Overview of Scanning Laser Ophthalmoscope
A scanning laser ophthalmoscope, or SLO, is a tool that sends a very thin beam of laser light into your eye to create a detailed picture of your retina and optic nerve head. The retina is the back part of your eye where images are processed, and your optic nerve is the “cable” that sends those images to your brain.
What makes the SLO special is that it uses a lot less light to create this picture than some other types of eye cameras like an indirect ophthalmoscope or a conventional camera for the back of the eye. This means it’s more comfortable for you when you have this procedure done. The first kind of SLO tool was invented by Robert H. Webb in 1980 and was called the “flying tv ophthalmoscope.”
Even though the SLO was ready for doctors to use by 1990, it wasn’t used very much at first because the pictures weren’t as clear as other tools, the equipment was big and it was expensive. But, new improvements in technology have made it much better. The new and improved SLOs have stronger lasers, smaller apertures (which are like small windows that the laser light goes through), and very sensitive detectors that pick up the light. This means the pictures of your retina and optic nerve are now very high quality.
The SLO has become a very important tool for eye doctors. Especially when it is used with something called adaptive optics, a technology that corrects for changes in your eye that cause blurry images. This even allows them to see individual cells in your retina while looking into your eye!
Anatomy and Physiology of Scanning Laser Ophthalmoscope
The retina, which is part of the eye, serves a valuable function: it interprets light so we can see. Think of it as the eye’s film – like in a camera – that captures a scene before translating it into an image. It extends from a part known as the optic disc to the Ora Serrata, and has ten layers, with the inner nine responsible for processing light and signals.
The tenth layer, the retinal pigment epithelium, is separated from the light-processing section by a tiny gap. The retina has a central section called the macula, home to a variety of pigments. The most sensitive area in the retina includes parts known as the fovea and foveola. They focus on capturing the details in the center of our vision.
Blood is supplied to the inner layers of the retina from the central retinal artery, that comes from another artery called the internal carotid artery. The outer parts of the retina get their nourishment from the choroidal circulation. In about 20% of people, a vessel known as the cilioretinal artery provides blood supply to the macula. Essential barriers in the retina control fluid movement to maintain the retina’s health; assessments of any health issues here involve a test called fundus fluorescein angiography (FFA). It’s the retina’s job to translate light into electrical signals, which are then sent to a brain area known as the occipital cortex.
The choroid is the layer of tissue between the retina and the shell of the eye. It is dark and full of blood vessels, stretching from the optic disc to an area known as the ciliary body. This receives blood mainly from vessels called the long and short posterior ciliary arteries. The choroid comprises several layers, including Bruch’s membrane and Choriocapillaries. It plays a role in controlling how warm the retina is, contributes to eye pressure regulation, and participates in a process known as the uveoscleral pathway.
The optic nerve head, which is an intrinsic part of the optic nerve, is visible during an eye exam. It appears oval and tends to be either yellowish or reddish. This area has nerve fibers linking it to ganglion cells, which get insulated at a part of the optic disc known as the lamina cribrosa. This area is key in understanding if a person has eye diseases like glaucoma – it tends to change shape under disease conditions.
Why do People Need Scanning Laser Ophthalmoscope
Scanning laser ophthalmoscopy is a tool used to take images of the eye. It’s a powerful technique used for spotting and monitoring the progression of eye diseases, particularly those that involve the vitreous humor and retina (the back of the eye), and the optic nerve head, which is the area where the nerves from your eye connect to your brain. One such disease is glaucoma, a condition that damages the optic nerve.
This imaging technique can be used for various types of eye examinations. Let’s take a look at some:
Multicolor Fundus Photography: This is used to check for conditions like:
– Diabetic retinopathy: damage to the eye’s blood vessels caused by diabetes.
– Vascular occlusions: blockages in the blood vessels of the eye.
– Retinal dystrophies: a group of genetic eye disorders causing progressive sight loss.
– Macular disorders, and other problems including tumors and inflammation.
Fundus Autofluorescence: This is used to detect conditions such as:
– Age-related macular degeneration: a condition causing loss of vision in the center of the visual field.
– Retinal dystrophies.
– Drug toxicities affecting the retina.
– White dot syndromes: conditions leading to white dots appearing in the retina.
– Posterior uveitis: inflammation of the back part of the eye.
– Central serous chorioretinopathy: a disease causing fluid build-up under the retina.
Fundus Fluorescein Angiography: This is used to examine conditions like:
– Diabetic retinopathy.
– Retinal vein occlusions: blockages in the veins of the retina.
