Gonioscopy in dogs is a specialized ophthalmologic procedure used to examine the iridocorneal angle (also known as the drainage angle or filtration angle) of the eye. This angle is crucial because it contains the structures responsible for draining aqueous humor from the eye, thereby regulating intraocular pressure (IOP).
Why is it performed?
The primary reason for performing gonioscopy in dogs is to assess the risk, type, and cause of glaucoma, a painful condition characterized by elevated IOP that leads to irreversible blindness. It helps differentiate between:
Primary Glaucoma:
Primary Open Angle Glaucoma (POAG): The angle often appears structurally normal but has impaired function.
Primary Angle Closure Glaucoma (PACG): The angle is physically narrowed or closed.
Goniodysgenesis (Pectinate Ligament Dysplasia – PLD): This is by far the most common inherited cause of primary glaucoma in dogs. The pectinate ligaments, which form a meshwork across the angle, are abnormally formed, appearing as broad sheets rather than fine strands, which obstructs aqueous outflow.
Secondary Glaucoma: Resulting from another eye disease. Gonioscopy can help identify the cause, such as:
Uveitis (inflammation) causing peripheral anterior synechiae (adhesions) or inflammatory debris blocking the angle.
Lens luxation/subluxation (the lens moving from its normal position and blocking the angle).
Intraocular hemorrhage.
Intraocular neoplasia (tumors).
Retinal detachment.
Other indications include:
Screening at-risk breeds: For inherited glaucoma before clinical signs appear.
Pre-surgical assessment: Before glaucoma surgeries or other intraocular procedures.
Trauma: To assess damage to the angle structures.
Congenital anomalies: Evaluation of other developmental abnormalities of the anterior segment.
How is it performed?
Sedation/Anesthesia: Most dogs require moderate sedation or general anesthesia to allow for proper examination without stress or movement.
Topical Anesthetic: Anesthetic drops are applied to the surface of the eye.
Coupling Solution: A viscous solution (e.g., methylcellulose gel, artificial tear gel) is applied to the cornea to fill the space between the goniolens and the eye, eliminating air bubbles and allowing for clear visualization.
Goniolens: A special contact lens (goniolens) is placed directly on the cornea. These lenses are designed to overcome the total internal reflection of light that normally prevents direct visualization of the angle.
Direct goniolenses (e.g., Koeppe lens): Allow direct visualization but require the dog to be supine and the examiner to use a biomicroscope or magnifying loupes.
Indirect goniolenses (e.g., Goldmann-type, Zeiss-type): Use mirrors to reflect the image of the angle to the examiner, often used with a slit-lamp biomicroscope.
Visualization: The examiner rotates the goniolens to view the entire 360 degrees of the angle.
Documentation: Findings are meticulously recorded, often with drawings, photographs, or detailed descriptions.
What structures are seen?
From posterior to anterior (from the iris root towards the cornea):
Ciliary body band: Dark, posterior structure.
Pectinate ligaments: A meshwork of fibers extending from the iris root to the anterior chamber angle. In dogs, these are a critical focus for assessing goniodysgenesis.
Trabecular meshwork: The actual sieve-like tissue responsible for aqueous humor outflow, often not as distinctly visible as in humans.
Scleral spur: A whitish ring where the sclera meets the ciliary body.
Iridocorneal angle: The junction where the iris meets the cornea.
Normal vs. Abnormal Findings in Dogs:
Normal: The pectinate ligaments should appear as fine, individual, relatively uniform strands or a fenestrated (hole-filled) sheet. The angle should be open.
Abnormal (Goniodysgenesis/PLD): The pectinate ligaments appear as broad, imperforate (solid) sheets or thick bands, effectively creating a barrier to aqueous outflow, leading to increased IOP and glaucoma. The severity can range from partial to complete obstruction.
Abnormal (Other):
Peripheral Anterior Synechiae (PAS): Adhesions between the iris and the cornea, closing the angle.
Inflammatory debris/Cells: Indicative of uveitis, blocking the drainage.
Blood (Hyphaema): Can physically block the angle.
Iris Bombe: Forward bowing of the iris due to pupillary block.
Tumors: Mass lesions in the angle.
Breeds Predisposed to Goniodysgenesis and Primary Glaucoma:
Many breeds are predisposed, highlighting the importance of gonioscopy for screening:
Basset Hound
Beagle
Flat-coated Retriever
Siberian Husky
Cocker Spaniel (American & English)
Samoyed
Border Collie
Boston Terrier
Great Dane
Poodle (especially miniatures and toys)
Shih Tzu
Shar-Pei
And many others.
In summary, gonioscopy is an indispensable diagnostic tool in canine ophthalmology, offering crucial insights into the anatomy and patency of the ocular drainage angle. It is vital for the accurate diagnosis, prognosis, treatment planning, and genetic counseling related to canine glaucoma.
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