C52247: Examination of the retina

One of an optometrist’s primary roles is to examine the internal structures of the eye for pathology. As is well documented, one of the key legal functions of a sight test is ‘to perform, for the purpose of detecting signs of injury, disease or abnormality in the eye or elsewhere… an intra-ocular examination, either by means of an ophthalmoscope or by such other means as the doctor or optometrist considers appropriate.’1 Indeed, the increasing breadth of practice afforded to optometrists especially with shared care means that knowledge of different forms of fundus examination is essential. While imaging techniques are becoming ubiquitous in primary care practice, a thorough fundoscopic examination should remain a key component of any examination of the eye and will be the focus of this article.

What is the fundus?

The fundus is a term used to describe the inner surface of the eye. There are differing views on the terminology used to describe the fundus but clinically speaking the main landmarks are the fovea, macula, central retina and peripheral retina. The exact definition of the macula varies, clinicians generally describe it as an area of diameter 1.5mm in the central region,2,3 while for anatomists and others the area is larger. For example, England’s NHS Diabetic Eye Screening Programme’s (NDESP) definition ‘that part of the retina which lies within a circle centred on the centre of the fovea whose radius is the distance between the centre of the fovea and the temporal margin of the disc’.4

The retina extends all the way to the ora serrata, a 2mm-wide scalloped junction between the retina and ciliary body. It lies roughly 23-24mm temporal to the optic disc, and 18.5mm nasally.2

History of retinal examination

The first written record of something beyond the pupil is Roman author Pliny the Elder’s description of the red reflex, but it was not until much later that deeper exploration into the anatomy behind this was conducted. In the 18th century French surgeon Jean Méry visualised the retinal vasculature of a cat when placed under water. In 1847 Charles Babbage invented an instrument to view the fundus but was discouraged from pursuing this when a distinguished ophthalmologist of the time could not obtain a useful image. A few years later in 1851 Hermann von Helmholtz invented the Augenspiegel (eye mirror), an ophthalmoscope that proved to be the first widespread instrument for examining the internal features of the eye. Over years many innovations have lead to the modern techniques described below.5

Examination of the retina – to dilate or not?

Fundus examination can be greatly enhanced by the use of topical mydriasis. Even in those patients with large pupils stereopsis and field of view are much greater when pupils are dilated, however, these benefits must be weighed against the potential risks as well as the inconvenience that it may cause the patient.

As a practitioner, it is worth asking yourself whether to dilate all patients that come to see you. There is a surprising paucity of literature on the subject. A retrospective review of the records of patients with good visual acuity and no risk factors by Pollack and Brodie6 found only 0.3% of patients had a clinically significant finding beyond the field of view of a direct ophthalmoscope when dilated (all of those being choroidal naevi in this particular study). It is worth noting that this study did not compare findings with indirect methods of examination.

With a growing emphasis in healthcare on carrying out the minimum appropriate intervention, be that investigation or treatment, one must also consider if it is best patient care to dilate every patient. Clinical judgement should be used when assessing your view of the patient’s fundus. Dilation should certainly be considered if the patient has particularly small pupils or your view is hampered by media opacities, or if the patient presents with symptoms, signs or risk factors for pathology.

The most common mydriatic drops used by optometrists are tropicamide (0.5% and 1%), an antimuscarinic which works by blocking the action of the iris sphincter muscle, and phenylephrine (2.5% and 10%), an alpha receptor agonist which contracts the iris dilator muscle. Dilation should be carried out with due care because of possible adverse reactions. Commonly, patients experience blurring and photophobia for several hours and if necessary the patient should be advised not to drive until the vision has cleared, and to bring sunglasses with them to their appointment. Intraocular pressure should also be measured before and a reasonable time after instillation to ensure there is no significant increase. There are also more severe adverse events which should be borne in mind:

• Acute angle closure – a large scale review in 2007 found a risk of acute angle closure as low as one in 20,000, with the risk of missing pathology vastly outweighing the risk of closure and some studies suggesting occludable angles actually being a poor predictor of risk. In fact, it has been suggested that because of the pathophysiology behind angle closure (ie pupil block) occurring in a mid-dilated state there may be more of risk when sitting in a darkened room than using mydriatic drops!8 However, from a medico-legal point of view it is advisable to check the depth of the anterior chamber using the Van Herick technique, Smith technique, or gonioscopy, measuring intraocular pressures before and a reasonable time after dilation and providing the patient with verbal and written advice.
• Potential systemic complications with phenylephrine – a recent large systematic review by Stavert et al 9 showed a slight short term increase in blood pressure and heart rate with phenylephrine 10% but no significant change with 2.5%, the latter being suggested as the practitioner’s first choice. It must be borne in mind that topical phenylephrine should not be used in patients taking antihypertensive medication (because it may reverse their action), monoamine oxidase inhibitors (due to the risk of adrenergic reactions) and tricyclic antidepressants (due to the risk of cardiac arrhythmia).10

