Azeem Siddique

Introduction

Alongside establishing a thorough history, the slit lamp examination forms the foundation of an ophthalmology consultation. Prior to ophthalmology placements, medical students and junior doctors have minimal exposure to the slit lamp microscope despite its value in visualising the eye and identifying ocular pathology. Understanding the structure and function of the slit lamp microscope is fundamental to carrying out an effective examination.

Background

The first slit lamp mechanism was invented by Swedish ophthalmologist Allvar Gullstrand in 1911 where the light from a hot filament was focused through a slit forming a rectangular beam (1). Five years later, German optician Carl Zeiss combined the slit lamp with a binocular microscope to allow magnified visualisation of the eye focused under the beam of light (2). A mechanism was developed in the 1930s to allow for the simultaneous focusing of the light source and the microscope (2). Further to this, the illumination system and microscope were mounted on a single column around which the light source could be rotated.

Upon these fundamental designs, the modern slit lamp was developed with its ability to alter magnification, filter the light source and control various mechanisms allowing for focusing and support. The slit lamp microscope is now used routinely in ophthalmology practice.

Structure and Function of the Slit Lamp

The slit lamp consists of three main components: an observation system, an illumination system and a mechanical system. In general, a binocular microscope is mounted upon a table with a separate source of light that faces a frame made up of a chin rest and forehead rest to support the patient.

The illumination source consists of a focal light source, usually a tungsten or halogen bulb, with a slit diaphragm mechanism and filtering mechanisms. This produces a beam of light for which properties such as height, width, intensity and colour can be altered using various mechanisms on the instrument. The direction of the beam can also be changed with the rotating illumination arm. The observation system consists of an objective lens and two eyepieces allowing for binocular, magnified visualisation of the eye. The mechanical system is composed of an adjustable table base, a joystick for moving the microscope, and a frame for patient positioning.

A beam of light, produced by the illumination source, is focused onto the eye through a slit. The height and width of the slit and the intensity of the light can be altered to adjust the amount of light used. The eye is viewed under magnification using the microscope viewing system. Certain filters can be applied to the light – a cobalt blue filter is used in combination with the instillation of fluorescein drops to allow for identification of corneal defects as well as for measuring intraocular pressure in Goldmann applanation tonometry. A red free light filter enhances the contrast allowing for greater clarity of blood vessels. It also allows pigmented cells to be differentiated from blood.

Slit Lamp Examination

Along with a structured method and practice of techniques, slit lamp microscopy can allow for the detailed visualisation of ocular anatomy as well as making particular measurements. Thorough cleaning of the equipment with anti-septic wipes as well as hand hygiene should be carried out prior to the examination.

The examiner should begin by setting up the apparatus to suit both themselves and the patient, taking into consideration both comfort and functionality. This can include adjusting the height of the table, and chin rest so as to avoid neck strain. Height adjustable chairs can also be helpful in maximising ease. Some patients will require particular consideration, such as those who are wheelchair bound. When table based slit-lamp microscopy is not viable, other options for examination include using a portable slit-lamp and the indirect ophthalmoscope.

The eyepieces should be focused to account for any refractive error and adjusted to the inter-pupillary distance of the examiner. Dimming the lighting in the consultation room is helpful however not always possible.

As with any other system of the body, a structured approach to examination is helpful. For example, working from anterior to posterior:

External eye examination:

• Eyelids
• Conjunctiva

Anterior segment examination:

• Cornea
• Anterior Chamber
• Iris and Pupil
• Lens

Posterior segment examination:

• Anterior Vitreous
• Dilated Fundal Examination

Eyelids, Lashes

Using a diffuse, wide beam allows for an initial gross assessment of the external eye. Inspecting the eyelids for their position may reveal entropion or ectropion, potentially a ptosis, or lid retraction. These features may allude to further pathological processes. Eyelid lesions include common benign cysts and rarer suspicious lesions including melanomas. In the case of trauma, there may be a laceration. In patients with dry eye, the Meibomian glands may not be as expressive as normally seen, or indeed may be absent. Gently compressing these glands should result in expression of lubricating oils. The lower eyelid can be gently pulled inferiorly while the upper eyelid can be inverted utilising a cotton bud in order to examine beneath them (3). This can be useful to check for any foreign bodies imbedded in the palpebral conjunctiva.

