Chapter 9: Patient comfort and supporting personal hygiene
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Source: Adapted from Aldwin et al. ([3]), Lin et al. ([109]).
Anatomy and physiology
The eye consists of three main parts: the orbit, the globe (eyeball) and the extrinsic structures.
The orbit
The orbit, or socket, is formed by seven bones of the skull and is lined with fat; it supports and protects the globe and its accessory structures (blood vessels and nerves) and provides attachments for the ocular muscles (Shaw and Lee [212]).
The globe
The globe is approximately 2.5 cm in diameter and can be divided into three layers (Figure 9.4).
- The outer layer or fibrous tunic is composed of the transparent cornea and the white sclera. The primary function of the outer layer, in particular the sclera, is protective and it gives shape to the eyeball. The cornea is a transparent tissue that forms a window at the front of the eye and its main function is to protect the eye by providing a physical barrier against pathogens. It is also responsible for refracting or bending light rays, focusing them as they pass through on their route to the retina (Eghrari et al. [56], Marsden [121]).
- The middle layer or vascular tunic is composed of the choroid, ciliary body and iris; the globe's vascular supply is provided by the choroid.
- The inner layer or nervous tunic is composed of the retina, which contains light‐sensitive cells called rods and cones. It is responsible for converting light rays into electrical signals, which are transmitted to the brain via the optic nerve. This area contains the macula lutea, also known as the yellow spot. The central fovea, the area of highest visual acuity, is also located here, as is the blind spot, the area of no visual field (Overby‐Canon [171]).
Figure 9.4 Horizontal cross‐section through the eyeball at the level of the optic nerve. The optic axis and axis of the eyeball are included.
The internal part of the globe
The globe is divided into two chambers by the lens (Figure 9.5): the anterior cavity (in front of the lens) and the vitreous chamber (behind the lens). The anterior cavity is divided into the anterior chamber and the posterior chamber by the iris. It contains a clear, watery fluid called the aqueous humour. The vitreous chamber is filled with a clear, jelly‐like substance called the vitreous body or vitreous humour. The vitreous humour fills the vitreous chamber, which, unlike the aqueous humour, is produced during foetal development and is never replaced (Tortora and Derrickson [228]). Together, these two fluid‐filled cavities help to maintain the shape of the eyeball and the intraocular pressure (Tortora and Derrickson [228]).
Figure 9.5 The anterior cavity in front of the lens is incompletely divided into the anterior chamber (anterior to the iris) and the posterior chamber (behind the iris), which are continuous through the pupil. Aqueous humour, which fills the cavity, is formed by ciliary processes and reabsorbed into the venous blood via the canal of Schlemm.
The aqueous humour is continuously secreted by the ciliary process (a part of the ciliary body), which is located behind the iris. This fluid then permeates the posterior chamber, passing between the lens and the iris, and flows through the pupil into the anterior chamber. From the anterior chamber, the aqueous humour drains into the scleral venous sinus (canal of Schlemm) and is absorbed back into the bloodstream (Figure 9.5).
The aqueous humour is the principal source of nutrients and waste removal for the lens and cornea, as these structures have no direct blood supply. If the outflow of aqueous humour is blocked, excessive intraocular pressure may develop, leading to the disease process known as glaucoma. This excessive pressure can cause degeneration of the retina, which may result in blindness (Lee [106]).
Extrinsic structures
The extrinsic structures of the eye protect the globe from external injury (Shaw and Lee [212]):
- Eyelashes: protect the eye from debris.
- Eyebrows: prevent moisture, in particular sweat, from flowing into the eye.
- Eyelids: the eyelid is made up of complex muscles for eye movement, glands for the production of tears and oil (which serve as a cleansing mechanism against dirt and foreign objects), and sensitive nerves for defence. The eyelids also protect the eyes from excess light (Rehman et al. [198]).
- Lacrimal (tear) apparatus: tears are produced in the lacrimal glands, located at the upper, outer edges of the eyes. They are excreted onto the upper surface of the globe and wash over the ocular surface by the action of blinking. The function of tears is to clean, moisten and lubricate the ocular surface and eyelids. Tears also provide antisepsis as they contain an enzyme called lysozyme, which is able to rupture the cell membranes of some bacteria, leading to their lysis and death (Forrester et al. [65]). The tears collect in the nasal canthus (inner, medial aspect of the eye), from which they drain into the upper and lower lacrimal puncta, which drain into the lacrimal sac. From here, the tears pass into the nasolacrimal duct and empty into the nasal cavity (Figure 9.6) (Tortora and Derrickson [228]).
Figure 9.6 Lacrimal apparatus.
Optic nerve
The optic nerve, which is responsible for vision (cranial nerve II), exits the eye to the side of the macula lutea at an area called the optic disc. This area is sometimes referred to as the anatomical blind spot. The optic nerve passes from the orbit through the optic foramen and into the brain. The two separate optic nerves meet at the optic chiasma and some optic nerve fibres cross over here to the opposite side of the brain. The nerves then continue along the optic tracts and terminate in the thalamus. From there, projections extend to the visual areas in the occipital lobe of the cerebral cortex (Tortora and Derrickson [228]) (Figure 9.7).
Figure 9.7 Visual pathways and visual fields. Source: Reproduced from MRI Questions ([139]) with permission of Elster LLC.
An additional blind spot or area of depressed vision, called a scotoma, may be indicative of a brain tumour. In pituitary gland tumours, for example, it is common for patients to develop bilateral defects in the field of vision due to invasion of the optic chiasm (O'Leary and Birkbimer [168]).
The ageing process
The eye changes with age. This process can start in the third decade of life, with most anatomical and physiological changes becoming more prevalent the older a person gets (Aldwin et al. [3]) (Table 9.4).
Table 9.4 Effects of ageing on the eye
| Anatomical changes | Physiological changes | Eye conditions common in the older population |
|---|---|---|
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