So what drug should an optometrist choose? For healthy adults tropicamide 0.5% is often enough, but in some cases (for example diabetic patients) or those that require maximal dilation (for example those that present with symptoms of retinal breaks) a combination of tropicamide 1% and phenylephrine 2.5% should be used. Instillation of a topical anaesthetic prior to the mydriatic can enhance their effect by reducing reflex tearing and increasing corneal permeability.

Examination of the retina – to indent or not?

Scleral indentation (or scleral depression) is used to view the extreme peripheral fundus to the ora serrata. It is contraindicated in those with recent surgery, penetrating trauma, those with tumours or scleral thinning (eg scleromalacia perforans) and should be used in extreme care in pseudophakes.11 However, the Royal College of Ophthalmologists 12 and American Academy of Ophthamology 13 recommend this technique is used in all patients who present with symptoms of retinal breaks despite relatively low strength of evidence.14

However, the College of Optometrists 15 and NICE 16 both recommend that dilated indirect ophthalmoscopy and assessing for Schafer’s sign (tobacco dust) are enough, and make no reference to scleral indentation. Again, clinicians should exercise their judgement on whether this is required and refer if appropriate.

Documentation of findings

Ensuring thorough recording of findings, both positive and negative is vital, particularly with the risk of medico-legal action. Each of the structures examined must be commented on including external eye, media, blood vessels, disc, macula and peripheral retina. The use of ticks and ‘NAD’ have fallen out of favour as they can be open to misinterpretation. Drawing abnormal findings can be useful but one must take care when using indirect methods of examination as the image is reversed and inverted. A handy method of overcoming this is by simply turning the page upside down and drawing the lesion as you see it. One must also remember that when the patient is looking away from the primary position the outermost part of the fundus is seen in the part of the lens closest to the centre (eg, when the patient looks up, the lower part of the practitioner’s view is the uppermost part of the superior retina). With a headset binocular indirect ophthalmoscope, a recording chart with two concentric circles (the outer representing the ora serrata and the inner the equator) split into 12 clock hours may be used 17 but its use in a primary optometric setting is arguable (Figure 1).

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While measuring in disc diameters may be useful as a quick and straightforward method of recording size and location of retinal lesions, it is not as accurate as other methods. 18 Measuring fundus lesions is relatively straightforward when using an indirect lens. Optic disc height or fundus lesions such as naevi may be measured by overlaying a slit beam over the lesion in question and reducing or increasing the length of the slit to match. The measurement may be read off the height scale (often found on the illumination housing) and converted to the actual size by multiplying it by the magnification factor of the lens (see Table 1). The illumination housing should be rotated for non-vertical measurements. For those working with a slit lamp without a beam height scale, other than using disc diameters, a retinal scale attachment may be added to a Volk lens for comparative measurements.

Direct Ophthalmoscopy

Direct ophthalmoscopy is a commonly performed technique and is often the first form of fundus examination taught on undergraduate optometry and medical courses in the UK. A study by Myint and colleagues 19 found that in the UK, this is the only fundoscopic examination method performed by a quarter of community optometrists.

Advantages

• Wide magnification (around 15x in an emmetropic eye)20
• Portable
• Cheaper overall cost
• Erect image
• Easier to use than other methods on less mobile patients

Disadvantages

• No stereopsis
• Field of view is restricted to about 5°, depending on pupil diameter, refraction and viewing distance21
• Patient comfort – in order to maximise field of view practitioner-patient distance must be kept to a minimum, some patients may find this proximity uncomfortable
• Poorer view in patients with media opacities
• Can be ergonomically poor for the practitioner
• Poor view of the anterior segment

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Indications and contraindications

Direct ophthalmoscopy still has its place in a modern optometric setting. It can be useful when a magnified view is required of the posterior pole of less mobile patients or younger patients who may be unable to reach the slit lamp. For those patients who are dilated, a headset or other modified indirect techniques may provide a better view, especially in very young children.

