Continuing Education Activity

Scanning laser ophthalmoscopy (SLO) is an investigative technique that uses a collimated laser beam for imaging the eye. It provides tomographic imaging of ocular structures in vivo. The use of coherent light sources also improves axial resolution. Newer applications of SLO, like multicolor and wide-field imaging, have revolutionized the diagnosis and management of retinal diseases. SLO also provides reproducible optic nerve head measurements in patients with glaucoma. Understanding the basic principles will help the clinician employ techniques in specific clinical scenarios. This activity summarises the basics of SLO, its advantages in day-to-day practice, and its diagnostic applications.

Objectives:

• Describe the principle and mechanism of Scanning laser ophthalmoscope.
• Outline the various non-invasive and invasive dye base imaging techniques performed using a scanning laser ophthalmoscope.
• Summarize the indications and utility of Scanning laser ophthalmoscope in day-to-day practice.
• Review the clinical significance of scanning laser ophthalmoscope.

Introduction

A scanning laser ophthalmoscope is an instrument that uses a collimated beam of laser light to image the ocular structures, especially the retina and optic nerve head. The SLO achieved a drastic reduction in the amount of light required to image the posterior segment of the eye in contrast to indirect ophthalmoscope and conventional fundus camera, thereby improving patient comfort during the procedure.[1] The "flying tv ophthalmoscope" was the very first SLO introduced in 1980 by Robert. H. Webb.[2] The incident highly radiant laser beam uses only the central 1 mm of the pupillary aperture, and the scattered light is collected back from the remaining pupillary aperture. This arrangement was termed "co-pupillary." 

Though SLO for clinical practice was introduced as early as 1990, it was not routinely utilized due to disadvantages like inferior resolution, bulky equipment, and cost. These disadvantages were overcome by a "confocal" arrangement where the scattered light rays from the ocular structures are detected at a focal point conjugate to the focus of the point illuminated.[3] The recent scanning laser ophthalmoscopes are equipped with high-powered laser beams, smaller confocal apertures, and extremely sensitive detectors, which has made them an invaluable tool in imaging the retina and the optic nerve in clinical practice. When combined with adaptive optics, it helps in the elimination of optical aberrations, thus making the visualization of individual photoreceptors possible in vivo.[4]

Anatomy and Physiology

The Retina

The retina is the light-sensitive inner coat of the eyeball which extends from the optic disc to the Ora Serrata. Histologically the retina is made of 10 layers. The inner nine layers of the retina, the internal limiting membrane, nerve fiber layer, ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer, outer nuclear layer, external limiting membrane, and layers of rods and cones are considered the neurosensory retina.[5] A potential space exists between the neurosensory retina and the tenth layer, the retinal pigment epithelium. The central 5.5 mm area of the retina is called the macula and contains xanthophyll carotenoid pigments. The most sensitive central part of the retina, the fovea, represents the central 5 degrees of the visual field, and the foveola is a central concave indentation that represents the central 1 degree of the visual field.[6] 

The inner layers of the retina receive blood supply from the central retinal artery, a branch of the ophthalmic artery arising from the internal carotid artery. The route retinal layers are nourished by the choroidal circulation. About 20 % of the individuals may have a cilioretinal artery, a branch of the short posterior ciliary arteries that may supply the macula.[7] The retinal milieu is maintained by the blood-retinal barriers, inner and outer, which regulate the movement of fluid and molecules from the vasculature to the retina.[8] Any pathology in the retinal circulation or the blood-retinal barriers can be assessed by fundus fluorescein angiography (FFA). The primary function of the retina is to convert light from the surroundings to electrical impulses and relay it via the optic nerve to the occipital cortex.[9]

