12.4 Slit Lamp Technique, Slit Lamp Lenses, Direct Ophthalmoscopy & Pen Light Examination
Key Takeaways
- Direct illumination with a narrow beam produces an optical section that localises a lesion in corneal depth.
- Retroillumination uses light reflected from the iris or fundus to reveal transparent defects.
- The 90 D lens gives a wider, less magnified fundus view and the 78 D lens gives a narrower, more magnified view; both images are inverted and reversed.
- The direct ophthalmoscope gives an upright, highly magnified image of a small field with no stereopsis.
- The pen light with an oblique beam estimates anterior chamber depth by the shadow cast across the nasal iris.
Slit lamp illumination techniques
The slit lamp is a binocular microscope with a variable, movable light source. Its power is not magnification alone but the relationship between the angle of illumination and the angle of observation. Six techniques cover essentially every anterior segment examination.
| Technique | Setup | Reveals |
|---|---|---|
| Diffuse illumination | Wide beam, diffuser in, low to moderate magnification, 30–45° | Overview of lids, conjunctiva, cornea; the orientation view |
| Direct focal / optical section | Narrow beam (0.5–1 mm), 45–60°, high illumination | A cross-section of the cornea; localises a lesion's depth — epithelial, stromal or endothelial |
| Parallelepiped | Beam 2–3 mm wide, 45° | A three-dimensional block of cornea; the workhorse for corneal detail |
| Retroillumination | Beam directed at the iris or through a dilated pupil onto the fundus, observing the structure against that returning light | Transparent abnormalities: iris transillumination defects, corneal or lens vacuoles, posterior capsular opacification, keratic precipitates, epithelial oedema |
| Specular reflection | Angle of illumination equals angle of observation, both about 30° from the axis | Corneal endothelial mosaic; guttata in Fuchs dystrophy |
| Sclerotic scatter | Beam aimed at the limbus, observation of the central cornea with the beam decentred | Subtle corneal haze, oedema, scars glowing against a dark background |
| Tangential / oblique | Very oblique beam | Surface elevation, iris masses |
Anterior chamber cells and flare. Use a 1 mm × 1 mm beam at maximum illumination, high magnification, an angle of about 45 degrees, in a darkened room, focused in the aqueous between cornea and lens.
- Cells are discrete particles drifting in convection currents — graded 0 to 4+ by the number visible in the beam field.
- Flare is a haze from protein, described as a beam of light visible in a smoky room — graded 0 to 4+ by how obscured the iris detail becomes.
Cells indicate active inflammation; flare may persist long after inflammation has resolved because a damaged blood-aqueous barrier leaks protein. That distinction determines whether treatment is escalated.
Slit lamp lenses
Non-contact fundus lenses (used at the slit lamp, held in front of the eye):
| Lens | Field of view | Magnification | Best for |
|---|---|---|---|
| 60 D | Narrowest | Highest | Detailed optic disc and macular examination |
| 78 D | Intermediate | Intermediate-high | The general-purpose disc and macula lens |
| 90 D | Widest | Lowest | Wide survey, small pupils, peripheral screening |
All produce an image that is inverted and laterally reversed — a lesion seen superiorly is inferior in the eye, and one seen nasally is temporal. This must be corrected mentally before it is drawn or described.
Contact lenses (placed on the anaesthetised cornea with a coupling gel):
| Lens | Use |
|---|---|
| Goldmann three-mirror | Central lens for the posterior pole; three mirrors at different angles for the mid-periphery, far periphery and the angle |
| Gonioscopy lenses (Goldmann, Zeiss four-mirror, Sussman) | Examining the angle; Zeiss and Sussman allow indentation gonioscopy to distinguish appositional from synechial closure |
| Macular contact lenses | High-resolution stereoscopic macular view |
| Laser lenses | Deliver and focus laser energy for retinopexy, capsulotomy or trabeculoplasty |
Contact lens use requires topical anaesthetic, a coupling agent, and afterwards a reminder not to rub the anaesthetised eye until sensation returns.
Direct ophthalmoscopy
The direct ophthalmoscope produces an upright, virtual image magnified about 15 times, over a field of only about 5 degrees — roughly two disc diameters — with no stereopsis.
Technique:
- Darken the room; dilate if permitted.
- Right eye to right eye, left eye to left eye, approaching from about 15 degrees temporal to the patient's line of sight to land on the disc.
- Set the aperture to a small round beam for an undilated pupil, larger for a dilated one.
- Start at about +8 to +10 D at arm's length to check the red reflex, then move in, reducing plus, until the fundus is focused.
- Find a vessel and follow it toward the wider branches — they converge on the disc.
- Examine in order: disc (margins, colour, cup-to-disc ratio), vessels (calibre, arteriovenous ratio, crossings), background (haemorrhages, exudates, drusen), then ask the patient to look directly at the light to bring the macula into view last, because it is the most uncomfortable.
Useful apertures and filters:
| Setting | Use |
|---|---|
| Small aperture | Undilated pupils |
| Large aperture | Dilated pupils |
| Red-free (green) filter | Nerve fibre layer defects, haemorrhages (appear black), vessel detail |
| Slit beam | Estimating elevation of a lesion |
| Grid / graticule | Estimating lesion size in disc diameters |
| Cobalt blue | Fluorescein staining of the cornea |
Direct versus indirect:
| Feature | Direct | Binocular indirect |
|---|---|---|
| Image | Upright | Inverted and reversed |
| Magnification | ~15× | ~2–5× |
| Field | ~5° | Wide |
| Stereopsis | None | Yes |
| Periphery | Poor | Excellent, with scleral indentation |
| Through media opacity | Poor | Better |
Red reflex assessment is the most valuable thirty seconds of direct ophthalmoscopy, especially in children. A white reflex (leucocoria) requires urgent referral; dark shadows suggest media opacity; asymmetry between the eyes suggests ametropia, deviation or an obstruction.
Pen light examination
A pen light performs more than illumination.
Oblique flashlight estimation of anterior chamber depth. Shine the beam from the temporal side, parallel to the plane of the iris, and observe the nasal iris:
| Appearance | Interpretation |
|---|---|
| Nasal iris fully illuminated, no shadow | Deep anterior chamber |
| A crescent shadow on the nasal iris | Shallow anterior chamber — the iris is bowed forward and casts a shadow |
| Shadow covering most of the nasal iris | Very shallow; narrow angle risk — do not dilate without clinician assessment |
This is a screening estimate; the definitive assessments are the Van Herick technique at the slit lamp (comparing the peripheral anterior chamber depth against the adjacent corneal thickness) and gonioscopy.
Other pen light uses: corneal light reflex tests (Hirschberg, Krimsky), the swinging flashlight test for an afferent pupillary defect, gross assessment of lids and conjunctiva, checking for foreign bodies, and eliciting photophobia.
A practical safety rule: before dilating any patient, estimate the anterior chamber depth. A shallow chamber with a history of haloes, brow ache or intermittent blur is a reason to have the clinician assess the angle before drops are instilled.
Which slit lamp illumination technique best reveals iris transillumination defects?
Compared with a 78 D lens, what does a 90 D lens provide?
A pen light directed from the temporal side parallel to the iris plane casts a crescent shadow across the nasal iris. What does this indicate?
What is the clinical significance of anterior chamber flare persisting after cells have cleared?
Which finding on direct ophthalmoscopy of a child requires urgent referral?