Auxiliary Lenses for Slit-Lamp Examination of the Retina

Function

There are different lenses for use with the slit lamp. They are either high-magnification or wide-angle lenses used to visualize in great detail or map a large area in a single view. The image formed is either virtual and erect (as with the central lens of Goldmann) or real and inverted (as with the +90D lens).

The power of a lens determines how far the lens should be held from the patient's eye to obtain a clear retinal image. Some lenses can be used in dilated pupils (most lenses) and others through small pupils (including 90D and SuperPupil XL lens). The cornea has a high refractive power; thus, it is possible to see only the anterior one-third of the eye using a slit-lamp biomicroscope.[1] Special lenses, thus, need to be placed in front of the slit-lamp objective lens to view the retina and vitreous.[2] 

There are two basic optical principles to view the retina. These principles are combined in certain lenses, like the Rodenstock contact lens.

• The first approach is to nullify the corneal power using a high-power minus lens or a contact lens. These include the Goldmann 3-mirror contact lens, Goldmann fundus contact lens, Zeiss 4-mirror goniolens, and Hruby lens (Figure).[3] The Goldmann 3-mirror contact gonioscopy lens has a minus power; it essentially nullifies the power of the eye (the eye has about +60 D of power) and provides an upright view of the posterior pole (Figure). It has a central lens and three mirrors. The central lens has a magnification of 1.08x and is used to examine the central 36 degrees retina. The larger trapezoidal mirror has a declination of 73 degrees and is used to view up to the mid periphery. The smaller rectangular mirror has a declination of 67 degrees and is used to view the peripheral retina. The smallest semicircular mirror has a declination of 59 degrees for examining the vitreous body, ora serrata, and gonioscopy. The main disadvantage of the Goldmann lens is its limited field of view which requires rotation of the lens to visualize the fundus.
• The second method uses the eye's refractive power (power of the cornea and the crystalline lens) as a convex lens component of an astronomical telescope (Keplerian telescope), similar to that used by the indirect ophthalmoscope. Lenses used in this method include the +60 D, +78 D, and +90 D funduscopic lenses (Figure). The +90D is a popular fundoscopy lens. It is the gold standard lens for retinal examination.[4] The small size with clear optics makes it user-friendly. This lens is the first choice for general examination and fundus imaging. The retina can be examined even through a small pupil and is preferred in patients who do not prefer dilatation.[5] Further, a magnified image can be seen by using the magnification system of the slit lamp. In addition, there are lenses that use a combination of both approaches. These lenses employ a corneal contact lens (minus power) and a high-power (plus power) spherical condensing lens. The panfundoscope contact lens and the Rodenstock contact lens (Figure) use this mechanism.[6]

These auxiliary slit lamp lenses are used to examine the retina. Most of these lenses have a double aspheric design to minimize peripheral image distortion and improve the depth of the field.[7] The serrations on the lens ring provide a stable grip and facilitate easy lens manipulation. With time the lenses have evolved with better designs and greater magnification and field of view. The fundus image magnification factor is the reciprocal of the laser spot magnification factor.

Issues of Concern

First generation (classic lenses):  These double-aspheric lenses were designed for small pupil examination and high magnification imaging. 

+90D: It is the most popular lens for examination at the slit lamp and is excellent for small pupils. The static field of view is 74 degrees, and the dynamic field of view is 89 degrees. A static or instantaneous field of view is the maximum field visible without tilting the lens.[8] The working (also called dynamic) field of view is seen with a 15-degree tilt to either side of the optical axis.[8] 

The working field of view is also limited by pupillary dilation, which is 7 mm in most calculations. The working distance is 7 mm. The image magnification is 0.76x, and the laser spot magnification is 1/0.76 or 1.32x. The image magnification factor is used for the calculation of optic disc size.[9]

+78D: It complements the +90D but with higher magnification for central retinal examination.

+60D: It gives a highly magnified view of the posterior pole. 

Second generation (super series): These Super Series lenses have advanced double-aspheric lens designs with high-quality glass for enhanced clarity. 

Super field lens: It has a 30% wider field of view than the 90D lens.

