November 26, 2018

How Body Position Affects Intraocular Pressure in Cats

Similar to humans and dogs, apparently healthy cats experience an increase in intraocular pressure when placed in dorsal recumbency.
By Natalie Stilwell, DVM, MS, PhD
Intraocular pressure (IOP) is an important indicator of ocular health that can also provide insight into certain systemic diseases. Ocular diseases, such as uveitis and glaucoma; blood pressure, activity level, and hormone levels can all affect IOP.

Important components that collectively determine IOP include the following
  • Aqueous humor volume
  • Vitreous humor volume
  • Choroidal blood volume
  • Scleral rigidity and compliance
  • Extraocular muscle tone
  • External pressure
Controlled studies performed in dogs and humans have demonstrated that head and body position significantly affect IOP. The exact mechanism of action for this effect remains unclear; however, blood pressure and gravity may contribute.

In a recent study published in Journal of Feline Medicine and Surgery, investigators evaluated the influence of body position on IOP in healthy adult cats.

Three Body Positions Examined
The study, conducted at the Islamic Azad University College of Veterinary Medicine in Iran, examined IOP in 8 intact male and 8 intact female cats ranging in age from 2 to 4 years. Before the study, investigators determined the cats to be apparently healthy via physical examination, complete blood cell count, and ophthalmic examination, which consisted of applanation tonometry, slit lamp biomicroscopy, and indirect ophthalmoscopy.

About 20 to 25 minutes before tonometry was performed, each cat was sedated with 100 µg/kg of intramuscular medetomidine. This medication was preferentially chosen, as an earlier study demonstrated it does not significantly affect IOP in apparently healthy cats. Next, 0.5% proparacaine ophthalmic solution was applied to the left eye, and IOP was measured in the left eye via applanation tonometry with a precalibrated Tono-Pen Vet tonometer (Reichert Technologies).

Cats were then placed in 3 positions for IOP measurement: sternal recumbency, right lateral recumbency, and dorsal recumbency. Only those IOP measurements with a 5% error or less were recorded, and IOP readings in the 3 body positions were compared statistically via t-test.

Body Position Influences Intraocular Pressure
Mean IOP readings were significantly higher when healthy cats were placed in dorsal recumbency (18.6 mm Hg) than in sternal (15.6 mm Hg) or right lateral (16.6 mm Hg) recumbency. These results correlated with a previous study performed in dogs. The authors noted that, despite being statistically significant, IOP readings in the 3 examined body positions stayed within the reference range of 9 to 31 mm Hg for cats. This suggests that placing an apparently healthy feline patient in dorsal recumbency is unlikely to cause issues, although those with ocular issues (such as deep ulcerative keratitis) should be handled with particular care.    

Take-Home Message
Manipulating healthy cats from sternal to dorsal recumbency increased IOP by a mean of 3.3 mm Hg. Although IOP values remained acceptable in all examined body positions, this study’s findings could have clinical implications for patients with borderline-high values and for those with compromised corneal integrity and subsequent risk of globe rupture with increased IOP.
Dr. Stilwell received her DVM from Auburn University, followed by a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.

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