A slit lamp (slit-lamp biomicroscope) is a stereoscopic biomicroscope that emits a focused beam of light with adjustable height, width, and angle. It allows three-dimensional observation and measurement of fine anatomical structures of the ocular adnexa and anterior segment. By using a handheld lens, the posterior segment can also be observed, and with a gonioscopy lens, the angle can be examined.
It is a cornerstone of ophthalmic examination and an important tool not only for ophthalmologists but also for emergency physicians and general practitioners. Slit lamps are widely available in emergency departments and are used for diagnosing ophthalmic emergencies and systemic diseases.
In 1823, Purkinje attempted to develop a handheld slit lamp. In 1863, De Wecker designed the first ophthalmic microscope. The precursor to the modern slit lamp was developed in 1911 by Swedish physicist Allvar Gullstrand in collaboration with Carl Zeiss.
In the 1930s, Swiss ophthalmologist Hans Goldmann improved Gullstrand’s slit lamp, establishing a parfocal design where the focal point of the light beam coincides with the focus of the microscope. Goldmann’s slit lamp was manufactured by Haag-Streit starting in 1958, becoming the first commercially available model.
Goldmann also developed gonioscopy prisms, and later David Volk developed lenses for posterior segment observation.
Slit-lamp microscopy is not a diagnostic tool specific to certain subjective symptoms, but a versatile instrument for all ophthalmic complaints. It is particularly useful for the following complaints.
Major Observation Sites of the Anterior Segment
Eyelids and eyelashes: Blepharitis, entropion, ectropion, hordeolum, chalazion, trichiasis
Conjunctiva and sclera: Redness pattern (conjunctival injection vs ciliary injection), discharge, papillae, follicles
Cornea: opacity, keratic precipitates (KP), ulcer, edema, stromal lesions
Anterior chamber: depth, flare, cells, hypopyon, hyphema
Observation of lens and posterior segment
Iris and pupil: iris neovascularization, pigment abnormalities, posterior synechiae, incomplete mydriasis
Lens: location, type, and degree of opacity (nuclear, cortical, posterior subcapsular, anterior subcapsular)
Intraocular lens (postoperative eye): IOL position, presence of posterior capsule opacification, IOL opacification
Anterior vitreous: floaters, hemorrhage, signs of infection
Posterior segment (using auxiliary lens): optic disc, macula, retina, blood vessels
The main types of cataract opacities are classified according to the WHO classification (3 main types). ① Cortical cataract: wedge-shaped or ring-shaped opacity progressing from the periphery to the center of the lens. ② Nuclear cataract: opacity and yellowing of the lens nucleus. Nuclear hardness is graded by the Emery-Little classification (1–5). ③ Posterior subcapsular cataract: opacity just beneath the posterior capsule. Even mild cases significantly affect visual function. In addition to these, there are also subtypes such as anterior subcapsular cataract, water clefts, retrodots, and fiber folds.
Risk factors for major diseases evaluated by slit-lamp microscopy are shown below.
| Observation target | Main risk factors |
|---|---|
| Age-related cataract | Aging, ultraviolet light, smoking, diabetes, obesity (high BMI), steroid use |
| Posterior subcapsular cataract | Atopic dermatitis, steroids, uveitis |
| Posterior capsule opacification | Diabetes, uveitis, congenital cataract, high myopia |
| Angle-closure glaucoma | Shallow anterior chamber, hyperopia, Asian ethnicity, elderly women |
| Anterior uveitis | Autoimmune diseases, infections, trauma |
A standard slit lamp microscope consists of the following four main parts.
Positioning
The patient places their chin on the chin rest and adjusts so that the outer canthus aligns with the height indicator. The forehead rest and chin rest are wiped with alcohol before use.
Focusing
Turn on the power, slide the entire stand toward the patient for coarse focusing. Use the joystick for fine adjustment (clockwise: upward movement, counterclockwise: downward movement).
Illumination Adjustment
Adjust light intensity, slit width, and slit height according to the purpose. Use cobalt blue filter (fluorescein staining), red-free filter (hemorrhage evaluation), and ND filter (fundus examination) as needed.
| Illumination method | Slit setting | Main use |
|---|---|---|
| Diffuse illumination | Wide beam/diffuser | Wide-field observation of ocular adnexa and ocular surface |
| Direct focal illumination | Narrow beam | Assessment of nuclear cataract severity and depth of opacity |
| Oblique illumination | 30–45° oblique | Evaluation of cortical cataract, water clefts, anterior subcapsular opacities |
| Transillumination | Frontal, slightly broad | Posterior subcapsular cataract, retrodots, intraocular lens position check |
| Tangential illumination method | Broad beam from nearly sideways | Observation of anterior subcapsular opacity and anterior lens surface |
Maximum pupillary dilation is essential for detailed observation of the lens. Without dilation, the pupillary light reflex prevents accurate assessment of the posterior cortical findings.
Observation with oblique illumination (30–45°)
First, widen the slit width and check the following.
For evaluation of nuclear cataract, observe with a slightly narrower slit width and constant width and light intensity. Be careful not to overestimate nuclear hardness with strong light intensity. Nuclear hardness is evaluated using the Emery-Little classification (1–5) and used to determine the difficulty of cataract surgery.