Super 66 lens: It has a higher magnification (compared to a +90D lens) for posterior pole examination. The image magnification and laser spot magnification factor is one that simplifies the calculation of optic disc size.

Third generation: This includes the lenses of digital series.  

Digital wide field (digital series): The lens has a 40% wider field of view than the 90D. 

Digital high mag lens: It has a very high magnification (1.30x) and provides high resolution for detailed posterior pole examination.

The details of commonly used lenses are given below:

90D lens: It is a widely used and the most popular slit lamp fundus examination lens. Its image magnification is 0.76x. Though the lens actually minifies the retinal image, a magnified image of the retina may be visualized using the magnification system of the slit lamp. The magnification of the fundus image is markedly influenced by axial ametropia and the distance of the lens from the examined eye. Therefore, a correction factor (p) is used to estimate the actual optic disc size.[10] Its small diameter ring is ideal for dynamic examination and easy manipulation within the orbit. It can be used on small pupils.

78D lens: This lens offers a higher magnification than the 90D lens without cutting down on the field of view. Thus it's an ideal balance of magnification and field of view. The double aspheric design offers a clear and large view of the optic disc and posterior pole. It is used for diagnosing glaucoma and other posterior pole abnormalities. Dilation is required to obtain optimum retinal imaging. The working distance is 8 mm. 

60D lens: It is a high-magnification fundoscopy lens. It is used for detailed examination of the macula and optic nerve head. It requires a fully dilated pupil and a longer working distance of 18 mm to obtain clear retinal imaging. The double aspheric design provides excellent detail for detecting subtle details of retinal abnormalities. It is a good choice for assessing capillary hemorrhages, age-related macular degeneration, and cup-to-disc ratios.

The Digital Series lens: This series incorporates advanced optical lens design and low-dispersion glass to reduce chromatic aberrations and enhance stereopsis. They have advanced antireflecting coatings to reduce reflections and glare up to 50% more than traditional coatings. These features may provide high-resolution imaging & superior optical clarity. 

The Digital Wide Field allows a clear view of the posterior pole up to the retinal periphery through a small, undilated pupil. With dynamic maneuvers, ora Serrata can also be visualized. It has a similar magnification (0.72x) as a 90 D lens (0.76x) but provides a 40% greater field of view (static field of view 103 degrees). The anti-reflective coating gives a glare-free image. The lens is made of glass with a high refractive index, providing precise and distortion-free imaging.

Digital high mag: It is also known as the 3rd generation 78D. It has the highest magnification among all the slit lamp lenses. This enables a highly detailed examination of the posterior pole. In addition, it provides high-resolution imaging of the macula and the optic disc with an enhanced stereoscopic view. This lens is a perfect choice for examining the optic nerve head, retinal nerve fiber layer, and macula.

Super 66: This lens is similar to 78 D.  However, it has 1.0x magnification, simplifying measurements of the optic disc without needing conversion.[11] Thus it is an excellent choice for examining and assessing optic nerve head health. The optical design enhances the stereoscopic view of the central retina to examine the subtle details and changes at the macula and the optic disc. The field of view is up to the equator, and with a dynamic examination, it extends up to the mid-periphery. This lens is a smooth transition for doctors experienced with the 78D, given the similar form factor and optical profile.

The digital 1.0x:  This lens has a similar magnification as of Super 66 lens. But it has a unique glass surface curvature and coating, which minimize photographic distortion and reflections. In addition, the high-index, high-resolution glass provides greater stereopsis and image clarity. Thus it is a good lens for slit lamp photography.

Super field: It is also known as 'Super 90D'. It provides wide-field imaging up to the mid-periphery and dynamic viewing up to the ora Serrata. It is suitable for pan-retinal examination. This lens has the magnification of a 90D lens. This combination of magnification and wide-field imaging allows quick and detailed examination of the retina. It has a 30 mm lens ring which provides a comfortable grip and easy manipulation. It can be used on small pupils too.

Super pupil XL: Its main application is in small pupil pan retinal examination. It can be used in a 2 mm pupil and may help in quick undilated screening exams.