The Choroid

The choroid is pigmented and highly vascular layer between the retina and the choroid. It extends from the optic disc posteriorly to the ciliary body anteriorly. The choroid receives its blood supply mainly from the long and short posterior ciliary arteries, which are end arteries.[10] The choroid consists of Bruch's membrane, Choriocapillaries, Sattler's layer with medium-sized vessels, Haller's layer with large choroidal vessels, and suprachoroid. The choroid also helps in the thermoregulation of the retina, participates in the uveoscleral pathway, and helps maintain intraocular pressure.[11]

The Optic Nerve Head

The intraocular part of the optic nerve, which is the second cranial nerve, comprises the optic nerve head or the optic disc up to the lamina cribrosa.[12] This is visible during ophthalmoscopy and is oval in shape, with a yellowish or reddish color. It consists of axons originating from the ganglion cell layer, and they get myelinated at the lamina cribrosa, where they exit the eye. The neuroretinal rim is the part of the optic disc that contains the axons and its thicker inferiorly and thinner temporally (ISNT) rule. The physiological cup is the central depression in the optic disc from the center of which the retinal vessels arise and branch.[13] In patients with glaucomatous damage, there is a thinning of the neuroretinal rim, which results in an increased size of the cup, causing an increased Cup disc ratio (CDR) which is an important clinical sign in patients with glaucoma.[14]

Indications

Scanning laser ophthalmoscopy has widespread applications for imaging the eye and can be classified as angiography, reflectance, and autofluorescence. Its main application is in detecting and monitoring the progression of the vitreoretinal disease and the optic nerve head in patients with glaucoma.

Contraindications

Scanning laser ophthalmoscope imaging of the eye is a non-invasive imaging modality with no contraindications. Dyes like sodium fluorescein or indocyanine green which are used to assess retinal and choroidal circulation, are relatively safe and commonly used in day-to-day practice.[21]  A previous history of anaphylactic reactions to these dyes is an absolute contraindication for administration. Both dyes are considered category C drugs by FDA and are contraindicated in pregnancy. Sodium fluorescein is metabolized and excreted by the kidneys. It can be used with caution in patients with cardiac and renal failure.[22]

In patients undergoing dialysis, it can be safely used as the dialysis procedure eliminates it. Indocyanine green is metabolized by the liver and is contraindicated in patients with liver diseases. In addition, it is contraindicated in individuals with uremia, iodide, and shellfish allergies.[23]

Equipment

The components of a scanning laser ophthalmoscope include a laser source, beam splitter, detector, scan unit, and imaging optics (Figure 1). The laser emitted from the laser source is collimated by the lens, and the collimated beam passes through a beam splitter and then into the beam scanner, which generates a raster line scan of the retina.[2] The reflected laser beam, along with the backscattered light, is sent back to the beam splitter, where only the deflected light is sent through the focal lens and a confocal aperture by the detector, which then generates the images.[24]

When a barrier filter for angiography is present just before the detector, thereby reflecting away the reflected laser beam and allowing only the excited wavelength to pass through. The scanning laser ophthalmoscope uses a laser beam of 490 nm for excitation and a barrier filter of 530 nm. For indocyanine green angiography,  a laser beam of 490 nm is used for excitation and a barrier filter of 830 nm.[25] The SLO continuously scans the fundus using a blue wavelength of 488 nm for fundus autofluorescence, and the images are obtained immediately. A barrier filter of 500 nm is used to block the reflected light.[26]

Personnel

The SLO can be operated by an ophthalmologist, optometrist, paramedics, or a trained ophthalmic photographer. While performing FFA or ICG, it is better to do it in the presence of an anesthetist to manage any unforeseen anaphylactic reactions, allergic reactions, or complications.[27]

Preparation

The patient is first clearly explained about the procedure. Pupillary dilatation is not mandatory since SLO can capture good-quality images in the non-mydriatic state. Proper sterilization of the head and chinrest is essential in between consecutive patients.[28] The patient is made to sit comfortably before starting the procedure. The patient is also explained they may have to look in different gazes as instructed by the photographer when a conventional 30- or 55-degree image is taken.[29]