Conjunctiva and Sclera

The conjunctiva are usually clear in appearance, overlying the white sclera, however can become hyperaemic in response to various conditions of the eye (3). Inflammation of the conjunctiva due to infection or allergy can cause injection as well as swelling known as chemosis. Redness also results from trauma to the ocular surface such as from abrasions or foreign bodies. Intraocular inflammation as well as raised intraocular pressure can result in a diffusely red conjunctiva. Subconjunctival haemorrhages can cause a concentrated area of blood giving the conjunctiva a deep red appearance. Scleral colour can also give a clue to other systemic conditions – for example, the yellowing of the conjunctiva in jaundice.

Cornea

The cornea can be viewed by focusing the light beam anteriorly onto the ocular surface and scanning across in a systematic fashion. The eyelids may require manual retraction to view the extent of the corneal surface. Swinging a thin, bright light beam to a wide angle can allow for visualisation of a corneal cross-section. Through this cross section the integrity of the corneal layers: epithelium, stroma and endothelium, can be assessed and any defects such as ulcerations can be observed. In normal circumstances the cornea is translucent (3). Haziness may indicate corneal oedema in response to inflammation. The addition of a fluorescein drop to the eyes can be useful in assessing the ocular surface. Viewed under a cobalt blue light filter, corneal defects will be highlighted in bright yellow. The Oxford Schema can be used to grade ocular surface staining depending on degree of uptake (4). The Siedel test refers to fluorescein stained aqueous humour leaking through an opening in the ocular surface can indicate a penetrating injury to the cornea (5).

Anterior Chamber

The anterior chamber can be visualised through the cornea and the aqueous humour it contains is usually transparent. To assess for inflammation in the anterior chamber, inspect for white blood cells and flare resulting from free protein material. To do this, adjust the light beam to a high intensity 1mm x 1mm from a 45 degree angle, increase the magnification and focus the viewing system just beyond the cornea and in front of the lens (6). If cells are present they will be visible as small white dots reflecting the light while flare will appear as haziness within the chamber. The degree of cells can be graded according the Standardisation of Uveitis Nomenclature (SUN) system (6). A hypopyon may be visible in the anterior chamber in severe intraocular inflammation – this is a visible white exudate settled at the base of the chamber. A collection of blood in the anterior chamber, known as a hyphema, may result from ocular trauma as well as from iatrogenic and idiopathic causes (7). An anterior chamber that does not display any signs of inflammation can be labelled as ‘quiet’.

Iris and Pupil

The iris should be inspected for any lesions, lacerations and prolapse. Posterior synechiae occur in uveitis when the iris adheres to the lens resulting in irregularities of the pupil shape (8). These can lead to raised intraocular pressure and further damage (8). Neovascularisation of the iris is known as rubeosis iridis and can occur in response to retinal ischaemia (8).

The pupil should be assessed for shape, size and symmetry prior to instilling any dilating eyedrops. Normal pupils should be central, round, equal in size and constrict in response to light. Fixed pupils may indicate ocular pathology or neurological abnormalities (9). Fixed dilated pupils pupils can indicate angle closure glaucoma. Irregularly shaped pupils are seen in uveitis however can also be the result of congenital abnormalities such as iris coloboma.  Medications can also cause pupillary abnormalities, from dilating eye drops such as tropicamide to pinpoint pupils in opiate overdose.

Lens

The lens can be examined through the pupil, which is made easier with the application of dilating eyedrops. The red reflex, a reflection of light from the retina, is usually visible when the light beam is shone through the pupil from a 90 degree angle. A cataract within the lens can block this resulting in absence of a red reflex. Similarly to the cornea, the lens can be viewed in cross-section. This allows the density and translucence of the lens layers and the posterior capsule to be assessed, particularly for any opacification.