Super vitreo fundus: It is another wide-field imaging lens. It is ideal for examining the peripheral retina for tears and detachments. It has a closer working distance of 4 to 5 mm. It is suitable to get through pupils of 3 to 4 mm, thus enabling a quick, undilated retinal screening. 

In the COVID era wearing a mask is mandatory. However, it leads to fogging of the lens and hinders visualization of the retina. A new shield with a wing and a flange to direct warm air currents away from the optical path may be used to avoid this issue. Other potential solutions include using tape at the upper portion of the mask, an antifogging noseband, pressing the upper part of the mask with the examiner's fingers, a fan, antifogging spray, and a polyvinyl chloride deflector attached to the lens.[12][13][14]

Clinical Significance

The different pathologies of the eye need different slit lamp lenses for proper visualization. The posterior pole disorders such as glaucomatous disc and maculopathy need a high magnification lens.[3] The digital high mag lens is useful for these indications. It provides a clear and magnified image of the macula and the optic disc. Retinal disorders such as lattice and holes are usually present in the periphery.

The superfield lens provides wide-field imaging up to the periphery. It is beneficial in these conditions. In many patients, the pupil is non-dilating. Visualization of internal structures is difficult in a constricted pupil. The super pupil XL lens can be used in a small pupil. It is essential to document the ocular signs. The clinical photographs should be clicked and stored. The digital 1.0x lens has a distinct curvature. Its special antireflective coating decreases the slit lamp glare and distortion. Its high-resolution image has greater clarity compared to other lenses. Thus it is a perfect lens for slit lamp photography and documentation.

It is important to examine the fundus of all patients with diabetes under slit-lamp with auxiliary lenses as indirect ophthalmoscopy with a +20D lens may miss early retinal changes (microaneurysms) due to diabetic retinopathy. However, the cost of these lenses is a challenge, especially in areas with poor resources. Most of the ophthalmological diagnostic work may be done with a +90D or +78D for slit lamp examination of the fundus and a +20D lens for examination of the retina with an indirect ophthalmoscope. A +28D lens is commonly used to examine the peripheral retina in babies, specifically in patients with retinopathy of prematurity.

These lenses can also estimate the size of the optic disc.[15] The correct size of the optic disc can be estimated by histopathologic examination, photogrammetry, scanning laser ophthalmoscopy, or interference fringe scale.[16][17][18] 

However, these methods cannot be applied in a routine clinical setting. The slit lamp lenses can easily determine the size of the optic disc by overcoming the high focal convergence of the cornea. However, these magnify the image by a certain factor. While evaluating the optic disc, the disc and cup size has to be noted while calculating the cup disc (C:D) ratio. The lens's magnification factor must be considered while estimating the true optic disc size.[19] 

The super 66 lens has a magnification factor of 1.0. This simplifies measurements of the optic disc size without the need for conversion. The magnification factor is also taken into consideration in retinal lasers.[8] Earlier, the retinal laser was done with a Goldmann contact lens (Ocular). It has a laser spot magnification factor of 1.08x. Thus for producing a 108-micron laser spot over the retina, a spot size of 100 microns should be chosen in the laser machine.

Retinal Laser Lenses

Laser is used in the treatment of various retinal disorders. A slit lamp with a contact lens delivers laser energy to the retina. Proper use of ophthalmoscopic contact lenses for retinal photocoagulation requires knowledge of their magnification, spot size, resolution, and field of view. 

Goldmann lens: It was the first lens introduced for retinal photocoagulation. The Goldmann fundus lens or the Goldmann three-mirror lens was used. It has a flat anterior surface and produces an erect, virtual image located near the posterior surface of the lens. The main disadvantage of the Goldmann lens is its limited field of view.[8][20]

Yannuzzi fundus lens: It has a concave corneal surface which produces an erect, virtual image at the anterior vitreous. The concave corneal surface transmits the lens pressure to the sclera without corneal distortion.

