Deciding on a gonioscope can be a difficult task, especially for the practitioner without much experience. With the overload of information that can be found online, including message boards, manufacturers’ descriptions, and retail websites, it can be confusing. There are several factors to consider, and with the price of lenses, it is not a decision to be taken lightly.
One of the most fundamental decisions to make when choosing a gonioscope is deciding on the basic design. The two basic varieties are the three-mirror and the four-mirror lenses. Within each category, there are various options to consider and several manufacturers available. However, by prioritizing your needs, it becomes a bit easier to choose a goniolens.
Many ophthalmologists find the three mirror variety of lens to be a great choice. These lenses include the Goldmann, Magna View, and Rich lenses. They offer magnificent views of the iridocorneal angle, and are great for surgical procedures of the angle such as trabeculoplasty. However, only one of the three lenses actually points toward the angle—the other two offer views of the anterior retina and the ora serrata. If views of these portions of the eye are required, the three mirror lens is the choice to make. However, if the lens is being used to examine only the iridocorneal angle, some practitioners find it cumbersome to rotate the lens while maintaining contact with the eye. Others, who prefer this type of lens but are only interested in the iridocorneal angle, may opt for a one- or two-mirror lens. Such lenses include Ocular Instruments’ Two Mirror Magna View, which offers 1.45x gonio magnification and Laserlight® HD anti-reflective coating for maximum light transmission and image brightness, and Volk’s G-1 trabeculum, which offers 1.5x magnification and a single 62° mirror.
In the four-mirror variety, all four mirrors point toward the iridocorneal angle. This allows physicians to quickly look at all segments of the angle without much rotation of the lens, significantly cutting down on examination time. Compared to three-mirror lenses, the four-mirror lenses have a relatively small and flat area of contact. Some physicians, especially those who are still perfecting their technique, may find that this smaller area of contact creates difficulties. Pushing too firmly on the lens can artificially open the angle. While this may be desired, as in indentation gonioscopy, it can also fool the practitioner into thinking that the angle is more open than it actually is. Makes of the four-mirror variety include the Zeiss, Volk G, Posner, and Susmann lenses. Volk’s popular G-4 series is available with a variety of options, including large and small rings to fit a variety of facial features, adjustable handles, and flanges for stability. Another difference between the three- and four-mirror lenses is the requirement of a viscous coupling solution. In general, three-mirror lenses actually create a vault over the cornea rather than coming into contact with it; as a result, they require the use of a methylcellulose viscous coupling solution such as Goniosol. This can be time consuming and may slow down the exam. In addition, the fluid can make subsequent visualization of the fundus, and other parts of the exam such as visual fields and fundus photography, difficult. However, in some cases, the benefits of the three mirror lens outweigh the cumbersome aspects of the use of a coupling solution. In addition, some manufacturers, such as Volk, are now offering designs that eliminate the need for a coupling solution altogether with their Advanced No Fluid (ANF+) contact option.
Once you have chosen the basic design of your lens, there are still choices to make. For instance, some manufacturers, including both Volk and Ocular Instruments, offer the option of a handle on their lenses. While some like the convenience offered by a handle, others, especially taller physicians, may find the handle awkward. In addition, it can be more difficult to stabilize the gonioscope without bracing your hand against the cheek.
Another option to consider is whether to choose a lens with a flange. A flange will provide stability compared to those with a flat contact surface. This is especially useful when performing laser procedures involving the angle. However, they will require the use of a viscous fluid medium, which comes with the disadvantages already discussed. In addition, the flange can make the lens difficult to seat in patients with small fissures and in patients who resist the lens by squeezing. Ocular Instruments’ Mini Gonio Laser has a small diameter flange that is convenient for eyes with small palpebral fissures.
Finally, it is important to consider the materials used to manufacture the lens. Although acrylic lenses are generally less expensive than their glass counterparts, many find glass lenses to offer better optics, clarity, and durability.
When choosing a gonioscope, one of the most useful tools is the ability to try a lens before buying it. This will give you the opportunity to consider all the advantages and drawbacks of a lens, and it also helps to avoid the costly mistake of purchasing a lens that you will not use. If possible, ask other physicians what goniolenses they use and give them a try. If this isn’t an option, consider contacting manufacturers and dealers directly; some will allow you to try a lens before buying.