There are 2 classifications of glaucoma – Primary and Secondary. Primary glaucoma occurs without previous trauma or ocular disease, and will eventually affect both eyes. Secondary glaucoma is a consequence of some other disease, such as ocular inflammation, lens dislocation, intraocular tumors, or trauma.
My first reaction was this can’t be primary as the numbers do not support a genetic disease, but yet here was science and the new tests available telling us it is primary. How is it I have bred this line time and time again for 15 years and this is our first time seeing this mutation?
Primary glaucoma is known to occur in certain breeds at a significant rate and is thought to have a genetic basis – it may be familial or hereditary. At the present time there is no proof that it is inherited in the Jack Russell, although with an increase of reported cases and genetic data available to participate in a research study, we may find in fact it is genetic in our breed.
Glaucoma is the increased pressure inside the eye due to a decrease in aqueous humor outflow. Aqueous humor is produced by the ciliary body cells and is secreted into the posterior chamber of the eye. It flows over the anterior surface of the lens, through the pupil and into the anterior chamber. The fluid exits through the flow holes in the pectinate ligament and is absorbed into the bloodstream. A delicate balance between production and absorption maintains the normal intraocular pressure. The production of aqueous humor is not regulated by the intraocular pressure, however. Blockage of the outflow pathway, either at the pupil or iridocorneal angle, results in increased IOP (glaucoma). In embryo the pectinate ligament does not undergo enough rarefaction to form normal pectinate ligaments and flow spaces.
There are 3 basic types of glaucoma:
- Open angle glaucoma
- Narrow angle glaucoma
- Mesodermal Dysgenesis
Early signs of Glaucoma
The early signs of glaucoma may consist of mild conjunctiva inflammation, increased tearing and pain; and sensitivity to light. The cornea can become cloudy as well. Your regular vet may not be aware that glaucoma is a predisposed disease in Jack Russell’s and if symptoms appear we should request that he/she measure IOP (intraocular pressure) as a part of the examination of the eye. This is of utmost importance in an effort to save the eye.
- Elevated IOP, above 50 mm Hg, with corneal edema (cloudiness), inflammation, and a dilated fixed pupil are all characteristic.
- Normal IOP ranges from 15 to 25 or 30 mm Hg (20 or below is preferred). If IOP is 30 to 50 mm Hg, the pupil may be moderately dilated and have a sluggish reaction to light.
The area of aqueous outflow, is referred to as the iridocorneal angle, and may be examined clinically with a corneal contact lens. Does your dog have a normal Iridocorneal Angle? How would you know? A gonioscopic exam.
Gonioscopy is a technique used to evaluate the iridocorneal angle, which will determine the predisposition for glaucoma. Both eyes should always be done. The test takes about 15 minutes and involves placing a dome-shaped contact lens (goniolens) on the cornea. This lens permits the ophthalmologist to directly view the drainage angle and determine if the dog has goniodysgenesis, a narrow angle, etc. Either will signal a predisposition for glaucoma. Medical therapy may then be initiated to prevent glaucoma or reduce risk of future problems.
The gonioscopic examination can be performed with the use of only a topical anesthetic, and can be done in conjunction with your CERF exam, but you should request this ahead of time. Remember that the gonioscopic exam must be done prior to the drops to dilate the pupils for the CERF exam being administered to your dog.
The gonioscopy only needs to be done one time and it may be done as early as 6 months of age, if the eye is large enough for the goniolens. If glaucoma should ever come into question in your breeding program, I highly recommend you have all your dogs checked with a gonioscopic examination, as well as your regular CERF exam.