Volk area centralis lens is an indirect contact lens that provides a good field of view with high magnification. The field of view is 70/84 (static/dynamic) degrees. Image magnification is 1.06x. Laser spot magnification is 0.94x (around 1).[21] This lens is used for focal laser of macular lesions (including microaneurysms and leaks in central serous chorioretinopathy) and macular grid.[22][23]

Volk HR centralis lens is another lens used to laser macular leaking lesions. The image spot magnification is around 1 (0.94x, to be exact). The image is real and inverted.

Volk PDT lens: The field of view is 115/137 degrees. Image magnification is 0.67x. Laser spot magnification is 1.5 x.

Volk transequator lens: It is designed for focal laser treatment and peripheral fundus evaluation. Its unique design provides a wide field of view up to the equator. The field of view is 110/132 degrees. Image magnification is 0.70x. Laser spot magnification is 1.44x. This lens may also be used for panretinal photocoagulation.

Mainster lens: It was introduced in 1986. It has a 58% greater field of view than the Goldmann lens. It has a biconvex, an aspherical anterior lens surface, and a concave lens surface to fit the corneal curvature. It produces an inverted, real image located in front of its biconvex aspheric anterior lens element.[24]         

Mainster standard lens focal/grid (ocular): This lens is designed for focal and grid laser treatment. The field of view is 90/121 degrees. Laser spot magnification is 1.05x. Its high resolution and high magnification provide clear retinal details. So, it is used to diagnose and treat choroidal neovascular membrane, macular edema, and branch retinal vein occlusion.[8]

Pan-retinal Photocoagulation (PRP) Lenses

Mainster wide field lens (ocular): It has an excellent ophthalmic resolution. It is used for pan-retinal photocoagulation. The field of view is 118/127 degrees. Image magnification is 0.68x. Laser spot magnification is 1.50x.

Mainster PRP 165 lens (ocular): This lens has the widest field of view available for pan-retinal photocoagulation. The field of view is 165/180 degrees. Image magnification is 0.51 x. Laser spot magnification is 1.96x (around 2x).

Rodenstock panfundoscopic lens: It was introduced in 1969 by Schlegel. It is used for pan-retinal photocoagulation. It gives a wider view and produces an inverted, real image located in its spherical biconvex anterior lens element. The field of view is 84% greater than a Goldmann, but lateral magnification is 24% less. The laser spot magnification is 1. 4x. One disadvantage is that it causes peripheral distortion, which results in laser beam astigmatism while treating the peripheral retina. These disturbing reflexes compromise the retinal image and cause oblong burns when treated through the periphery of the lens.

Volk quadraspheric lens: It has four aspheric surfaces with superior antireflective coatings that reduce astigmatism across the entire field of view. It can be used in a small pupil. It produces an inverted and reversed image. The laser spot magnification is 1.97x (around 2x), and the image magnification is 0.51x.

Volk super quad 160 lens: This lens offers a vast field of view. Its 0.5x image magnification provides simultaneous visualization of the posterior pole to the peripheral retina providing a greater margin of safety even during extreme wide-angle pan-retinal photocoagulation. The field of view is 160/ 165 degrees, image magnification is 1.97x (around 2x), and laser spot magnification is 2.0x. This has become the ideal lens for treating peripheral retinal holes and tears, proliferative diabetic retinopathy, and ischaemic retinal vein occlusion.

Other Issues

Table 1.

Table 2. Magnification and field of view of different types of retinal laser contact lenses

Table 3. Comparison of parameters of ophthalmoscopic retinal laser contact lens[8]

Enhancing Healthcare Team Outcomes

Any patient presenting with a retinal pathology should receive treatment from an ophthalmologist and a multidisciplinary team. Slit lamps are commonly available in emergency departments. However, any patient with an ocular problem often presents to the primary physician. The clinician with knowledge of auxiliary lenses for slit lamps can diagnose common ocular diseases and ocular emergencies. Thus interdepartmental communication will ensure better patient management.

The primary physicians can follow up with these patients. The nursing will be the first medical staff to come in contact with the follow-up patients. They can assess treatment progress and report any issues to the primary care clinician. This collaborative, interdisciplinary approach to care can ensure better patient outcomes.