– Wet age-related macular degeneration: the advanced stage of age-related macular degeneration.
– Other conditions leading to new, weak blood vessels developing under the retina.
Indocyanine Green Angiography: This is used in the diagnosis of conditions such as:
– Wet age-related macular degeneration.
– Polypoidal choroidal vasculopathy: a condition causing abnormal blood vessel growth under the retina.
– Central serous chorioretinopathy, and other conditions causing thickness or swelling of the layer below the retina.
– Choroidal tumors and inflammation diseases of the eye.
Adaptive Optics: This allows us to view the cone photoreceptors (cells in your eyes that let you see color) in healthy and diseased eyes.
In addition, scanning laser ophthalmoscopy is used in monitoring glaucoma. It can help detect changes in the optic nerve, which often precede defects in the field of vision.
When a Person Should Avoid Scanning Laser Ophthalmoscope
Imaging of the eye with a scanning laser ophthalmoscope is a safe process that doesn’t involve any invasions into the body, and generally, everyone can undergo this imaging. Doctors use certain dyes, like sodium fluorescein or indocyanine green, to better see the blood flow in the retina (the back part of the eye) and choroid (layer of blood vessels and connective tissue in the eye). These dyes are usually safe and they are often used.
However, if you’ve previously had a severe allergic reaction to these dyes, then they should not be used. Also, these dyes are not recommended to be used during pregnancy. The sodium fluorescein dye is broken down and removed by the kidneys. It can be used carefully in patients with heart problems or kidney failure. If you are a patient undergoing dialysis, the dye can be safely used as it gets removed during the dialysis process.
On the other hand, indocyanine green dye is broken down by the liver, hence, it should not be used if you have liver diseases. It’s also not recommended if you have uremia (a condition where waste products are present in the blood), are allergic to iodide (a kind of salt), or have a shellfish allergy.
Equipment used for Scanning Laser Ophthalmoscope
A scanning laser ophthalmoscope, a tool used to examine your eyes, is made up of a few key parts, including a laser light source, a device to split the beam of light, a scanner to create detailed eye image, a detector, and lenses for focusing the light.
Here’s how it works: a laser ray starts from the device, is focused by a lens, and travels through a beam splitter. The split beam then enters the scanner, which creates a detailed map of your eye’s retina. As the laser light reflects off your retina, it also creates a scattered light. These two types of light, the reflected and the scattered, travel back to the beam splitter. Here, only the deflected light moves through the lens and onto a small hole (a confocal aperture), before reaching the detector. The detector then transforms this light information into the images of your eye’s interior.
If there’s a special filter placed in front of the detector, it helps by reflecting the laser light away and only allowing through the light of a specific wavelength. The scanning laser ophthalmoscope uses a blueish laser with a wavelength of 490 nm, and a filter that only allows light of 530 nm wavelength through. For a type of eye scan called indocyanine green angiography, the same laser light is used, but this time, the filter only lets through light with a longer wavelength of 830 nm. Additionally, for another type of imaging, fundus autofluorescence, the device continually scans the back of your eye (the fundus) using a laser with a wavelength of 488 nm. The resulting images are available immediately. To block reflected light, another filter of 500 nm is used.
Who is needed to perform Scanning Laser Ophthalmoscope?
The eye examination known as SLO, can be done by different types of eye doctors and professionals. This includes an ophthalmologist (a medical doctor who specializes in eye health), an optometrist (a health care professional who can examine the eye for certain problems), paramedics (healthcare workers who respond to medical emergencies), or a trained person who can take special pictures of your eye. If a special type of eye imaging test called FFA or ICG is being performed, it’s generally safer to have an anesthetist around. They are doctors who specialize in giving anesthesia, which is a medicine that either relieves pain or puts you to sleep. This is to make sure that they can quickly handle any unexpected severe allergic reactions or other problems that might happen during the test.
Preparing for Scanning Laser Ophthalmoscope
The first step to the procedure is to make sure the patient understands what is going to occur. It isn’t always necessary to dilate, or widen, the pupils as the SLO (a type of imaging device) can take good-quality pictures without dilation. It’s very important to clean the head and chin rest between each patient.
Once the patient is seated comfortably, they are informed that they will need to look in different directions based on the photographer’s instructions, especially when a regular 30- or 55-degree picture is taken.
When it’s required to use dye for the pictures (a procedure called angiography), the patient is asked to give written agreement after understanding the procedure as well as the risks associated. The doctor also asks about any previous severe allergic reactions to dyes or medications. If they have serious heart, kidney, or other illnesses, the doctor will need clearance from another physician. Although it’s usually performed with the patient going home the same day, it’s important for the patient to have someone with them. In high-risk cases, your anesthetist – a specialist in providing anesthesia – is informed first, and the procedure is performed in the presence of the patient’s companion.