When performing dye-based angiography, informed written consent is obtained. The patient is clearly explained about the procedure and duration of angiography and the possible risks of performing the procedure. A thorough systemic history is elicited, including any history of anaphylaxis or allergic reactions to dyes or other medications.[9] In case of advanced cardiac, renal, or other systemic illness, physician clearance is obtained beforehand. Though it is commonly performed in the outpatient department, it is necessary that the patient has an attendee. The anesthetist is informed before starting the procedure, which is done in the presence of the attendee in high-risk patients.[30]

The crash cart is kept ready and checked if all the emergency medications are available before every patient undergoes angiography. Premedications with antihistamines or corticosteroids can be done in patients with a history of hypersensitivity reactions. The patient can have a light meal, preferably 2 to 4 hours before the procedure, to avoid vomiting commonly seen with sodium fluorescein. It is preferable to have well-dilated pupils to reduce artifacts. Few control images are taken, and the focus is adjusted. An intravenous line is secured, and the arm is placed comfortably on the armrest.[31]

Technique or Treatment

For non-invasive procedures like multicolor and ultra-widefield fundus imaging, autofluorescence, OCT, and OCTA, patients are asked to remain still without blinking and to fix using an internal or external fixation target to focus on the area of interest.[32] For dye-based angiographic procedures, the timing of dye injections is coordinated with the image capture. Images are captured at different time intervals helping in visualizing the circulation of the dye through the vasculature and may help us pick up various pathologies. Based on the time from dye injection, the angiography is divided into different phases, and the photographer tries to highlight the significant phase in the involved eye to aid in diagnosis.[33]

Complications

There are no complications with non-invasive fundus imaging.

Complications associated with Fundus Fluorescein Angiography 

Minor complications include yellowish discoloration of urine and rarely skin. The patient is counseled pre-procedure and advised to hydrate adequately. The commonest side effect includes nausea which may be associated with vomiting. This can be avoided by pre-medicating the patients with anti-emetics in highly susceptible patients, avoiding doing the procedure on the full stomach, and a slow injection of the drug over 5 to 10 seconds. Extravasation of the dye into the surrounding tissue can occur, causing severe pain and rarely tissue necrosis. Other mild side effects include itching, urticaria, and pruritus, which can be managed using antihistamines. Severe side effects include vasovagal syncope, sudden hypotension, fainting, cardiopulmonary arrest, and sudden death.[26]

Complications associated with Indocyanine Green Angiography

Though various adverse events have been documented during ICG, including nausea, vomiting, rashes, and anaphylactic reactions, they are generally rare compared with FFA. The incidence of these reactions is higher in patients with uremia and must be used cautiously in such instances. Patients with iodide allergy are more prone to anaphylactic reactions.[26]

Clinical Significance

Multicolor Fundus Photography

Multicolor imaging of the fundus uses lasers of three different wavelengths to capture an image of the retina in contrast to the bright flash of light used to capture routine color fundus images. The blue (488 nm), green (515 nm), and infrared (820 nm) wavelengths depict information from different retinal depths.[34] The blue wavelength penetrates up to the inner retina and depicts information from the vitreoretinal interface - the retinal nerve fiber layer and the macular surface. The green wavelength penetrates up to the inner retinal layers highlighting details about retinal vasculature and the exudation of blood, lipids, and fluid in these layers.[35] The longer infrared wavelength penetrates up to the choroid and the outer retina and may depict changes in these layers. The field of view it provides is usually 30- or 55 degrees.[36] 

The advantages of multicolor imaging include high contrast images, better resolution, reduction in image noise due to eye tracking mechanism, better comfort to the patient as laser beams are used for image capture instead of a bright flash of light, and the ability to image in a miotic pupil. The disadvantages include that it requires a slightly more extended fixation period and is highly operator dependent on acquiring artifact-free images.[37] 