Filters

There are several filters available on most modern direct ophthalmoscopes. The slit can be used to assess retinal elevations, a fixation cross can be used to assess eccentric fixation, green filters act as a red-free filter and is useful in examining the vasculature, haemorrhages or retinal nerve fibre layer, while the cobalt blue filter can be used in conjunction with sodium fluorescein in assessing corneal pathology.

How to…

1 Dilate the pupils, if necessary and safe to do so

2 Dim the room lights

3 Adjust the height of the patient’s chair so that you do not have to bend or stretch too far

4 In order to maximise your field of view by getting as close the the patient as possible, ask them to remove their spectacles (unless they have a moderate to high spectacle prescription, where magnification may become an issue)

5 Ask the patient to fixate a specific point straight ahead of them

6 Hold the ophthalmoscope with your right hand against your right eye when examining the patient’s right eye. Some practitioners prefer to keep their contralateral eye closed, while some find it more comfortable to keep it open

7 Dial up approximately +10D (adding your own spectacle prescription) and use a medium aperture circular white beam at moderate illumination

8 At about 10cm, view the red reflex and move yourself vertically and horizontally to assess the media. NB If an opacity remains stationary, this suggests it is around the pupil plane, whereas opacities seen moving against the direction of gaze are located on the posterior lens or further back

9 Move closer to the patient while reducing the power of the lens wheel until the fundus becomes focused. The final power will be a combination of the patient’s and practitioner’s refractive error

10 If a blood vessel is seen, move along until you reach a bifurcation, this will act as a compass needle pointing in the approximate direction of the disc

11 Carefully examine the disc, then follow each branch of blood vessels to examine each quadrant in a systematic fashion

12 The patient should be asked to look in eight positions of gaze to ensure the peripheral fundus is examined. Ensure the fine blood vessels remain in sharp focus and adjust the lens wheel accordingly

13 Reduce the aperture size and then move temporally from the disc to examine the macula

Direct ophthalmoscopy may also be conducted at the slit lamp. The use of high-powered non-contact negative lenses, such as that developed by Hruby in the 1940s has been described as providing an easy to master, stereoscopic, non-contact method of fundus examination; however, field of view is only slightly better than that of the direct ophthalmoscope (1-2DD), examination is limited to the posterior pole and good patient cooperation is required.22

Slit Lamp Binocular Indirect Ophthalmoscopy

Slit lamp binocular indirect ophthalmoscopy (BIO), often simply called ‘Volk’ after the synonymous American lens manufacturer is a commonly performed technique in optometry and ophthalmology in the UK.

El Bayadi developed a technique in the early 1950s, using a plano-convex +60D lens to obtain a stereoscopic image with reasonable field of view (approximately 6DD or 40°).23 However, this did not reach widespread use due to the spherical optics and subsequent aberrations. Volk began to develop lenses for indirect ophthalmoscopy in the 1970s, but it was not until as late as the early 1980s that ophthalmologists began using them at the slit lamp.24 Once a specialist technique, the use of slit lamp BIO is an increasingly popular form of fundoscopic examination. Slit lamp BIO is a compulsory element of the pre-registration period visits and OSCEs, and the College of Optometrists’ guidelines state that as a minimum a dilated fundus examination with an indirect lens should be carried out on those presenting with photopsia or floaters. Despite this, a study by Myint and others 19 found that only three-quarters of community optometrists use this technique either as the sole method of examination, or combined with direct ophthalmoscopy.

With this in mind, the technique should now be a key component in the armoury of optometrists in their routine (and not so routine) examinations of internal ocular structures. It provides many benefits to the examiner with very few shortcomings once mastered. Practitioners who are new to the technique are encouraged to attempt it alongside their standard procedures on all patients, especially those with large pupils initially to gain confidence.

Advantages

• Stereoscopic
• Wider field of view
• Patients may find it more comfortable as the practitioner is out of their personal space
• Magnification is relatively independent of ametropia and can be altered with lens choice
• Better view through media opacities
• Relatively inexpensive when a slit lamp is available
• Provides easy measurement of fundus lesions

Disadvantages

• Needs a slit lamp therefore not portable
• Takes practice to perfect
• Brightness can make the experience uncomfortable
• Patient needs to be able to reach slit lamp – this can be difficult in particularly young children, patients with back or neck problems etc.
• The image is inverted and reversed which may confuse practitioners new to the technique
• Can be difficult in small pupils

