Abstract
Objectives
The objective of this study was to investigate the effect of body position on intraocular pressure in clinically normal cats.
Methods
Sixteen clinically normal cats without any ocular abnormalities were used in this study. Cats were sequentially placed in three different recumbency positions (sternal recumbency, right lateral recumbency and dorsal position). For each of the three body positions, intraocular pressure (IOP) was recorded in the left eye using a Tono-Pen Vet tonometer (Reichert).
Results
Mean IOP readings in cats in the sternal recumbency, right lateral recumbency and dorsal positions were 15.6 ± 4.1, 16.6 ± 6.4 and 18.6 ± 6.8 mmHg, respectively. A significant increase was observed in IOP in cats in the dorsal position compared with the IOP observed in cats in sternal recumbency (P = 0.01) and right lateral recumbency (P = 0.04). There was no significant difference between the IOP recorded in the lateral recumbency and sternal recumbency (P = 0.17) positions.
Conclusions and relevance
Body position has a significant effect on the IOP of cats. IOP increases when cats are placed in dorsal recumbency.
Introduction
Measurement of intraocular pressure (IOP) is commonly performed during routine ophthalmic examinations because IOP is an important indicator of ocular health and disease states. Measuring the IOP in animals with ocular disease, such as uveitis and glaucoma, is an important diagnostic procedure. Despite many recent advances, tonometry still remains one of the basic criteria for diagnosing glaucoma. 1 The ability to obtain an accurate measurement for IOP is critical in diagnosing and managing glaucoma in cats. The effect of body position changes on IOP is well recognised in humans.2–4 Studies in horses and dogs have indicated that the position of head and body has a significant effect on the IOP. Komáromy et al studied the effect of head position on IOP in horses. 5 In 2008, another study compared the IOP of dogs in dorsal recumbency, sternal recumbency and sitting positions. Findings from this study revealed that mean IOP in dorsal recumbency was significantly higher than in sternal recumbency. 6
There is no published information regarding the positional effect on IOP in cats. Therefore, the objective of this study was to investigate the effect of body position on the IOP in clinically normal cats.
Materials and methods
The study was approved by the Iran Society for Prevention of Cruelty to Animals in accordance with Iranian ethical codes for studies on laboratory animals.
Sixteen adult, mixed-breed, intact cats (eight males and eight females) were used in this study. Cats ranged in age from 2–4 years. There was no historical evidence that the cats had previous or current ophthalmic problems. Prior to the study, complete physical examinations, complete blood counts and ophthalmic examinations, including applanation tonometry, biomicroscopy and indirect ophthalmoscopy of both eyes, were performed.
At the start of the trial, the cats were sedated with intramuscular medetomidine (100 μg/kg). The time interval between the injection and tonometry varied between 20 and 25 mins. Measurements were taken in a sternal recumbency, right lateral recumbency and dorsal recumbency. IOP was measured using applanation tonometry with the Tono-Pen Vet tonometer (Reichert) after topical instillation of 1 drop of proparacaine 0.5% ophthalmic solution (proparacaine hydrochloride 0.5% [Paracain; Sunways]). The tonometer was factory calibrated prior to initiation of the study and was hand-calibrated prior to data collection. IOP was measured in the left eyes of all animals. Only the IOP readings with a 5% error (5% displays on LCD screen of Tonopen) were recorded.
Data were analysed using the SPSS for windows version 12.0 (IBM). A paired-samples t-test was used to compare the IOPs obtained from the cats in the three positions. A P value <0.05 was considered statistically significant.
Results
Mean ± SD IOP readings in sternal recumbency, right lateral recumbency and the dorsal position were 15.6 ± 4.1, 16.6 ± 6.4 and. 18.6 ± 6.8 mmHg, respectively (Figure 1). A significant increase was observed in the IOPs in the dorsal position compared with the IOPs obtained when the cats were in sternal recumbency (P = 0.01) and right lateral recumbency (P = 0.04). There was no significant difference between the IOPs recorded when the cats were in lateral recumbency and sternal recumbency (P = 0.17).
Mean intraocular pressure (IOP) readings in cats in the sternal recumbency, right lateral recumbency and dorsal positions
Discussion
In the present study, we evaluated IOPs in three different positions in 16 sedated cats. The use of intramuscular medetomidine for immobilising animals in this study might have some bearing on the IOPs obtained. However, a recent investigation reported that administration of intramuscular medetomidine had no significant effect on the IOP of normal cats. 7
In the present investigation, a significant increase was seen only when the IOP obtained in dorsal recumbency was compared with both measurements in sternal and lateral recumbency. We found that dorsal recumbency produced the greatest increase in IOP (of 3.3 mmHg) compared with the mean values measured in sternal recumbency.
The effect of body position on IOP in normal dogs has been reported in one study, which concluded that mean IOP in dorsal recumbency was significantly higher than that in sternal recumbency. 6
However, in all recumbency positions, the IOP remained within expected physiological levels for the species. The reported range for IOP obtained by applanation tonometry in normal cats is 9–31 mmHg. 8
By definition, IOP occurs when pressure is exerted by the components of the eyeball against the tunica fibrosa, which surrounds these components. IOP is determined by the volume of intraocular of aqueous humor, choroidal blood volume, vitreous humor volume, scleral rigidity and compliance, extraocular muscle tone and external pressure. There are several factors that can influence IOP, for example ocular disease, blood pressure, time of day (diurnal variation), activity level, circulating hormone levels, anesthetics and body position, to name a few.9,10
The effect of body position on IOP has long been recognised in humans. The exact mechanism of action in which body position could alter the IOP in cats is unclear. However, previous studies in humans have suggested that IOP readings were altered as a result of body position through one or more of the following mechanisms: changes in episcleral venous pressure, choroidal vascular volume, gravity or shift of body fluid, increase intraperitoneal pressure and, in turn, central venous pressure.11–16
Conclusions
Body position has a significant effect on the IOP of cats. IOP increases when cats are placed in dorsal recumbency.
Footnotes
Conflict of interest
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.