Imaging the eye and measuring refractive error





Objectives


After working through this chapter, you should be able to:




  • Explain why it is difficult to see the back of the eye and the anterior angle without the help of special devices



  • Explain how direct and indirect ophthalmoscopy works



  • Explain how direct and indirect gonioscopy works



  • Explain retinoscopy and how it works



  • Explain how applanation tonometry works



Introduction


In this chapter, we’re going to use some of the knowledge we gained in previous chapters in order to start to understand some of the basic principles behind imaging the inside of the eye (which as you might imagine, requires special optical systems), before moving on to discuss how we can use optical physics to determine a person’s refractive error and how applanation tonometry works.


Please note that this book is not designed to be a guide for how to perform these techniques safely in clinical practice; instead consider this an interesting summary of the underlying principles to help you understand the techniques.


Imaging the eye


To start with, let’s think about imaging the inside of the eye. Now, when I say ‘imaging’ here, I’m referring to the ability to produce an optical image of the inside of the eye. The reason for needing to do this would be to assess the health of a patient’s eye, so it is an essential part of an optometrist’s daily practice. However, if you’ve ever met another human being before you’ll know that we can’t just look directly into a person’s eye, because in normal circumstances the pupil (the aperture in the iris leading to the back of the eye) appears black.


Why can’t we see inside the eye anyway?


Let’s begin by considering the reduced human eye that we learned about in chapter 5 ( Fig. 15.1 for a refresher). This reduced eye assumes that the overall power of the eye will be +60.00D, and the retina (the sensory tissue at the back of the inside of the eye) will exist at the secondary focal point of the eye (+22.22 mm). This means that we can assume that light reflecting from the retina (forming an image of the retina) will possess a vergence of −60.00D when it reaches the front surface of the cornea, and that therefore the light leaving the patients’ eye will have parallel vergence. Following this, we can start to understand that an in-focus image of the retina will be ‘at infinity’ (we can confirm this using vergence equations from chapter 2 ). This means that in fact, we absolutely should be able to see the retina from outside the eye, providing we can line ourselves up with the exit pupil of the eye (see chapter 13 for a reminder of this idea). However, there are two issues:



  • 1.

    The inside of the eye is very dark, and we can only see illuminated objects (see chapter 1 for a review of this).


  • 2.

    The human pupil is usually very small (4.3 mm ), which will severely restrict our field of view .




• Fig. 15.1


Reduced model eye with power, distances, and refractive indices labelled appropriately.


Logically, then, if we could illuminate the inside of the eye somehow, and overcome the issue of the field of view, then it should be possible to see inside the eye.


Ophthalmoscopy


One method for viewing the back of the eye is called ophthalmoscopy ( ophthalmo-, eye; -scopy, view ), and there are two main types: direct ophthalmoscopy and indirect ophthalmoscopy .


Direct ophthalmoscopy


Direct ophthalmoscopy requires the use of a hand-held device called an ophthalmoscope. This device sends parallel (zero vergence) light into the eye, which will then focus on the retina, thanks to the focusing power of the cornea and lens. The light then travels along the same path back out of the eye, which means it leaves the eye as parallel light again. This parallel light can then approach the clinician’s eye, which will form a lovely, focused image of the patient’s retina on the clinician’s retina and allow them to see exactly what’s going on inside the patient’s eye ( Fig. 15.2 ).




• Fig. 15.2


Breakdown of how direct ophthalmoscopy images the patient’s retina (right) to allow the clinician to see what it looks like (left).


The nice thing about direct ophthalmoscopy is that it produces a virtual (upright) image, so if the top of the image looks a bit suspect, then it means the top of the retina looks a bit suspect, and it’s easy to relate the two (unlike with indirect ophthalmoscopy which produces a real, inverted image – see section ‘Indirect Ophthalmoscopy’ for more details).


Assuming that the clinician has corrected vision, and the patient is emmetropic (no refractive error), then the clinician will not need a lens of any power in order to see the patient’s retina. Instead, the clinician will just need to get very close to the patient – we’re talking 1 to 2 cm away – in order to be able to see through the patient’s pupil. However, due to the size of the pupil, the clinician will still only be able to see a small amount of the retina at any one time (small field of view), which means they will need to change their viewing angle to see other parts of the retina (a bit like viewing through a keyhole – Fig. 15.3 ). This is why, if you’ve ever had this done, clinicians will wiggle around and do the ophthalmoscopy dance in front of you – they’re attempting to see all the parts of your retina. The clinician will also usually ask the patient to move their gaze (upwards, up and to the right, rightwards, down and to the right, etc.) to help them view as much of the back of the eye as possible.




