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The Department of Health and the National Eye Care Steering Group have identified glaucoma as one of the four pathways for greater involvement of optometrists in providing primary care co-management.3 In recent times, optometrist-led glaucoma clinics have developed within hospital eye departments as have shared-care schemes between community optometrists and local eye departments.4,5 To participate in such schemes optometrists undergo extensive further training and examination by supervisory consultant ophthalmologists to prove their clinical competency in making safe decisions about which glaucoma patients are stable and which merit re-referral back to hospital care.
The Department of Health has plans to extend further the role of suitably trained optometrists to include the right to independently prescribe a small range of eye drops and, as part of a shared-care scheme, to prescribe a larger range of topical medications, possibly including those used to treat glaucoma. The aim of this article is to introduce the topical medication commonly prescribed in the management of glaucoma.
Glaucoma
Glaucoma is the leading cause of irreversible blindness in the developed world.6 Although the vast majority of patients with glaucoma do not go blind, many lose useful vision and it accounts for an estimated 12 per cent of all cases of blind registration in the UK. The majority of glaucoma cases are the result of primary open-angle glaucoma (POAG) its prevalence in Caucasian populations is estimated at between 1.1 per cent and 2.2 per cent of the adult population, increasing with advancing age.
Glaucoma can be defined as a progressive optic neuropathy showing characteristic morphological changes of the optic nerve head and retinal nerve fibre layer in the absence of other ocular disease and congenital abnormalities.7 A major risk factor for the development and progression of POAG is raised intraocular pressure (IOP) and reduction of IOP remains the mainstay of glaucoma treatment. Recent studies have shown that treated glaucoma patients with a 'target IOP' set in the low teens have the lowest rate of progression. This IOP reduction is achieved through either medical or surgical means.
Aqueous humour dynamics
To understand the fundamentals of the medical management of POAG, it is necessary to be familiar with the dynamics of aqueous production and outflow. The role of aqueous humour is to bathe the crystalline lens and corneal endothelium in a nutrient medium, to remove the by-products of metabolism and to maintain a level of IOP that keeps the eye inflated.
Aqueous production
Aqueous humour is produced by the ciliary body, the ring of tissue extending from the ora serrata posteriorly to the iris root anteriorly. The ciliary body is composed of loosely arranged collagen fibres, blood vessels and nerves, interwoven with the predominating smooth muscle, the ciliary muscle. Anatomically there are two distinct areas of the ciliary body: the anterior pars plicata and the posterior pars plana. The anterior pars plicata comprises approximately 70 radially arranged major ciliary processes which project into the posterior chamber. These ciliary processes have three main components:
- A double layered epithelium lining the processes
- A highly vascularised and fenestrated capillary core
- The stroma, composed of mucopolysaccharide ground substance and collagen, that separates the capillary network from the epithelium.
Aqueous humour is derived from the blood plasma of the capillaries in the ciliary processes via three mechanisms: diffusion, ultrafiltration and active transport (secretion). Most aqueous production is from the latter mechanism which occurs as a result of the active transport of sodium ions across the ciliary epithelium into the posterior chamber. Thus primary aqueous enters the posterior chamber where its composition is altered by the iris, ciliary body and crystalline lens, before passing through the pupil into the anterior chamber. It leaves the anterior chamber via the iridocorneal angle.
Aqueous outflow
The majority of aqueous leaves the eye through the trabecular meshwork (Figure 1), percolating into Schlemm's canal and from there into the collector channels, finally draining into the episcleral veins. The rate of outflow is determined by the hydrostatic pressure head and resistance of the meshwork to flow. This is often called the 'conventional' outflow pathway and accounts for approximately 90 per cent of the outflow process. A small amount of aqueous exits the eye by passing through the interstitial spaces of ciliary muscle and choroid, or the suprachoroidal space and out of the eye through the sclera or the perivascular spaces surrounding the emissarial channels in the sclera. This 'unconventional' or uveoscleral outflow pathway is pressure-independent.
Pharmacological principles of ophthalmic drugs
Many of the drugs used in the medical management of glaucoma exert their actions by modulating the activity of the autonomic nervous system (ANS). The ANS regulates involuntary actions within the body, mainly the control of smooth muscle, cardiac muscle and exocrine glands. It is divided into the sympathetic and the parasympathetic nervous systems and differs from the somatic (voluntary) nervous system by possessing a synapse outside of the central nervous system (CNS).
The parasympathetic nerve supply to the eye originates from the oculomotor nucleus in the CNS, travelling via the oculomotor nerve to synapse in the ciliary ganglion. Post-ganglionic parasympathetic nerve fibres then enter the eye as the short ciliary nerves to innervate the ciliary muscle and iris sphincter pupillae muscle. Other parasympathetic nerve fibres enter the orbit with the trigeminal nerve to innervate the lacrimal gland. The sympathetic nerve supply of the eye comes from the cervical and upper thoracic segments of the spinal cord, synapsing at the superior cervical ganglion. Some post-ganglionic sympathetic nerve fibres pass through the ciliary ganglion, entering the eye as short ciliary nerves to allow ocular vasoconstriction, others bypass the ciliary ganglion and enter the eye as long ciliary nerves innervating the iris dilator pupillae muscle. Another branch of sympathetic nerves travel via the oculomotor nerve to M