To be prepared for any emergencies, a crash cart filled with emergency medicines is prepared before each angiography. Patients who have had severe allergic reactions before can be given antihistamines or corticosteroids as a preventative measure. It’s best for the patient to eat a light meal 2 to 4 hours before the procedure to limit any nausea from the sodium fluorescein, a dye used in the procedure. It can be helpful to have the pupils dilated wide to minimize any errors in the pictures. Some initial pictures are taken to set the focus, then an IV line is set up, and the patient’s arm is positioned comfortably on the armrest.
How is Scanning Laser Ophthalmoscope performed
When doctors are doing eye imaging procedures that are not invasive, like multicolor and ultra-widefield eye pictures, autofluorescence (measuring natural light that the eye generates), OCT (Optical Coherence Tomography, which is a scan that provides images of your eyes), and OCTA (a type of OCT that shows the blood flow in your eyes), they need you to keep still and don’t blink. They will give you an internal (inside the machine) or external (outside the machine) point to look at to help focus on a specific area.
For imaging techniques that require an injected dye (angiographic procedures), the dye is injected and pictures are taken at different times. This helps doctors to track how the dye moves through the blood vessels in your eye and can help them spot any issues. The timing of the dye injection is carefully planned to align with the image capture. Depending on when the dye is injected, angiography – which is an x-ray examination of your blood vessels – can be separated into different stages. The eye specialist tries to focus on the crucial stage for further analysis in the affected eye.
Possible Complications of Scanning Laser Ophthalmoscope
Taking pictures of the back of your eye (non-invasive fundus imaging) doesn’t lead to any complications.
But, there are two other tests that can lead to some side effects:
1. Fundus Fluorescein Angiography: This method injects a special dye into your bloodstream to highlight the blood vessels in your eyes. This could lead to some minor side effects like yellow urine or skin. You might also feel nauseous and might throw up. To prevent this, doctors suggest drinking lots of water before the test, not eating much before the test, and inject the dye slowly. Sometimes, the dye can leak into the skin and cause severe pain or tissue damage, though this is rare. Other side effects can include itching, hives, or irritations which can be treated with allergy medication. More serious complications include fainting, low blood pressure, and rare cases of cardiac arrest or sudden death.
2. Indocyanine Green Angiography: This test also uses a dye, but the side effects are generally less common. They can include nausea, vomiting, rashes, or severe allergic reactions. If you have a severe kidney disease (uremia) or are allergic to iodine, you might be more at risk for these reactions. These risks are rare compared to Fundus Fluorescein Angiography.
What Else Should I Know About Scanning Laser Ophthalmoscope?
Multicolor fundus photography is a way to take a picture of the back of your eye (or the retina) using lasers of different colors: blue, green, and infrared. These lasers give us detailed information about different layers of the retina, which can help doctors identify problems. The perks of this method include clearer images, more comfort for patients since it doesn’t require a bright flash of light, and the ability to take pictures even when the pupil is small. However, using this method requires the patient to keep their eye still for a little longer and depends mainly on the expertise of the person taking the images.
Fundus autofluorescence is another way to examine the back of your eye. It takes advantage of natural substances in the retina that shine or “fluoresce” under certain conditions, primarily a substance called lipofuscin. This technique can provide valuable insights into various eye conditions by identifying areas with too much or too little lipofuscin.
Contrast-enhanced angiography, such as fundus fluorescein angiography and indocyanine green angiography, involve injecting a dye (like sodium fluorescein or indocyanine green) into your bloodstream to visualize the blood flow in your retina. This technique helps in imaging your eye’s blood vessels and identifying any disturbances in blood flow.
Adaptive optics is an advanced technology used to correct blurry images caused by imperfections or “aberrations” in the eye. It enables us to examine individual light-sensitive cells (photoreceptors) in the retina. Though this technology is still under research, it may eventually help track disease progression and evaluate the effectiveness of therapies like gene therapy.
Ultrawidefield imaging is a method that captures a complete, wide-angle picture of the retina – up to 200 degrees, depending on the device. This method is particularly useful for children or anxious patients who may have difficulty focusing their gaze. Its biggest advantage is that it can take extensive images even when the eye is not dilated, making it helpful for patients who cannot or do not want their eyes dilated. However, it may produce false color images and lower resolution for the center of the back of your eye.