Observation by retroillumination
A slit beam is directed from the front onto the dilated pupillary margin, and the entire lens image is evaluated using the light reflected from the fundus. The findings to be assessed are as follows.
Retroillumination is particularly useful for diagnosing posterior capsule opacification. Slightly widen the slit beam and direct it obliquely into the fundus, observing the posterior capsule with reflected light from the retina. Faint Elschnig pearls and fibrous opacities on the posterior capsule surface can be identified. Even if normal under direct illumination, opacities may be detected for the first time with retroillumination (especially in eyes with multifocal intraocular lenses, where mild posterior subcapsular cataracts causing vision loss are easily overlooked). After Nd:YAG laser posterior capsulotomy, the opening range is also confirmed with retroillumination.
The slit lamp microscope is used as a diagnostic tool in the following ways:
Anterior chamber evaluation (assessment of acute angle-closure glaucoma)
Anterior chamber depth can be assessed using the van Herick technique, in which a slit beam is directed at the peripheral cornea at a 60° angle to observe the distance between the corneal endothelium and the iris. If this distance is less than one-quarter of the corneal thickness, the anterior chamber is shallow and referral to an ophthalmologist is necessary.
Assessment of anterior chamber inflammation
The slit beam is narrowed to about 1 mm wide and 3 mm high to evaluate the presence of cells (floating leukocytes), flare (protein exudation), hypopyon, and hyphema. Having the patient perform rapid horizontal eye movements (saccades) stirs the aqueous humor, making findings clearer.
The slit-lamp microscope is used not only for diagnosis but also for outpatient procedures.
Application to silicone oil-induced pupillary block
After vitreoretinal surgery, silicone oil (SO) may migrate into the anterior chamber and cause pupil block. For this complication, an outpatient procedure under a slit lamp has been reported 1).
A 51-year-old male underwent phacoemulsification + vitrectomy + silicone oil tamponade for tractional retinal detachment due to proliferative diabetic retinopathy. On postoperative day 1, silicone oil migrated into the anterior chamber; on day 2, intraocular pressure rose to 60 mmHg and the anterior chamber became shallow. Anterior segment OCT confirmed pupil block by silicone oil. Under a slit lamp with the patient seated, viscoelastic material (OVD) was injected through a side port, pushing the iris posteriorly, allowing aqueous humor to re-enter and reform the anterior chamber. Subsequently, a transcorneal peripheral iridectomy was performed with a 20-gauge MVR blade, relieving the pupil block and normalizing intraocular pressure to 12 mmHg. 1)
The advantages of this method include avoiding the supine position (which facilitates silicone oil migration into the anterior chamber) and operating room procedures, requiring no special laser equipment, and being applicable even in cases with severe corneal opacity 1).
Cataract is a general term for opacification diseases caused by modification and insolubilization of lens proteins. Various factors, primarily aging (such as ultraviolet radiation, oxidative stress, glycation, deamidation, and methionine oxidation), cause water-soluble proteins (α-, β-, and γ-crystallins) to become insoluble and aggregate, scattering light and leading to opacification.
Mechanism of Nuclear Cataract
With aging, the lens increases in anteroposterior thickness (approximately 0.02 mm/year), and the scattered light intensity of each layer rises. In a normal lens, backscattered light from the posterior embryonic nucleus is strong, but when nuclear cataract develops, backscattering from the anterior embryonic nucleus increases. The color of the nucleus changes from white to pale yellow, then to yellowish-brown, and finally to dark brown.
Nuclear cataract causes myopic shift. If an elderly patient suddenly finds it easier to see near objects, progression of nuclear cataract should be suspected.
Oxidative Stress and Decline in Antioxidant Defense
Normal lens contains high concentrations of reduced glutathione (GSH), which controls oxidative aggregation of crystallins. With aging, GSH decreases and superoxide dismutase (SOD) activity declines, leading to increased production of oxidized glutathione (GSSG) and progression of protein aggregation.
Silicone oil has a lower specific gravity than water, so it tends to migrate into the anterior chamber in the supine position 1). When silicone oil in the anterior chamber occludes the pupil, pupillary block prevents aqueous humor from flowing into the anterior chamber, causing shallowing of the anterior chamber and a rapid rise in intraocular pressure. This risk is highest in aphakic eyes 1). Both open-angle mechanisms (infiltration of silicone oil into the trabecular meshwork, inflammation, worsening of preexisting glaucoma) and closed-angle mechanisms (extensive peripheral anterior synechiae, pupillary block) contribute to elevated intraocular pressure 1).
In recent years, systems integrating high-resolution cameras, OCT, and digital analysis into slit-lamp microscopes have become widespread. Anterior segment OCT (AS-OCT) has become an important modality complementing slit-lamp microscopy in diagnosing pupillary block, angle configuration, and intraocular lens malposition 1).
The indications for outpatient slit lamp procedures are expanding. Outpatient iridectomy for silicone oil-induced pupillary block is one example 1). By keeping the patient in a seated position, the risk of additional silicone oil migration into the anterior chamber is minimized, and the procedure can be completed without using an operating room, which is significant 1).
For patients who are difficult to examine with a standard table-mounted slit lamp (e.g., wheelchair users or bedridden patients), a handheld slit lamp is a useful alternative.
