Anatomy of a Slit Lamp Biomicroscope

The slit lamp consists of:

1. An illumination arm which offers a range of settings to facilitate examining the eye. This swivels on a pivot to allow for the light beam to illuminate the eye from different angles.
2. A viewing arm with a binocular microscope.
3. A joystick which allows both fine and gross horizontal displacement of the slit lamp on the table; twisting the top of the joysticks allows for vertical displacement of the slit lamp.

Patient support frame:

1. Forehead band
2. Canthus indicator
3. Chin rest
4. Chin height adjustor

Illumination arm:

• Slit length scale (mm)
• Light filters
• Slit height adjustment and blue filter
• Illumination mirror
• Slit width adjustment

Viewing arm:

1. Oculars
2. Adjustment for examiner refractive error
3. Magnification changer (some models have this on the side of the microscope)

Base:

1. Patient hand rest
2. Joystick
3. Light intensity control
4. Base locking screw

Illumination filters

Switch the controls for several illumination settings that may be used for different purposes:

1: Open – no filter, maximal brightness.

2: Grey – reduced maximum brightness to decrease patient discomfort.

3. Red-free – to facilitate the visualisation of blood vessels and blood.

4. Cobalt blue (twisting the setting screw anticlockwise all the way) – used with fluorescein dye on the tear film. Useful for detecting damage to the cornea or foreign bodies in the eye.

5. Diffuser (flipped upwards next to illumination mirror): produces even lighting across the whole field, used for the general overall examination of the eye and adnexa.

Positioning

1. Ensure the patient’s chin is on the rest, their forehead against the band, and the chin height adjusted so their lateral canthus aligns with the black canthus indicator.
2. Adjust the patients chair and the slit lamp table height for an ideal patient position. Apply the brakes to the slit lamp wheels.
3. The examiner should adjust their own chair for comfort, allowing easy access to the joystick and slit-lamp controls.

Note: The examiner should adjust the oculars to the correct inter-pupillary distance to facilitate optimal stereopsis during the examination. Adjustments can also be made for refractive errors.

Systematic slit lamp examination

A comprehensive slit lamp examination should be done systematically to ensure that all components of the eye are thoroughly examined².

External exam

Prior to utilising the slit lamp, conducting a thorough gross examination of the eye and the ocular adnexa is important.

Look for:

• Eyelid disorders such as ptosis.
• Nerve palsies such as oculomotor nerve palsy.
• Peri-orbital pathologies such as skin carcinoma.

Eyelids and eyelashes

Ensure thorough examination of both upper and lower eyelids, the lids can be reflected for better visualisation of the lid margins. Remember to look at the eyelids in a closed position to assess for proper apposition.

Common pathologies to look out for:

• Hordeolum (stye)
• Chalazion
• Blepharitis
• Ectropion

When examining eyelashes look for:

• Loss of lashes
• Misdirected lashes (trichiasis)
• Deposits on the lashes

Conjunctiva and sclera

Assess all areas of the conjunctiva and sclera:

• Get the patient to look in all four directions.
• Examine the lower tarsal conjunctiva, on the posterior surface of the lids, by retracting the eyelids and asking the patient to look up.
• The upper tarsal conjunctiva can be seen by everting the upper lids.

Observe for:

• Signs of injection (enlarged conjunctival vessels). Circumferential injection to the edge of the cornea (”ciliary flush”) could indicate acute glaucoma or anterior uveitis³.
• Discharge, which may suggest infectious of allergic conjunctivitis⁴.
• Foreign bodies.

Cornea

For optimal visualisation of the cornea and its layered anatomy, a cross-sectional view is preferred. This enables the superficial epithelium, stroma, and deep endothelium layers to be appreciated².

To achieve a cross-sectional view the examiner should:

• Use a narrow beam.
• Position the illimitation arm at an oblique angle.
• Move the beam slowly across the cornea to detect any irregularities in transparency of thickness, which could indicate underlying pathology such as foreign bodies, abrasions, or ulcers.

Anterior Chamber

The anterior chamber is normally filled with transparent aqueous humour. Any reflective substance visible within this chamber is typically indicative of pathology⁵. To aid in visualising blood, the red-free filter can be employed, which causes blood to appear as black.

Examples of substances indicative of pathology:

• Hypopyon – purulent material secondary to severe infections
• Hyphema – blood in the anterior chamber, often secondary to trauma

Iris

• When examining the iris, observe its surface, with particular attention to its margin.
• Note any raised lesions, irregularities in pupil shape, or abnormal pigmentation.

Lens

• For optimal examination of the lens, the pupils should be dilated.
• The anatomy also has layers which are better appreciated at an oblique angle.
• The lens should normally be clear, a cloudy or opaque area may be indicative of a cataract.

Fluorescein stain

The fluorescein stain serves as a valuable tool for detecting foreign bodies on the eye and assessing corneal integrity; it enhances the ability to discern the morphology of corneal abrasions or ulcerations⁶.

To use the fluorescein stain:

• Administer fluorescein drops or utilise the impregnated strips.
• Adjust the light filter on the illumination arm to cobalt blue.

Posterior chamber

You are not expected to be able to examine the posterior chamber of the eye using the slit lamp if you are not an ophthalmologist or optometrist. However, please see the article about how to use the direct ophthalmoscope to visualise the fundus.

Written by Dr Abeku Koomson (FY2) and reviewed by Mrs Uppugonduri (Ophthalmology Consultant)

References

1.     Haag-Streit. Manual instruction for use BM 900 [Internet]. Köniz: Haag-Streit UK; 2019 [cited 2024 May 20]. Available from: https://uk.haag-streit.com/2%20Products/General%20diagnostics/Slit%20lamps/BM%20900/Instructions%20for%20use/English/1500_7200492_04190_IFU_BM_900_01_en_web.pdf

2.     Kaur K, Gurnani B. Slit-Lamp Biomicroscope. [Updated 2023 Jun 11]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK587440/

3.     Frings A, Geerling G, Schargus M. Red Eye: A Guide for Non-specialists. Dtsch Arztebl Int. 2017 Apr 28;114(17):

4.     Azari AA, Arabi A. Conjunctivitis: A Systematic Review. J Ophthalmic Vis Res. 2020 Aug 6;15(3):372-395.

5.     Slit Lamp Examination. EyeWiki. Available from: https://eyewiki.aao.org/Slit_Lamp_Examination

6.     Wipperman JL, Dorsch JN. Evaluation and Management of Corneal Abrasions. Am Fam Physician. 2013;87(2):114-120.