• Fig. 15.3


Figure showing clinician using a direct ophthalmoscope to look at the patient’s fovea (A) and then adjusting their position to look at the inferior (bottom) part of the patient’s retina (B).


However, this method only allows the clinician to use one eye at a time (due to how close the clinician needs to be to the patient), which can be an issue if the clinician has amblyopia or any other condition affecting the vision of a single eye.


The other, slightly complicated factor to think about is that if the patient (or clinician) has a refractive error, then the light from the ophthalmoscope will be focused incorrectly (either behind or in front) of the patient or clinician’s retina, and so the ophthalmoscope will need to be adjusted to account for that.


Indirect ophthalmoscopy


In contrast to direct ophthalmoscopy, indirect ophthalmoscopy allows the clinician to use two eyes ( stereoscopic viewing ) and utilises a high-power plus lens (e.g. +20.00D, +90.00D) called a condensing lens . Light is shone through the condensing lens and focused on the patient’s retina, and, just like before, the light will reflect back out of the eye following along the same path and passing back through the condensing lens ( Fig. 15.4 ). This lens then forms an image of the retina which is magnified but inverted (upside down and flipped left to right), which is passed to the clinician’s retina in order for them to see the image. The field of view will be determined by the power of the condensing lens – so clinicians will usually have a preferred type of lens.




• Fig. 15.4


Breakdown of how indirect ophthalmoscopy images the patient’s retina (right) to allow the clinician to see what it looks like (left).


The advantages of the indirect ophthalmoscopy technique (compared to direct) are that the clinician can view the patient’s retina using both eyes, and thanks to the power of the condensing lens, the field of view will be much greater. This method can be utilised using either a head-mounted binocular indirect ophthalmoscope (abbreviated as BIO) or with a slit-lamp biomicroscope (abbreviated as SL). If using a slit-lamp biomicroscope, then another thing clinicians need to think about is that they can’t move themselves very easily to view different parts of the patient’s retina. To get around this, they will ask the patient to move their gaze (upwards, up and to the right, rightwards, down and to the right, etc.) to help them view as much of the fundus as possible.


Gonioscopy


Now, let’s imagine that we’re not as interested in the back of the eye, but instead we’d be interested to see the anterior angle . We haven’t discussed much anatomy and physiology in this book, so you’ll just have to trust me when I say that the anterior angle (the point where the inside surface of the cornea becomes the limbus and connects to the iris – shown by an arrow in Fig. 15.5 ) is an important site for aqueous drainage . Aqueous humour is the name for the fluid that exists in the front part of our eye, and it helps to nourish certain structures and also helps to maintain a certain intraocular pressure (IOP) . IOP is important because if the pressure goes too high then this can lead to damage which can ultimately lead to sight loss – so it’s useful for clinicians to view the anterior angle and check that it’s healthy in order to determine whether the aqueous is able to drain effectively.




• Fig. 15.5


Diagram showing cross-section of an eye (left) with zoomed-in section (red dashed line) to show anterior angle (black arrows).


However, the problem is that it’s not possible to see the anterior angle from outside the eye, as total internal reflection occurs. If you recall back in chapter 9 we discussed the idea that when moving from a medium with a higher refractive index into a medium with a lower refractive index, if the angle of incidence of light leaving the object exceeds the critical angle for that medium, then all the light from the object will be reflected back into the higher refractive index medium (which is appropriately called total internal reflection). This is what happens with the anterior angle in the eye (as seen in Fig. 15.6 ), where due to the refractive index difference between the aqueous and cornea (1.336 and 1.376), relative to the air (1.00), the light from the anterior angle meets the corneal surface at an angle that exceeds the critical angle (i c ). This causes it to reflect back into the anterior chamber – making it seemingly invisible to anyone outside of the eye.




• Fig. 15.6


Illustration showing cross-section of the anterior eye. Light from the anterior angle (see Fig. 15.5 ) suffers from total internal reflection due to the refractive index difference between the inside of the eye (relative to the outside).


In order to get around this, clinicians need to utilise a goniolens – a special lens for viewing the anterior angle. In general terms, these lenses come in two forms to allow clinicians to perform: direct gonioscopy ( Fig. 15.7 A) and indirect gonioscopy ( Fig. 15.7 B). Both techniques require a lens to be placed directly onto the front surface of the eye (so you’ll be relieved to know that the eye will be anaesthetised first).


Mar 12, 2023 | Posted by in OPHTHALMOLOGY | Comments Off on Imaging the eye and measuring refractive error
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