Gonioscopy describes the use of a goniolens (also known as a gonioscope) in conjunction with a slit lamp or operating microscope to gain a view of the iridocorneal angle, or the anatomical angle formed between the eye's cornea and iris. The importance of this process is in diagnosing and monitoring various eye conditions associated with glaucoma.
The goniolens allows the clinician - usually an ophthalmologist or optometrist - to view the irideocorneal angle through a mirror or prism, without which the angle is masked by total internal reflection from the ocular tissue.
The mechanism for this process varies with each type of goniolens. Three examples of goniolenses are the:
Koeppe direct goniolens: this transparent device is placed directly on the cornea along with lubricating fluid, to avoid damaging its surface. The steeper curvature of this goniolens' exterior surface optically eliminates the total internal reflection problem and allows a view of the iridocorneal angle. Unfortunately it requires the patient to be lying down, and so it cannot be so easily used with an ordinary slit lamp in an optometric environment. In an ophthalmological setting, an operating microscope is one available option.
Goldmann indirect goniolens: this truncated-cone like device utilises mirrors to reflect the light from the iridocorneal angle into the direction of the observer (as shown by the schematic diagram). In practice the image comes out roughly orthogonal to the back surface (nearer the practitioner), making observation and magnification with a slit lamp easy and reliable. The small, curved front surface does not rest on the cornea, but instead vaults over it, with lubricating fluid filling the gap. The border of the front surface rests on the sclera. While the view obtained is smaller than that of the Koeppe goniolens, it can be used with the patient sitting upright, and other mirrors within the device can be used to obtain views of other parts of the eye, such as the retina and the ora serrata.
Zeiss indirect goniolens: this instrument uses a similar method to the Goldmann, but employs prisms in the place of mirrors. Its four symmetrical prisms allow visualisation of the iridocorneal angle in four quadrants of the eye simultaneously, and works well with a slit lamp. Most importantly, the size and shape of the instrument - a smaller front surface that rests on the cornea without requiring lubricating fluid, only the patient's tear film - allows for indentation gonioscopy, which can be used for further diagnosis.
There are many other goniolenses available for use, including modified versions the aforementioned, which prove valuable for surgical use (goniotomy).
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