Ophthalmic Exam Lenses
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Overview
An ophthalmological examination classically consists of eight points.[1] The use of ophthalmic lenses in this process is critical to conducting a thorough assessment. Volk and Ocular Instruments are the current industry leaders in the manufacture of lenses for slit lamp examination, indirect ophthalmoscopy as well as gonioscopy lenses.
Lens Selection
The shape and dioptric power of a lens is adapted to produce varying fields of view and image magnification, and to allow for different working distances.[2] For example, slit lamp lenses are commonly “aspheric” and “condensing,” high power plus lenses (78D and 90D).[3] This gives them the ability to form an inverted image of the retina between the lens and the ophthalmologist. Lenses with lower dioptric power (28D and 20D) provide greater magnification to assist in evaluating the retina via indirect ophthalmoscopy, particularly helpful in pediatric populations, patients with nystagmus and when scleral indentation is required.[3] Thus, the distinguishing characteristics of each lens lends way for selection based on practitioner preference and the primary objective of the examination being undertaken.
Slit Lamp Lenses
Both the 90D and Digital Wide Field (Volk 90D, Volk Digital Wide Field and Ocular Maxfield 90D) are commonly regarded as general-purpose lenses for slit lamp examination.[4] They offer a good field of view, satisfactory image magnification and the ability to examine small pupils. These lenses also allow panretinal fundus examination from the central to the periphery, including the ora serrata.[4] In contrast to the Volk 90D, the Digital Wide Field provides a greater field of view, with comparable magnification but at a shorter working distance.[2] The Ocular Maxfield 90D has the widest dynamic field of view with a similar working distance to the Digital Wide Field (Table 1). The Digital Wide Field is also constructed of a high index glass with an optical coating that generates bright images with minimal distortions and aberrations.[4] The shorter working distance lenses, however, have the disadvantage of requiring more frequent cleaning due to build up of oils from proximity to patient eyelashes.[2]
Higher magnification lenses for slit lamp use include the 78D, 66D and Digital 1.0x. The 78D lenses provide a good balance of magnification and field of view. Although they also provide better stereopsis than the 90D lenses, pupil dilation is typically required.[2] They are ideal for general diagnosis and treatment. They are also particularly suited to checking for glaucoma and other posterior pole anomalies. The Volk Super 66, Volk Digital 1.0x are both 66D lenses that allow high resolution imaging of the central retinal for detailed optic disc measurements without the need for a scaling factor.[4] Unlike the Volk 66D lenses, the Ocular MaxField 66D uses 0.91x magnification and therefore requires upscaling to accurately measure disc size.
Compared to the Ocular MaxField High Mag 78D, the Volk Digital High Mag 55D are lenses with 1.3x magnification and excellent stereopsis.[5] They both require a properly dilated pupil for visualization of the macular fluid and for eliciting stereo views of the optic disc and retinal nerve fiber layer. This makes them great for glaucoma screening. Given their smaller field of view, these lenses are especially suited for further analysis of a known abnormality and less suited as a general scanning tool.[2]
| SLIT LAMP LENSES | ||||
| Manufacturer | Lens | Field of View | Image magnification | Laser spot magnification |
| Volk | 90D | 74° / 89° | 0.76x | 1.32x |
| Volk | 78D | 81° / 97° | 0.93x | 1.08x |
| Volk | Super 66 | 80° / 96° | 1.0x | 1.0x |
| Volk | Digital 1.0x | 60° / 72° | 1.0x | 1.0x |
| Volk | Digital Wide Field | 103° / 124° | 0.72x | 1.39x |
| Volk | Digital High Mag | 57° / 70° | 1.30x | 0.77x |
| Ocular Instruments | 90D | 94° / 153° | 0.75x | 1.34x |
| Ocular Instruments | 78D | 98° / 155° | 0.77x | 1.30x |
| Ocular Instruments | 66D | 91° / 144° | 0.91x | 1.10x |
| Ocular Instruments | High Mag 78D | 88° / 154° | 0.98x | 1.02x |
| INDIRECT LENSES | ||||
| Manufacturer | Lens | Field of View | Image magnification | Laser spot magnification |
| Volk | 20D | 60° | 3.13x | 0.32x |
| Volk | 28D | 69° | 2.27x | 0.44x |
| Volk | Pan Retinal 2.2 | 73° | 2.68x | 0.37x |
| Volk | Digital Clear Field | 72° | 2.79x | 0.36x |
| Ocular Instruments | 20D | 50° | 2.97x | 0.34x |
| Ocular Instruments | 28D | 58° | 2.11x | 0.47x |
Indirect Lenses
For the purposes of indirect ophthalmoscopy, the main lenses of interest are the 20D and Pan Retinal 2.2. They are both great all-round lenses for general diagnosis and treatment. Although the Pan Retinal 2.2 uses a larger lens and requires marginally longer working distances, it has a slightly wider field of view at a similar magnification.[4] This makes it efficient at scanning the posterior pole as well as the periphery. However, the larger size of the lens and longer working distance may make it more difficult to use for providers with smaller hands, and on patients with anatomic eatures such as a prominent nose, compared to smaller alternatives.
