Ординатура / Офтальмология / Английские материалы / The Ophthalmic Study Guide for Nurses and Health Professionals_Field_2009
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The ophthalmic study guide
system is responsible for regulating the intraocular pressure, as the mechanism is pressure sensitive.
Ahmed (2004) states that the uveoscleral drainage route is responsible for draining an estimated 5–15% of the aqueous via the stroma and vessels of the iris root and ciliary body, regardless of the intraocular pressure. Through the combination of the conventional route
and the uveoscleral route, about 1% of the |
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aqueous drains away per minute. It is |
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important to have some understanding of |
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these two drainage routes in order to |
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understand the principles behind medical |
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treatments for intraocular pressure. |
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The trabecular meshwork is also |
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responsible for removing cellular debris |
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from the anterior chamber by phagocyto- |
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sis and is able to facilitate the removal of |
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some debris into the canal of Schlemm |
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(Buller et al., 1990). It is thus generally |
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able to clear red blood cells (hyphaema) |
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and inflammatory products like hypopyon |
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(pus in the anterior chamber). |
Figure 2.7 Aqueous drainage. |
Vitreous humour
The vitreous occupies the space at the back of the eye, between the lens and the retina. It is a transparent gel-like substance that maintains the shape of the eye and holds the neural and pigment layers of the retina in position. It is composed of around 98% water and contains a network of collagen fibrils that maintain its shape. Unlike the aqueous, the vitreous is not continually produced and drained.
The retina
Critical to an understanding of retinal detachment is the fact that embryonic eyes develop from two optic vesicles which are derived from the embryonic forebrain. The retina and optic nerve develop from the optic cup, which folds in on itself; its outer wall develops into the retinal pigment epithelium, and the inner wall becomes the sensory layer of the retina. The retina therefore originally comprised two layers – the nervous layer and the pigment layer. Because of the nature of its embryonic development, the retina is thus an outward extension of the brain, to which it remains connected via the optic nerve (see Snell and Lemp, 1998).
The layers of the retina
The anterior surface of the neural layer of the retina is a thin, transparent sensory layer that lies in contact with the vitreous humour. It contains two types of photoreceptors – rods and cones. The pigment layer is a single layer of cells, which absorbs the light energy entering the eye. It delivers nutrition to the photoreceptors in the neural layer and has a key role in the chemical activity of the neural layer, which includes digesting, and recycling specialised cell products from the neural layer. It secretes a range of growth factors to maintain the visual function of the retina (Strauss, 2004).
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Basic anatomy and physiology of the eye
Figure 2.8 The layers of the retina
Key learning for the beginner
The fundus is the term used by ophthalmologists to describe the posterior part of the eyeball. It can be examined with an ophthalmoscope or a slit lamp using additional handheld lenses. Within the fundal area are:
●the optic disc, where the optic nerve, the central retinal artery and the central retinal vein enter and leave the eye
●the macula, which is situated in the posterior part of the retina of each eye, and looks a slightly darker red than the surrounding retina. It is responsible for the clearest and most detailed colour vision.
In retinal detachment the neural layer of the retina peels away from the pigment layer. As you will already have noted from the above discussion, the retina developed as two distinct layers.
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The ophthalmic study guide
To do …
Find out why people with retinal detachments are sometimes asked to maintain their heads in specific positions preand postoperatively.
Why do some retinal patients require treatment more urgently than others?
What are the functions of a) rods and b) cones?
The optic pathways
Many patients tend to think that we ‘see’ with our eyes, so all disturbances with our vision must necessarily fall within the remit of the ophthalmic department. This is not necessarily so. Consider briefly the development of the embryo, already discussed under the heading ‘retina’. The eyes begin to develop 22 days after conception, with the formation of optic grooves in the early brain tissue, from which eventually two ‘buds’ (optic vesicles) may be seen on either side of the forebrain. Eventually these vesicles expand laterally, making stalk-like connections with the developing nervous system. These ‘stalks’ eventually become the optic nerves.
The optic nerves
Like the brain, the optic nerves are covered with the pia, arachnoid and dura maters. As you can see in Fig. 2.10, the optic nerves partially cross over at the optic chiasma. This partial separation of the nerve fibres from each eye allows the left side of the brain to interpret the left visual field, and the right side of the brain to interpret the right visual field. This enables the slightly different information coming from each eye to be interpreted together, to facilitate depth perception, and results in the highly complex process that psychologists call visual perception. This involves not just ‘seeing’ but also being able to make sense of what has been perceived.
Any lesion occurring in one or both optic nerves as they leave the eye will cause a loss of vision. The optic chiasma lies just above the pituitary gland, so any enlargement of this gland will put pressure on the crossed fibres, causing a bilateral temporal loss of vision (bitemporal hemianopia). You can find out more about loss of vision due to damage to the optic pathways in Miller et al. (2007).
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Basic anatomy and physiology of the eye
Figure 2.9 The optic pathways.
To do …
Give three possible causes of partial loss of vision occurring as a result of optic nerve damage at the chiasma or following the separation of the nerve fibres as they proceed to the visual cortex of the brain.
