Fig. 14.1
Schematic of the eye with ocular structures labelled
Table 14.1
Description of ocular structures (anterior to posterior)
Structure | Comments |
|---|---|
Cornea | A transparent, avascular structure that joins the sclera at the limbus. It receives its nutrients from the tear film. The outer layer is densely innervated with pain fibres, whilst the inner layer is responsible for maintaining corneal hydration and transparency |
Anterior chamber | This is the space between the cornea and the iris. It is filled with aqueous humour, which is secreted by the ciliary body. Aqueous drains out of the eye via the trabecular meshwork found in the drainage angle (angle between cornea and iris). From here, it drains into the canal of Schlemm |
Iris | A circular, pigmented contractile muscular diaphragm. Sympathetic innervation causes dilation and parasympathetic innervation causes constriction. It divides the anterior and posterior chambers of the eye |
Lens | This is just behind the iris and is attached to the ciliary body via zonules. With the cornea, it provides refractive power to the eye. When looking at near objects (accommodation), the circular ciliary muscle contracts, reducing tension in the zonules by making the lens assume a more globular shape. This enables the focus of near rays on the retina |
Vitreous | A gel-like material that fills the posterior chamber. It is attached to the retina most strongly at the annulus of Zinn posteriorly and the ora serrata anteriorly |
Retina | A multi-layered neural layer, stretching from the ora serrata to the optic nerve. It contains rod and cone photoreceptors that detect light, and transmit signals, via other neurons, to the optic nerve. The macula is the central portion of retina that contains the fovea at its centre. This has a very high density of cone photoreceptors, which gives high central visual acuity |
Choroid | Vascular layer that nourishes the outer retina. The inner retina is supplied by the retinal vessels |
Sclera | Tough outer layer of connective tissue to which the extraocular muscles attach. Posteriorly, the sclera is encased in Tenon’s layer. Anteriorly, it is covered by the conjunctiva |
Extraocular Muscles
Each eye has six extraocular muscles responsible for eye movements. Figure 14.2 shows a schematic of the muscles responsible for each eye movement. Underaction or overaction can lead to strabismus (squint) which can be corrected with squint surgery. The six muscles include four rectus muscles and two oblique muscles.
Fig. 14.2
Schematic diagram of the extraocular muscles responsible for different directions of gaze. SR superior rectus, LR lateral rectus, IR inferior rectus, MR medial rectus, IO inferior oblique, SO superior oblique. Note that this is a simplified scheme. The actions differ depending on the direction of gaze; for example the oblique muscles may act to abduct the eye in the primary position, and the superior and inferior recti adduct the eye when in the primary position
The rectus muscles arise from the annulus of Zinn and insert near the limbus.
The medial rectus pulls the eye medially (adduction)
The inferior rectus pulls the eye down (infraduction or depression)
The lateral rectus pulls the eye laterally (abduction)
The superior rectus pulls the eye upwards (supraduction or elevation)
The oblique muscles arise from the orbital walls and insert temporally on the globe.
The superior oblique arises from the sphenoid bone and travels through the trochlea (u-shaped cartilage) before inserting superotemporally on the globe. The trochlea is located on the superonasal part of the anterior orbit, creating a pulley-like structure.
The superior oblique depresses and abducts the eye.
The inferior oblique arises from the inferonasal part of the anterior orbit, and inserts posterotemporally on the globe. It elevates and abducts the eye.
The superior oblique and superior rectus also intort the eye (superior limbus rotates medially), and the inferior oblique and inferior rectus extort the eye (the superior limbus rotates temporally). The extraocular muscles are all supplied by the 3rd cranial nerve (oculomotor nerve) except for the lateral rectus (6th cranial nerve, abducens) and the superior oblique (4th cranial nerve, trochlear). The 3rd nerve also innervates levator palpebrae superioris which elevates the upper lid.
Eyelid Anatomy
The upper and lower lids are very important in protecting and lubricating the corneal surface. The structure can be simplified to four layers from anterior to posterior, listed in Table 14.2.
Table 14.2
Basic components of the eyelid (anterior to posterior)
Structure | Comments |
|---|---|
Skin | This overlies loose connective tissue. At the lid margin, there are 2–3 rows of eye lashes with secretory glands at the follicles (glands of Moll and Zeiss) |
Orbicularis oculi | This is a large muscle with different portions responsible for closure of the eyelids. It is supplied by the 7th cranial nerve (facial), and hence lid closure can be diminished in facial nerve palsy |
Tarsal plate | This is a thickened plate of connective tissue and serves to maintain the lid shape. It contains Meibomian glands which secrete the lipid layer of the tear film. Blockade of this secretion leads to a chalazion |
Tarsal conjunctiva | This lines the inner surface of the tarsal plate and is continuous with the bulbar conjunctiva at the conjunctival fornix |
Lid opening is achieved by the upper lid retractors (a large voluntary muscle called levator palpebrae superioris, and a sympathetically-innervated smaller muscle called Muller’s muscle) and the lower lid retractors (more rudimentary than the upper lid retractors).
