The Slit Lamp in Ophthalmic Practice
The slit lamp (biomicroscope) is the central instrument of the ophthalmic examination. It combines a microscope with an adjustable slit light source, allowing magnified, three-dimensional examination of all anterior segment structures and, with adjunct lenses, the posterior segment. As a CPOA, you set up the slit lamp, position patients, assist with procedures, and may perform basic slit lamp assessments during preliminary work-up.
Slit Lamp Components
- Illumination arm: projects the slit beam of light onto the eye; adjustable in height, width, and angle
- Observation arm (binocular microscope): provides binocular, stereoscopic view at various magnifications (typically 6x, 10x, 16x, 25x, 40x)
- Joystick: moves the instrument forward/backward and side-to-side; the table height is adjusted separately
- Magnification drum: changes the microscope magnification
- Slit height and width controls: adjust the beam dimensions
- Beam angle control: changes the angle of the illumination relative to the observation axis
- Filters: cobalt blue (for fluorescein), red-free (green) for vascular detail, neutral density
- Chin rest and forehead rest: position and stabilize the patient's head
Patient Preparation and Positioning
- Clean the chin rest and forehead rest with a disinfectant wipe and fresh chin rest paper
- Adjust the table height so the patient sits comfortably with their chin in the chin rest
- Align the patient's outer canthus (lateral corner of the eye) with the alignment mark on the chin rest post
- Ask the patient to look straight ahead at the fixation target inside the instrument
- Adjust the binocular eyepiece separation for your interpupillary distance and focus each eyepiece
- Dim room lighting to improve visibility
💡 Clinical Tip: Proper chin rest height alignment is critical. If the patient's eye is below the alignment mark, you are looking up into the eye; if above, you look down. Correct alignment ensures the illumination and observation axes intersect at the eye plane.
Illumination Techniques
Different illumination techniques visualize different structures:
| Technique | Setup | Best For |
|---|---|---|
| Diffuse illumination | Wide beam, low angle; general survey | Overview of eyelids, conjunctiva, cornea |
| Direct focal illumination (parallelepiped) | Narrow beam with angle; illuminates a 3D cube of tissue | Corneal detail, cells and flare in AC, lens evaluation |
| Optic section | Very thin, tall beam; side-by-side angle | Cross-section of cornea, lens layers; evaluates depth of corneal pathology |
| Sclerotic scatter | Beam at limbus; light scatters across cornea | Subtle corneal edema, scar, foreign body |
| Specular reflection | Angle of incidence = angle of reflection; observer and beam same side | Corneal endothelial reflex (guttata evaluation) |
| Retroillumination | Reflects from iris or retina; illuminates from behind | Lens opacities, corneal deposits, iris transillumination defects |
| Cobalt blue + fluorescein | Cobalt blue filter; fluorescein instilled | Corneal epithelial defects, tear breakup time, applanation tonometry |
Basic Anterior Segment Assessment
During preliminary slit lamp work-up, the CPOA may assess:
- Eyelids: eyelid position (ptosis, ectropion, entropion), eyelid margins (meibomian gland orifices, blepharitis), lash direction
- Conjunctiva: injection pattern (bulbar vs. palpebral; diffuse vs. perilimbal), chemosis, follicles, papillae (upper tarsal conjunctiva requires lid eversion)
- Cornea: clarity, surface smoothness, epithelial defects (fluorescein staining), infiltrates (white/grey stromal opacities), pannus, scarring
- Anterior chamber: depth, clarity, cells and flare (requires narrow beam at high magnification)
- Iris: color, symmetry, pupil shape and size
- Lens: clarity (nuclear color, cortical spokes, PSC opacity)
⚠️ Common Mistake: Reporting "clear cornea" without using fluorescein and cobalt blue filter means epithelial defects may have been missed. Fluorescein staining is essential for a complete corneal assessment, especially in contact lens wearers or patients with pain.
Lid Eversion for Upper Tarsal Examination
Assessing the upper tarsal conjunctiva (for papillae, follicles, or foreign body) requires everting the upper eyelid:
- Ask the patient to look down and keep looking down
- Grasp the upper lash margin between thumb and forefinger
- Place a cotton swab or thumb above the upper tarsal plate as a fulcrum
- Gently pull the lashes down and flip the lid upward over the fulcrum
- Examine the exposed upper tarsal conjunctiva
- Ask the patient to look up to restore the lid to normal position (or simply release)
Tear Breakup Time (TBUT)
TBUT assesses tear film stability. After instilling fluorescein and asking the patient to blink once, the examiner uses the cobalt blue filter to observe when the tear film begins to break up (dark spots or streaks appear). Normal TBUT is >10 seconds; below 10 seconds suggests dry eye or evaporative tear instability.
Key Takeaways
- The slit lamp is a binocular microscope + slit illumination for magnified 3D ocular examination
- Proper patient alignment: outer canthus at the alignment mark on the chin rest post
- Illumination techniques vary by structure: parallelepiped for general detail; optic section for depth; cobalt blue + fluorescein for epithelial defects
- Evert the upper lid to examine the upper tarsal conjunctiva for papillae or foreign bodies
- TBUT normal = >10 seconds; short TBUT suggests dry eye or Meibomian gland dysfunction
- Always use fluorescein for complete corneal assessment