Loss of vision occurring as a result of damage to the optic pathways is not generally an ophthalmic problem, but patients who complain primarily of a loss of vision are generally seen in the ophthalmic department where the health of the retina may be ascertained and the visual fields recorded for diagnostic purposes. Many ophthalmic departments run joint ophthalmic neurological clinics.
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The ophthalmic study guide
References and further reading
Ahmed, E. (2004). A Textbook of Ophthalmology. India: Prentice-Hall.
Bradley, M., Miccoli, L., Escrig, M. and Lang, P. (2008). The pupil as a measure of emotional arousal and autonomic activation. Psychophysiology, 45(4), 602–07.
BMJ Publishing Group and Royal Pharmaceutical Society of Great Britain (2008). British National Formulary. Biggleswade, UK: RPS Publishing.
Buller, C., Johnson, D. and Tschumper, R. (1990). Human trabecular meshwork phagocytosis. Investigative Ophthalmology and Visual Science, 31(10), 2156–63.
Marsden, J. (2008). An Evidence Base for Ophthalmic Practice. Chichester: John Wiley.
Miller, N., Newman, N., Biousse, V. and Kerrison, J. (2007). Walsh and Hoyt’s Clinical Neuro-Ophthalmology: The Essentials, 2nd edn. London: Wolters Kluwer Health.
Moore, J., Graham, J., Goodall, E., et al. (2009). Concordance between common dry eye diagnostic tests. British Journal of Ophthalmology, 93(1), 66–72.
Shunsuke, T., Shen’Ichiro, Y., Chrisato, M., et al. (2005). Protective effect on the corneal epithelium and remaining efficiency at the anterior chamber for three different kinds of viscoelastic devices. Japanese Journal of Ophthalmic Surgery, 18(3), 409–12.
Snell, R. and Lemp, M. (1998). Clinical Anatomy of the Eye, 2nd edn. Oxford: Blackwell Science.
Strauss, O. (2004). The retinal pigment epithelium in visual function. Physiological Reviews, 85(3), 845–81.
Useful web resources
Ageing Eye.net
http://www.ageingeye.net
See the Eye Digest for more detailed information about the ageing eye.
BBC Health: Ask the Doctor
http://www.bbc.co.uk/health/ask_the_doctor/
National Library of Medicine (US) Medline Plus
http://medlineplus.gov
For information on presbyopia.
NetDoctor UK
http://www2.netdoctor.co.uk
For information on acetazolamide.
Patient UK
http://www.patient.co.uk
For information on dry eyes and carbonic anhydrase inhibitors.
VetMed Resource
http://www.cababstractsplus.org/veterinarymedicine/
Article by T. Caceci on the anatomy and physiology of the eye and development of the eye in invertebrates.
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Basic anatomy and physiology of the eye
SELF TEST answers on page 203
1. Label this diagram of the eye.
2.What are the tiny openings in the corners of the eyelids called?
3.Where are the reflex tears produced?
4.What is the function of mucin, produced by the goblet cells of the conjunctiva?
5.The conjunctival glands of Krause and Wolfring, situated in the upper and lower fornices, are responsible for secreting what?
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The ophthalmic study guide
6.The cornea is composed of five layers. What is the inner, endothelial layer responsible for?
7.What is the uveal tract composed of?
8.Name the two muscles of the iris.
9.What are the main functions of the ciliary body?
10.What is presbyopia?
11.What anatomical structures are involved in the ‘conventional’ drainage route from the eye?
12.What is the name of the other route for the drainage of aqueous from the eye?
13.Name the two main layers of the retina
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Basic anatomy and physiology of the eye
14.What four key structures are at ‘the fundus’?
15.What are the cones in the retina responsible for?
16.The fibres from the optic nerve cross over at an area lying just above the pituitary gland. What is this called?
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Chapter 3
Basic refraction
Dorothy Field
Sight is our most important sense. Without sight, we would be unlikely to survive without help. The quality of a person’s vision significantly affects the way they manage their daily life, as many common tasks like cooking are more difficult if, for example, the long-sighted cook has no reading glasses; similarly a customer in a restaurant cannot read a menu without reading glasses. Understanding how light is refracted within the eye is key to understanding the visual needs of our patients, particularly when it comes to obtaining accurate intraocular lens readings and understanding spectacle prescriptions and associated visual difficulties.
Essential learning
If you normally work with cataract patients, it is essential that you begin to get a grasp of the following:
●The vocabulary common when discussing refraction.
●How the eye works to focus images on the retina.
●Some of the pitfalls associated with inaccurate measurements prior to cataract surgery, or the insertion of a lens of the incorrect power (see Chapters 5, 6 and 7).
What is refraction?
Refraction of light by the eye to produce good, unaided vision (by focusing the light on to the retina) is dependent on the power of the cornea, the lens and the length of the eye. All the individual transparent areas of the eye contribute to bending and focusing the light rays on the retina. Snell and Lemp (1998) state that the power of the whole normal (emmetropic) eye is about 58 dioptres (see Dioptre in Appendix 2); the cornea has a refractive power of about 42 dioptres and is responsible for more than two-thirds of this refractive ability. In addition it should be recognised that the tear film, aqueous humour, lens and vitreous all contribute to refraction of the light rays so that they normally come to a focus at the macula.
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