Fundus Autofluorescence

Fundus autofluorescence is a non-invasive technique that utilizes the fluorescent properties of inherently occurring substances in the retina. Lipofuscin is the predominant fluorophore of the retina and is composed of over ten bisretinoid compounds, which are byproducts of the vitamin A cycle. Among them, N-retinyl-N-retinylidene ethanolamine (A2E) is the best-characterized component of lipofuscin.[38] Melanin and rhodopsin are other naturally occurring fluorophores requiring longer excitation wavelengths. The confocal optics also helps in avoiding scattered light. Hyperautofluorescence indicates increased production of lipofuscin, as is lipofuscinopathies and dystrophies, abnormal accumulation of lipofuscin, or retinal pigment epithelium defects where the masking effect is reduced.[39] Hypoautofluorescence may occur due to the absence or decreased production of lipofuscin or due to blockage by other material overlying it. Fundus autofluorescence is an invaluable tool in diagnosing and monitoring the progression of various vitreoretinal conditions.[40]

 Contrast-enhanced Angiography - Fundus Fluorescein Angiography and Indocyanine Green Angiography

Fundus fluorescein angiography is a technique of imaging the retinal vasculature by using sodium fluorescein dye. Indocyanine green angiography utilizes a higher wavelength that penetrates the retinal pigment epithelium and helps in imaging the choroidal vasculature. The transit of the dye through the retinal vasculature gives an idea of circulatory disturbances in the retina and choroid. When coupled with ultra-widefield imaging can give an idea of the dye transit through the entire retina and choroid.[41][42]

Adaptive Optics

Adaptive optics technology eliminates the higher and lower-order aberrations that result in the ability to image individual photoreceptors. It has been successfully integrated with fundus photography, SLO, and optical coherence tomography. It helps assess the cone density and distribution in the macula in normal individuals and diseased states like inherited retinal diseases. Hence, it may be utilized to evaluate the extent of damage caused by the disease's progress, monitor progression, and assess response to therapeutic interventions like gene therapy. The utility of adaptive optics is still in the research stage and is yet to be introduced for day-to-day clinical use.[43]

Ultrawidefield Imaging

Ultra-widefield imaging enables to capture of the entire retina up to 200 degrees, depending on the commercial models used. This is especially useful in children and anxious patients who might be unable to fix in different gazes to image the different parts of the retina. The ability to capture such an extensive field in a non-mydriatic state is an added advantage and may be useful in patients in whom dilatation is contraindicated or unwilling for dilatation. It has a few disadvantages, like pseudocolor imaging and low posterior pole resolution. Though binocular indirect ophthalmoscope is the gold standard of examination of the retina, wide-field imaging may document and monitor progress in patients with vitreoretinal and choroidal pathologies.[44]

Enhancing Healthcare Team Outcomes

An interprofessional approach is essential in performing the investigative procedures in scanning laser ophthalmoscopes. When an optometrist or trained ophthalmic photographer is performing the procedure, it is necessary to focus on the areas of interest and highlight them accordingly. Hence a basic understanding of anatomy and pathophysiology will help in this purpose.[45] The photographer needs to precisely capture the areas of interest and critical phases in the angiogram. Moreover, an entire team of ophthalmologists, anesthetists, nurses, or mid-level ophthalmic professionals is required to perform invasive procedures like angiography. Coordination among the team members and preparation to manage complications will help in the smooth performance of these procedures.[46]

Nursing, Allied Health, and Interprofessional Team Interventions

The nursing, allied health staff, and interprofessional team play a key role in patient recruitment to the scanning area, explaining the procedure, comforting the patient as well as counseling the patient. They also help plan the intervention at the correct point of the team when the patient is systemically fit and can undergo the procedure.

Nursing, Allied Health, and Interprofessional Team Monitoring

The nursing, allied health staff, and interprofessional team also help in monitoring patient parameters while the scanning is being performed. In case of any emergency and need for code blue, they play a key role in managing the patients.