How to…

1 Clean the slit lamp with isopropyl alcohol 70% as recommended by the College of Optometrists

2 Dilate the pupils if necessary, ensuring it is safe to do so

3 Ensure the patient is comfortable on the chin rest and aligned with the canthal marks

4 Dim the room lights

5 The illumination system should be aligned with the biomicroscope

6 Set the magnification to low and use a parallelepiped beam of low intensity, moderate height and moderate width

7 Ask the patient to fixate on your ear, or at a fixation light straight ahead

8 Line up the beam with the pupil and place the lens in the path of the beam several millimetres from the cornea. Hold the lens in your left hand to examine the right eye and vice versa. The hand may be steadied by resting your little finger against the forehead rest, and your elbow may be placed on the lens case.

9 Pull the biomicroscope back until a red reflex is seen then further back to focus

10 The optic disc should be in the approximate area of focus if the patient is fixating your ear

11 First examine the optic nerve head, altering the position of the lens, the beam and magnification if required

12 Work in a systematic manner ensuring all areas of the posterior pole are examined

13 The patient should be asked to look in eight positions of gaze so that all areas of peripheral retina are covered (eg to examine the superior retina, the patient should be asked to look upwards). Move the illumination system with their gaze. Bothersome reflections may be combated by tilting the lens away from the beam slightly (e.g. tilting the lens upwards when the patient looks up). The patient’s eyelid often inhibits a good view of the inferior fundus and it is often best to raise them slightly with your third or forth finger. Ensure the fine blood vessels remain in sharp focus and adjust the joystick accordingly

14 The vitreous may be assessed by pulling the slit lamp further back

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Lens choice

When choosing a lens, the key features to consider are magnification and field of view (see Table 1). Other things to consider may be cost, lens diameter (from a practitioner handling point of view) and usability on undilated pupils. Often practitioners have a few at hand for different uses. The author uses a Superfield for general examination due to its wide field of view and larger diameter (versus the 90D) and successful use on all but the smallest of pupils, combined with a Super 66 for a high magnification view of the disc and macula, and where measurements are required.

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Hints and tips:

• This technique may be used on surprisingly young children. Getting the patient to kneel on the chair and hold onto the headrest tightly, explaining how you are going to use a ‘small magnifying glass’ to have a closer look often wins them round
• The edge of a raised retinal lesion such as a pigment epithelial detachment can be made more obvious when the slit beam is placed over it. The edge of such a lesion appears to glow
• A yellow filter may be attached to the most common lens types. This can aid patient comfort and purports to protect against phototoxicity
• Once slit lamp biomicroscopy is mastered the Watzke-Allen test can be used as a simple and effective subjective method of distinguishing full thickness macular holes from its differential diagnoses. A thin slit beam is centred over the macula (in vertical and horizontal meridians) and the patient is asked what they can see.
• Those with a full thickness macular hole may report a broken or narrowed beam, while those with a lamellar or pseudohole will report it as distorted or normal25
• As with the direct ophthalmoscope, several filters on the slit lamp can be used to examine different features for example the red-free filter can be used when examining the retinal nerve fibre layer, vasculature and haemorrhages

Monocular indirect ophthalmoscopy

The Welch-Allyn Panoptic is a monocular indirect ophthalmoscope that provides a fields of view of 25° and, according to the manufacturers, has an increased magnification of 26% over a standard direct ophthalmoscope. Despite technically being an indirect system, lenses within the system invert the practitioner’s view to one of an erect image. The axial point of light entering the eye lies on the cornea and allows for a larger field of view.26

Studies into ease of use, sensitivity and specificity are mixed but largely favourable in patient comfort due to patient-practitioner distance and shorter examination time.27

Headset Binocular Indirect Ophthalmoscopy

Originally, the development of headset BIO in the 1940s by Charles Schepens was to aid the examination of patients with retinal detachment, however it has become commonplace in other clinical settings especially paediatric examination.28

Advantages

• Rapid method of obtaining an overall view of large areas of the fundus
• Better view through media opacities
• Stereoscopic
• Wide field of view (can be 45°)
• Can be used in conjunction with a scleral indenter

Disadvantages

• Not widely available in practice
• The headset may be scary to patients
• The image is inverted and reversed
• Low magnification (approximately 3x) – not good for examination of the central fundus as small lesions may be missed
• Mydriasis is usually required
• It is recommended that the patient is in the supine position which may be problematic in a standard optometric setting
• Potential risk of phototoxicity

