiogram and dynamic CT studies have found more vascular compromise on the side ipsilateral to the mouth gag.4,5 This could be because the distance between the angular process of the mandible and tympanic bulla is smaller on the ipsilateral side, thereby amplifying the compressive forces on the maxillary artery.4
In addition, all cats are not equally susceptible to ischemic injury.3 One study found that maximal opening of the mouth by a mouth gag produced electroretinography (ERG) changes consistent with vascular compromise in one of 6 cats, whereas 4 of 6 cats assessed with magnetic resonance angiography had vascular compromise.3 The reason cats are unequally affected is unknown, although it is hypothesized that some cats have enough variation in their vascular anatomy to allow collateral blood flow or altered blood flow through the basilar arteries.5
Clinical blindness and neurologic signs appear to develop sporadically as postoperative complications in cats; however, the incidence is unknown. Deafness is also possible. Maximal opening of the mouth can alter the brainstem auditory evoked response (BAER) test (see Brainstem Auditory Evoked Response Test), which suggests that maximal opening of the mouth impairs blood flow to the auditory parts of the brain, which are also supplied by the maxillary artery.5 It is likely that multiple factors (eg, the degree and duration of mouth opening, type of mouth gag used, anatomic variation, anesthetic protocols, degree of systemic oxygenation and blood pressure) are involved in clinically affected cats.4 Recovery is variable, ranging from days to months, and some cats may have permanent neurologic deficits or blindness.2