Anterior Vitreous

The vitreous can be examined through the pupil by focusing the light beam beyond the lens and increasing magnification. The vitreous contents is subject to gravity and tends to settle however asking the patient to look up, down and straight again moves the contents of the vitreous cavity allowing observation. As with the anterior chamber, signs of inflammation include cells and flare. Schafer’s sign or ‘tobacco dust’ is the presence of pigmented cells from the retinal pigmented epithelium in the vitreous (10). These appear as small brown specs amongst the vitreous humour and can be sign of retinal tear or detachment.

Dilated Fundal Examination

In combination with a handheld biconvex lens, the slit lamp microscope can be used to visualise the fundus. Lenses of varying power can provide differing magnification and fields of view. The lens is held up around 10mm away from the eye. The light beam is aimed directly through the lens and pupil from a 90 degree angle to the patient. The focus is the adjusted to be able to see a partial, aerial image of the fundus. The view obtained is inverted vertically and horizontally due to the nature of the lens. Through systematic adjustment of the viewing system and the lens, much of the fundus can be visualised including the optic disc, vascular arcades and the macula. The peripheral retina can also be visualised by asking the patient to look to the extremes of their vision, vertically, horizontally, and diagonally.

Optic disc appearances including colour, contours and cup to disc ratio can be observed. Retinal and macular abnormalities can be identified such as haemorrhages, exudate, drusen, neovasculaisation, tears and detachments.

Conclusion

Doctors and medical students often enter ophthalmology placements for the first time with little to no experience of using a slit lamp microscope. Understanding its structure, function and a coherent approach to ophthalmic examination will allow for quicker familiarisation with the equipment and its potential as a diagnostic tool.

References

1. Timoney, P.J., Breathnach, C.S. Allvar Gullstrand and the slit lamp 1911. Ir J Med Sci 182, 301–305 (2013). https://doi.org/10.1007/s11845-012-0873-y

2. Ehinger, B. and Grzybowski, A. (2011), Allvar Gullstrand (1862–1930) – the Gentleman with the Lamp. Acta Ophthalmologica, 89: 701-708. https://doi.org/10.1111/j.1755-3768.2011.02235.x

3. Bell FC. The External Eye Examination. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Boston: Butterworths; 1990. Chapter 114. Available from: https://www.ncbi.nlm.nih.gov/books/NBK218/

4. Bron, Anthony J. FCOphth, FMedSci; Evans, Victoria E. BOptom(Hons); Smith, Janine A. MD. Grading Of Corneal and Conjunctival Staining in the Context of Other Dry Eye Tests. Cornea: October 2003 – Volume 22 – Issue 7 – p 640-650

5. Cain W, Sinskey RM. Detection of anterior chamber leakage with Seidel’s test. Arch Ophthalmology. 1981 Nov; 99(11): 2013.

6. Jabs DA, Nussenblatt RB, Rosenbaum JT; Standardization of Uveitis Nomenclature (SUN) Working Group. Standardization of uveitis nomenclature for reporting clinical data. Results of the First International Workshop. Am J Ophthalmol 2005;140(3):509-16.

7. Gragg J, Blair K, Baker MB. Hyphema. [Updated 2022 Sep 19]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK507802/

8. Duplechain A, Conrady CD, Patel BC, et al. Uveitis. [Updated 2022 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK540993/

9. Wilhelm H. Disorders of the pupil. Handb Clin Neurol. 2011;102:427-66. doi: 10.1016/B978-0-444-52903-9.00022-4. PMID: 21601076.

10. Tanner V, Harle D, Tan J, Foote B, Williamson TH, Chignell AH. Acute posterior vitreous detachment: the predictive value of vitreous pigment and symptomatology. Br J Ophthalmol. 2000 Nov;84(11):1264-8. doi: 10.1136/bjo.84.11.1264. PMID: 11049952; PMCID: PMC1723305.