by Noelle La Croix, DVM, Dip. ACVO
Glaucoma is the progressive dysfunction and death of retinal ganglia. Once a retinal ganglion cell has died it cannot normally be regenerated. The only-evidence based treatment shown to reduce the risk of progressive ganglion cell loss and blindness is lowering of intraocular pressure (IOP). However, despite adequately controlled IOP measured within the examination room, glaucoma may worsen. Circadian IOP changes, head position (seated vs. supine), and blood flow can affect ocular perfusion. In humans increased mean ocular perfusion pressure (MOPP) is associated with the progression of glaucoma.
Measurements of IOP only reflect pressures measured at that brief moment in time. However, both IOP and blood pressure follow circadian rhythms. Variations in healthy human IOP are typically within 6 mmHg throughout the day. Open angle glaucoma patients show greater variations (12 – 14 mmHg in humans, 6 – 10 mmHg in beagles). In these patients, peak IOP are typically found in the morning. Canine patients with closed angle glaucoma have not been studied for IOP variations throughout the day.
Within my practice, we have been able to assess variations in daily IOP for a single 13-year-old Maltese with closed angle glaucoma (Figure 1). This single-eyed dog was owned by a veterinary technician who measured the dog’s IOP every 4 hours for 3 weeks using a rebound tonometer (TonoVet) that does not require the instillation of anaesthetics. In this dog IOP routinely peaked at 2 AM (65 mm Hg) and regressed to normal levels (15 – 18 mmHg) by the early afternoon (Figure 2). Interestingly, the 2 AM pressure spikes were seemingly unaffected by aggressive treatment with antiglaucoma medications. The dog was reported as comfortable despite the pressure spikes, the owner refused surgical intervention, and the dog eventually lost vision.
In humans increased MOPP has also been shown to significantly contribute to progressive glaucoma with resultant blindness. Blood flow within an organ is defined as the difference between arterial blood pressure and venous blood pressure within it. The mean arterial blood pressure (MAP) is defined as diastolic blood pressure plus one-third of the difference between diastolic and systolic blood pressure. For eyes, IOP is a measurement of diastolic pressure. The MOPP is defined as IOP subtracted from MAP. Changes in head positioning will affect the MAP as it relates to the eye. The MAP measured when an eye is at the level of the heart (supine) will be greater than that measured when the eye is above the heart (upright), thus affecting MOPP. Importantly, in both humans and primates, decreases in MOPP are associated with greater risks of glaucoma induced ganglion cell death.
In dogs, systemic blood pressure typically decreases at night. A nightly increase in a glaucomic canine’s IOP, coupled with a decrease in systemic blood pressure, decreases MOPP. This decrease in MOPP may increase the risk of ganglion cell death. The MOPP may further decrease in upright dogs walked in the evening.
The circadian rhythm of IOP influences measurements within the examination room. The MOPP can affect the health of retinal ganglia. The rationale for treating canine glaucoma cannot therefore be solely based upon individual measurements of IOP.
The next article will discuss surgical strategies shown to lower IOP. If you have further questions concerning canine glaucoma, please feel free to consult with a veterinary ophthalmologist.
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006