ophthalmology Ophthalmology (, ) is the branch of medicine that deals with the diagnosis, treatment, and surgery of eye diseases and disorders. An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Following a ...

, gonioscopy is a routine procedure that measures the angle between the iris and the

cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...

(the iridocorneal angle), using a goniolens (also known as a gonioscope) together with a

slit lamp In ophthalmology and optometry, a slit lamp is an instrument consisting of a high-intensity light source that can be focused to shine a thin sheet of light into the eye. It is used in conjunction with a biomicroscope. The lamp facilitates an ...

or operating microscope. Its use is important in diagnosing and monitoring various eye conditions associated with

glaucoma Glaucoma is a group of eye diseases that can lead to damage of the optic nerve. The optic nerve transmits visual information from the eye to the brain. Glaucoma may cause vision loss if left untreated. It has been called the "silent thief of ...

The goniolens or gonioscope

The goniolens allows the clinician - usually an

ophthalmologist Ophthalmology (, ) is the branch of medicine that deals with the diagnosis, treatment, and surgery of eye diseases and disorders. An ophthalmologist is a physician who undergoes subspecialty training in medical and surgical eye care. Following a ...

optometrist Optometry is the healthcare practice concerned with examining the eyes for visual defects, prescribing corrective lenses, and detecting eye abnormalities. In the United States and Canada, optometrists are those that hold a post-baccalaureate f ...

- to view the irideocorneal angle through a mirror or prism, without which the angle is masked by

total internal reflection In physics, total internal reflection (TIR) is the phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely refl ...

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

along with lubricating fluid (to avoid damaging its surface). The steeper curvature of this goniolens' exterior surface optically eliminates the

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

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

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 The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber. In the development of t ...

. 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

. 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 of the aforementioned, which prove valuable for surgical use ( goniotomy).

Interpreting the gonioscopic image

* Iridocorneal angle width: The width of the iridocorneal angle is one factor affecting the drainage of

aqueous humour The aqueous humour is a transparent water-like fluid similar to blood plasma, but containing low protein concentrations. It is secreted from the ciliary body, a structure supporting the lens of the eyeball. It fills both the anterior and the po ...

from the eye's

anterior chamber The anterior chamber ( AC) is the aqueous humor-filled space inside the eye between the iris and the cornea's innermost surface, the endothelium. Hyphema, anterior uveitis and glaucoma are three main pathologies in this area. In hyphema, ...

. A wide angle allows sufficient drainage of humour through the

trabecular meshwork The trabecular meshwork is an area of tissue in the eye located around the base of the cornea, near the ciliary body, and is responsible for draining the aqueous humor from the eye via the anterior chamber (the chamber on the front of the ...

(unless obstructed), whereas a narrow angle may impede the drainage system and leave the patient susceptible to acute angle-closure

. Gonioscopy indicates the angular width of the iridocorneal angle by the number of ocular structures visible above the rim of the iris. Generally the more structures visible, the wider the angle. However, not all structures may be easily discriminated, especially the faint

Schwalbe's line Schwalbe's line is the anatomical line found on the interior surface of the eye's cornea, and delineates the outer limit of the corneal endothelium layer. Specifically, it represents the termination of Descemet's membrane. In many cases it can b ...

at the top of the stack. Further information is obtained if a very narrow slit lamp beam may be shone upon the angle, as the angle width is generally proportional to the separation of the corneal beam and iris beam when they meet in the angle. * Anterior synechiae: Anterior synechiae are simply strands of the iris attaching to the iridocorneal angle or surrounding tissue. This may be exacerbated by ocular

inflammation Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...

, which can render the angle 'sticky' with inflammatory cells and substances, or by structural defects in the iris which lead to strands floating free into the

, as may occur with iris atrophy and congenital iris defects. Gonioscopy allows a direct view of these synechiae, and is thus especially helpful for the more subtle cases. * Indentation gonioscopy: An extension of the above two concepts, indentation gonioscopy involves the applied pressure of the goniolens against the eye, acutely raising the

intraocular pressure Intraocular pressure (IOP) is the fluid pressure inside the eye. Tonometry is the method eye care professionals use to determine this. IOP is an important aspect in the evaluation of patients at risk of glaucoma. Most tonometers are calibrated t ...

in the

and subsequently opening up the iridocorneal angle mechanically, allowing a greater understanding of the nature of the anterior synechiae. In the absence of synechiae, indentation gonioscopy may reveal the area where the cornea and iris are truly anatomically attached, as compared to where they are simply apposed against each other. *

Sampaolesi line Sampaolesi line is a sign which may be observed during a clinical eye examination. During gonioscopy (where the structures of the eye's anterior segment are examined), if an abundance of brown pigment is seen at or anterior to Schwalbe's line, a Sam ...

: Hyperpigmentation at, or anterior to,

, which can signify

pigment dispersion syndrome Pigment dispersion syndrome (PDS) is an eye disorder that can lead to a form of glaucoma known as pigmentary glaucoma. It takes place when pigment cells slough off from the back of the iris and float around in the aqueous humor. Over time, the ...

pseudoexfoliation syndrome Pseudoexfoliation syndrome, often abbreviated as PEX and sometimes as PES or PXS, is an aging-related systemic disease manifesting itself primarily in the eyes which is characterized by the accumulation of microscopic granular amyloid-like prote ...

Etymology

The word originates from the

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

words (, "angle") and (, "to see"). (As an aside to help you remember, the suffix "-gon" in "polygon" and its relatives comes from the same root word, so "polygon" literally means "many angles".)

The goniolens or gonioscope

Interpreting the gonioscopic image

Etymology

References

Further reading