How to…

1 Dilate the pupils if necessary, ensuring it is safe to do so

2 Ensure the headset is comfortable, adjusting the top and side bands as required

3 Bring the observation system down and adjust the interpupillary distance, spot location and illumination

4 Dim the room lights

5 Place the light onto the patient’s pupil so the red reflex is seen

6 Hold the condensing lens with the more convex side facing you, often the lens manufacturers mark the side that should face the patient. Place the lens in a couple of centimetres in front of the eye resting a finger or two on the patient’s forehead. If reflections are a problem tilt the lens slightly

7 Pull the lens away so the reflex fills the lens and retinal features become focussed.

8 Work in a systematic manner ensuring all areas are examined. If the patient is able, get them to look in eight positions of gaze, tilting the lens accordingly. When looking at different areas it is important to keep a direct line between the viewing system and condensing lens.

Lens choice

Lens choice impacts both magnification and field of view. Lower powered lenses such as the Volk +15D provide a higher magnification (4.11x) but smaller field of view (36° static/ 47° dynamic) while a higher powered lens such as the Volk +30D provides a lower magnification (2.15x) and larger field of view (58° static/ 75° dynamic). The latter has been suggested to be better for paediatric assessment (Volk catalogue). Perhaps the best compromise for general screening is the +20D lens (which provides 3.13x magnification and a field of view of 46° (static) and 60° (dynamic).28

Indications and contraindications

While still more common in ophthalmology than primary optomtetric practice, this is a handy technique to master when examining patients particularly children, those who cannot reach the slit lamp (for example those in a wheelchair) and those that require a thorough examination of the peripheral retina.

Modified monocular or binocular indirect ophthalmoscopy

Some practitioners advocate the use of a modified technique for using the condensing lenses mentioned above. The headset may be cumbersome when examining younger children and scary to the patient, thus the use of a direct ophthalmoscope in conjunction with a condensing lens held in front of one eye (for a monocular view) or on the bridge of the nose (for a binocular view when dilated) is a useful method of examining the fundus from a distance. The technique is similar to that of headset BIO but the lens wheel of the direct ophthalmoscope is only focused for the practitioner’s refractive error when using the monocular technqiue.22

Contact Lens Ophthalmoscopy

Originally developed by Goldmann in the 1930s, the fundus contact lens is an important part of an ophthalmologist’s arsenal in assessing patients who are suspected of having peripheral retinal pathology whilst also providing a detailed, stereoscopic view of the macula. It is also used in laser treatments for some retinal conditions. While data on optometrists’ use of this exact technique is sparse, a recent survey29 found that only 15% of optometrists use gonioscopy in their practice, a technique with an almost identical skillset.

Advantages

• The central lens provides a greater level of stereopsis compared with non-contact slit lamp biomicroscopy
• Some lenses have the ability to cover the entire retinal surface
• Some lenses have an inbuilt gonioscopy lens
• Unlike headset BIO, the patient does not need to be supine

Disadvantages

• Static method of examination
• Contact method is invasive and may be daunting for the patient
• Requires good patient cooperation
• Requires anaesthesia and often a coupling agent
• Like all reusable contact lenses there is a theoretical risk of vCJD – sterilisation or disposable equipment is required
• Not widely taught and there is a perception that the technique is difficult to learn
• The image is reversed but not inverted, this may be confusing initially

How to use the Goldmann 3-mirror

The following ‘How to’ will look at the best known of the fundus contact lenses- the Goldmann 3-mirror. The Goldman 3-mirror is useful as it combines a central lens for viewing the macula, with 2 mirrors for assessing the peripheral fundus as well as a gonioscopy mirror (Figure 4).

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1 Prior to using this technique, it is important to rule out any contraindications. This technique should not be used:

a Immediately following ocular surgery

b If there is active ocular surface disease

c If there has been recent ocular trauma

2 Clean the slit lamp with isopropyl alcohol 70% as recommended by the College of Optometrists

3 Disinfect the lens with 1% hypochlorite solution and rinse with sterile saline

4 Dilate the pupils if necessary, ensuring it is safe to do so

5 Instil a topical anaesthetic

6 Ensure the patient is comfortable on the chin rest and aligned with the canthal marks

7 Dim the room lights

8 The illumination system should be aligned with the biomicroscope

9 Use a low magnification and medium slit beam height and width

10 The cup of the lens is filled with a coupling fluid such as Viscotears (Alcon Laboratories (U.K.) Ltd.)

11 The patient looks up and the lens is placed in the lower fornix

12 As the patient returns to primary gaze, the practitioner should apply firm pressure to the lens and the patient should be encouraged not to back away