Lacrimal System
The lacrimal gland is a bi-lobed structure located in the superolateral part of the anterior orbit. Parasympathetic innervation leads to increased tear production (Fig. 14.3).
Fig. 14.3
Schematic anatomy of the lacrimal system
Tears drain via the upper and lower puncta (the majority via the lower puncta) into the lacrimal sac via the upper and lower canaliculi. The lacrimal sac drains via the nasolacrimal duct into the nose. One treatment for dry eyes is to reduce tear drainage by occluding the lacrimal puncta (temporarily with punctal plugs or permanently with punctal cautery). Blockage of the lacrimal drainage system can lead to a watery eye. Treatment of this depends on the site of blockage (discussed later).
Core Procedures
Cataract Surgery
Cataract is opacification of the lens of the eye, and the only treatment is surgical. The lens is composed of crystalline material bounded anteriorly by the anterior lens capsule, and posteriorly by the posterior capsule. The usual symptom is the gradual onset (over years) of blurring of vision (Table 14.3).
Types of Cataract
Table 14.3
Common types of cataract
Type | Description |
|---|---|
Nuclear sclerotic | Opacification of the central part of the lens. This may increase the refractive index of the lens, giving increased myopia (short-sightedness) as well as blurring of vision |
Cortical | Radial spoke-like opacification of the lens. This often leads to increased glare, frequently impairing ability to drive at night |
Posterior subcapsular | Opacity that is mainly in the posterior part of the lens, just anterior to the posterior capsule. This tends to impair reading vision particularly, and can be associated with exposure to steroids |
Risk Factors
Age (the commonest risk factor)
Diabetes
Intraocular inflammation (uveitis)
Steroid exposure
Trauma or ocular surgery
The Procedure
Cataract surgery is performed routinely and is not usually an urgent procedure. Rarely, the cataract can cause significant shallowing of the anterior chamber which can lead to secondary angle closure glaucoma. This is known as phacomorphic glaucoma and would require urgent treatment.
Modern cataract surgery is performed using an operating microscope under local anaesthesia with small (less than 3 mm) sutureless incisions. Some of the main steps are summarised below.
- 1.
A circular tear is carefully made in the anterior capsule, giving access to the lens. This tear is called a continuous curvilinear capsulorhexis, and is frequently the most difficult stage in the operation for new trainees.
- 2.
The main part of the lens is broken up (“phacoemulsification”) into pieces using a probe that vibrates at ultrasound frequencies.
- 3.
The lens pieces are sucked out, taking care not to rupture the posterior capsule.
- 4.
A flexible synthetic replacement intraocular lens (IOL) is injected through the small incision and opens within the lens capsule.
- 5.
An antibiotic is injected (commonly cefuroxime).
- 6.
A pad and shield are placed over the eye (or just a clear shield if the operation was performed under topical anaesthesia).
The “phaco machine” is a sophisticated piece of equipment used in modern cataract surgery. The “phaco probe” is held in the surgeon’s dominant hand, and is used to sculpt and emulsify the cataract using ultrasound energy, and it also aspirates the lens fragments. A second instrument, the “chopper”, is held in the surgeon’s non-dominant hand to manipulate the lens pieces. The phaco machine controls the ultrasound power, the rate of fluid entering the eye to maintain a deep anterior chamber, and rate of aspiration of fluid or material out of the eye. During the operation, the surgeon controls the rates of irrigation and aspiration and phacoemulsification power using a foot pedal. The other foot controls the operating microscope. Thus, during cataract surgery, both hands and feet are used simultaneously, just as in driving a car.
Post-Operative Care
The pad and shield are removed later the same day or the following morning. The patient is usually given steroid drops (dexamethasone or prednisolone) and antibiotic drops (e.g. chloramphenicol) to administer 4 times daily. The shield is usually worn at night for a week post-operatively. The antibiotic drops may be stopped after 1–2 weeks, and the steroid drops are usually continued for a month. Patients are reviewed around 2 weeks post-operatively, and can visit an optician subsequently to check whether there may be a need for glasses.
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