MADISON, WIS. — A study conducted by a University of Madison-Wisconsin veterinary ophthalmologist showed an increase in intra-ocular pressure (IOP) in dogs while pulling on a collar, confirming a correlation between glaucoma and collar stress.
Measurements of IOP were taken in eyes of 26 dogs before and after pulling on a collar during a six-month span. The study concludes that dogs with weak or thin corneas or glaucoma should wear a harness instead of a collar.
"We have often recommended harnesses over collars, and I wanted to know if it was a justifiable recommendation. After we started the study, a paper came out in the British Journal of Ophthalmology showing that tight neckties increase IOP in humans," says Dr. Ellison Bentley, dipl. ACVO, a University of Madison-Wisconsin professor.
Bentley collaborated with Drs. Amy Pauli, dipl. ACVO, Kathryn Diehl, dipl. ACVO, and Paul Miller, dipl. ACVO.
The human study showed IOP rose due to pressure on jugular veins from wearing tight neckties. Another study measured IOP in the visual field loss of wind instrument players related to the number of life-hours of playing.
Compression of the jugular vein might result in vascular engorgement of the anterior uvea and choroid and an increase in the choroidal blood volume, Bentley's study shows, potentially negating the usefulness of a harness.
"Depending on the design of the harness and where buckles are placed, it could be as harmful as a collar," Bentley warns.
Healthy sled dogs were used in the study, Bentley says. One drop of 0.5 percent proparacaine was administered to both eyes. IOP was measured with applanation tonometry.
All resting eyes measured <25 mm Hg and did not have abnormalities.
The dogs were fitted with nylon collars and harnesses. After 10 seconds of pulling on the leash, IOP was measured in both eyes. One minute later, IOP was measured again, Bentley says.
"There were some variances in the way different breeds pull, therefore a difference in their IOP."
Greater elevation in IOP of older dogs also was noted in the study. Alaskan Malamutes and Siberian Huskies experienced less change in IOP than the other dogs in this study, which could be attributed to the way the dogs brace their shoulders to pull.
It is possible that even small increases in IOP can have a negative effect on the eye, Bentley notes. Short-term IOP elevation can alter perfusion of the optic nerve and retina.
Other studies in dogs and cats showed increases in IOP can cause short-term dysfunction of retinal photoreceptors.
Dogs in this study exhibited significantly heightened IOP versus baseline values when a force was applied to the neck via a leashed a collar, but values were not significant with a leashed harness, Bentley says.
More research is needed to determine if IOP increases are greater in glaucomatous dogs while pulling against a collar or a harness.
The study was published in the American Animal Hospital Association's Trends May/June 2006 issue.
Bentley is working on another study on glaucoma.
"We are looking at factors which may predict how soon dogs will get glaucoma in the contralateral eye after development of glaucoma in one eye, among other things," she says.