Medium dioptric lenses (Volk 28D, Volk Digital Clear Field, Ocular Instruments 28D) are best used for patients with small pupils.[3] These lenses provide a larger field of view at the expense of magnification. Compared to the 20D lenses which offer better views of the fundus, the 28D lenses are more ergonomic with a wider field of view.[2] Practitioners with smaller hands may prefer the smaller size and shorter working distance of the 28D lens. The Digital Clear Field handles like a slightly bigger 28D lens; so, when viewing the far peripheral retina, a classic 28D lens may be easier to manipulate in the hands.
Gonioscopy
Volk three mirror gonio lenses have three mirrors that are all angled at different orientations. One mirror is specifically at 60 degrees for viewing the iridocorneal angle, another at 66 degrees for imaging from the equator to the ora serrata and the final mirror at 76 degrees for visualizing the mid and peripheral retina.[4] The views and angles are similar for the “Goldmann” type lens by Ocular.
Three mirror gonio lenses by Volk also feature flange and non-flange contact options. The flange version requires use of a coupling fluid as it vaults over the cornea and is ideal for dynamic and indentation gonioscopy.[4][6] Notably, the viscous fluid used in flange gonio lenses may obscure later visualization of the fundus.[6] The non-flange style is well designed for performing gonio laser as it provides optimal stability and control on the cornea.
Some gonio lenses also feature four mirrors. They have the added advantage in that they do not require a viscous contact fluid for full lens visualization.[2] Furthermore, since all four lenses point at 64 degrees to the iridocorneal angle they require minimal rotation for a full assessment of the angle.[6] This saves an appreciable amount of examination time compared to their three mirror counterparts.
Conclusion
With the myriad of ophthalmic examination lenses available, deciding between them may be daunting for beginner ophthalmologists. Both Volk and Ocular Instruments feature an array of products that compete so closely on features, the differences are almost indiscernible. Therefore, it is often useful to have a toolbox of different lenses for a range of applications. With time, a personal workhorse may be identified.
- ↑ Rupp, J.D. The 8-point eye exam, American Academy of Ophthalmology. Available at: https://www.aao.org/young-ophthalmologists/yo-info/article/how-to-conduct-eight-point-ophthalmology-exam2023, (Accessed: 13 February 2024).
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Xu, D. Volk Ophthalmic Lens Review, EyeGuru. Available at: https://eyeguru.org/blog/volk-lens-review/#Digital_Wide_Field_and_90D 2019, (Accessed: 15 February 2024).
- ↑ 3.0 3.1 3.2 Walling, P.E. et al. Condensing lenses: Sharpen your skills in choosing and using, Review of Optometry. Available at: https://www.reviewofoptometry.com/article/condensing-lenses-sharpen-your-skills-in-choosing-and-using 2017, (Accessed: 14 February 2024).
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Volk Optical Catalogue. Available at URL: https://www.volk.com/
- ↑ Ocular Instruments Product Data Sheet. Available at URL: https://ocularinc.com/
- ↑ 6.0 6.1 6.2 Lee, W.W. Choosing a gonioscope, OphthalmologyWeb. Available at: https://www.ophthalmologyweb.com/Tech-Spotlights/36495-Choosing-a-Gonioscope/ 2011, (Accessed: 15 February 2024).