13 While maintaining pressure on the lens with one hand pull the slit lamp back to focus on the mirrors with the other

14 Work in a systematic manner, examining the area covered by the equatorial mirror and the peripheral mirror. The pars plana can be visualised by using the gonioscopy mirror. Remember that the area in question is upside down and opposite to the position of the mirror

15 The illumination column should be moved and also vertically tilted (by unscrewing the locking screw on the slit lamp) to obtain the best view

16 Examine the posterior pole using the central lens

17 Lightly press on the lower lid and pull the lens to remove

18 Check the cornea for damage following removal. Superficial punctate epithelial erosions are common and may require temporary ocular lubricants for symptomatic relief

Remember, unlike indirect ophthalmoscopy the image is reversed but not inverted. It is advisable to carry out any other fundus examination procedures (eg Volk BIO) prior to contact lens-based examinations. This is because, due to the use of and persistence of coupling fluid the practitioner’s view is often

hampered.

Lens choice

As mentioned the Goldmann 3-mirror is commonly used because of its versatility. The central lens shows a 30° upright image, the equatorial and peripheral mirrors allow viewing of the fundus to the ora serrata and anterior chamber structures. An excellent view of the macula and disc can be obtained by using the Ocular Mainster High Mag (magnification 1.25x, field of view 75°)30 or Volk Area Centralis (magnification 1.06x, field of view 70°).31

With the requirement for sterilization, disposable versions of some lenses have been developed. There are also sterile, disposable covers for reusable lenses. Both of these options may be too costly to become widespread in primary practice.

Summary

Modern optometrists should use various methods for assessing the retina of their patients. Each of the methods have their own merits and downsides and careful consideration should be taken in choosing the most appropriate method in each clinical situation. Practitioners are particularly encouraged to try slit lamp BIO if they do not currently use this method, as it provides great benefits over the traditionally popular direct ophthalmoscopy and very few disadvantages once mastered.

Ceri Probert is an optometrist who practises in south Wales

References

1 Opticians Act 1989. c. 4.

2 Forrester, JV. 2015. The eye: basic sciences in practice. 4th ed. Edinburgh: Elsevier

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8 Liew, G et al. 2006. Fundoscopy: to dilate or not to dilate? The risk of precipitating glaucoma with mydriatic eye drops is very small. British Medical Journal 332(7532), pp. 3-3.

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11 Shuey et al. 1995. Scleral indentation: a review of the procedure and indications for use. Clin Exp Optom 78(3), pp. 106-109.

12 Turner, G. 2013. Management of acute retinal detachment. London: Royal College of Ophthalmologists

13 American Academy of Ophthalmology. 2014. Preferred practice patterns: management of acute retinal detachment. San Francisco: American Academy of Ophthalmology

14 Shukla, SY et al. 2015. Reassessment of Scleral Depression in the Clinical Setting. Ophthalmology 122(11), pp. 2360-2361.

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17 Rosser, D. 2010. Binocular indirect ophthalmoscopy part 2. Optician Magazine. 27 August 2010, pp. 16-21

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23 El Bayadi, G. 1953. New Method of Slit-lamp Micro-ophthalmoscopy. British Journal of Ophthalmology 37(10), pp. 625-628.

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25 Kanski, JJ and Bowling, B. 2011. Clinical Ophthalmology A Systematic Approach. 7th ed. Philadelphia: Elsevier Saunders

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27 Damasceno, E de F et al. 2009. Teaching ophthalmology at university medical undergraduate course: Comparative study of direct ophthalmoscopy between conventional ophthalmoscope and wide field ophthalmoscope (panoptic) as evaluation method. The Brazilian Journal of Ophthalmology 68(4), pp. 231-236.

28 Rosser, D. 2010. Binocular indirect ophthalmoscopy part 1. Optician. 30 July 2010, pp. 14-18.

29 Dabasia, PL et al. 2014. A survey of current and anticipated use of standard and specialist equipment by UK optometrists. Ophthalmic and Physiological Optics 34(5), pp. 592-613.

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31 Volk Inc. 2016. Product Catalog [Online]. Available at: http://www.volk.com/media/wysiwyg/pdf/brochures/Volk-Product-Catalog.pdf [Accessed: 31 March 2016].