Ophthalmology: Sub-specialty—Glaucoma

Ophthalmology: Sub-specialty—Glaucoma

Glaucoma: the basics

Glaucoma is not just one eye condition.

Aims

  • Understand the group of conditions classed as glaucoma.
  • Know the symptoms and signs of acute angle-closure glaucoma to enable urgent referral for treatment of this ophthalmic emergency.

Definitions

Glaucoma affects 60 million people worldwide: 8.4 million bilaterally blind from chronic glaucoma. Commonest cause of irreversible blindness in the world, affecting 2% of people over 40 years of age and 4% of people over 70 years. Glaucoma is a multifactorial optic neuropathy with characteristic acquired loss of optic nerve fibres causing structural change in the optic nerve and functional change with loss of visual field. The intraocular pressure (Figure 39.1) may be, but is not necessarily, elevated.
Intraocular pressure (IOP): the ‘normal’ range of IOP is defined as 10–21 mmHg. Patients with IOP within the ‘normal range’ may develop glaucoma (often referred to as ‘normal tension glaucoma’), and some patients with IOP above the ‘normal range’ do not develop optic nerve damage (ocular hypertension). Elevated IOP is still a risk factor for the development of glaucoma, and when glaucomatous damage is observed treatment is reduction of IOP regardless of the absolute presenting value.

Ocular hypertension (OHT)

Patients with raised IOP but no signs of glaucomatous optic neuropathy have OHT. It is common to find that they have thicker than average corneas. In the Ocular Hypertension Treatment study 10–20% of patients with OHT developed glaucoma, and 80% did not develop structural or functional damage.

Skills to obtain

  • Use of direct ophthalmoscope to evaluate disc colour, contour and cup-to-disc ratio.

Classification

The glaucomas are a diverse group of eye conditions (at least 60 types) defined in diagnostic groups:

  • primary or secondary: the presence or absence of causative factors
  • open-angle or angle-closure: the anatomy of the drainage angle
  • acute or chronic: speed of onset
  • age of onset: congenital, juvenile or adult.

Primary open-angle glaucoma (POAG)

Commonest glaucoma in Caucasian and Afro-Caribbean populations.
The exact pathogenesis is unknown; various factors are implicated, including elevated IOP and altered blood supply to the optic nerve.
Genetics play a role, with nine loci in the human genome associated with glaucoma. Risk factors include elevated IOP, thin cornea, positive family history and Afro-Caribbean descent.

Symptoms

Glaucoma is asymptomatic until advanced with marked optic disc cupping (Figure 39.2) and extensive visual field loss. Central vision is preserved until a late stage.

Signs
  • Usually, but not necessarily, raised IOP (Figure 39.3).
  • Normal open angle on gonioscopy.
  • Characteristic optic disc changes including progressive thinning of the optic disc neurosensory rim indicating loss of nerve fibres. Typically, the inferior and superior rims thin first and may give the appearance of a notch in the optic disc rim.
  • The cup–disc ratio increases over time due to nerve fibre loss on the retinal surface, which come together to form the rim of the optic nerve head. As the rim thins, the cup size relative to the disc enlarges. This may happen asymmetrically between the two eyes.
  • The cup–disc ratio change denotes anatomical or structural damage.
    This has functional effects that manifest as defects in the visual field, typically arcuate scotomas and nasal steps. The patient does not usually notice these defects which are picked up on automated visual field testing and analysis.
Treatment
  • Aim to reduce the IOP; medically (topical drop treatment is most commonly used), with laser treatments or surgically.
  • Patients with established disease require lifelong follow-up.

Acute angle-closure glaucoma (AACG)

This is an ophthalmic emergency. Patients with AACG can go blind if there is a delay in diagnosis and management. If it occurs in one eye, the fellow eye is also at risk.

Risk factors
  • Age: AACG is more common in elderly patients, particularly those with significant increase in anteroposterior size of their lens as their cataract develops.
  • Race: angle-closure glaucoma is the commonest glaucoma in Chinese.
  • Hypermetropia (longsightedness).
Symptoms
  • Sudden-onset headache with a painful red eye, nausea, vomiting, halos around lights and decreased vision.
Signs
  • Reduced visual acuity.
  • Mid-dilated fixed pupil on the affected side.
  • Hazy view of the iris and pupil in the affected eye from a cloudy oedematous cornea.
  • Red eye.
  • Rapidly raised IOP over a few hours. The speed of IOP rise in AACG is important. In POAG there may be high but asymptomatic IOP because the IOP has risen gradually over a period of months.
  • Closed angle on gonioscopy.
Treatment
  • Urgent treatment to reduce the IOP and prevent recurrences.
  • Medical treatment with intravenous acetazolamide, a powerful suppressor of aqueous production, then oral Diamox to maintain the effect.
  • Topical aqueous suppressors (e.g. beta-blockers and alpha-agonists).
  • Pilocarpine drops to constrict both pupils which alters the iris and ciliary body configuration and opens the drainage angles.
  • Prevent further attacks with Yag laser peripheral iridotomy (PI) or surgical iridectomy.
  • Treat the second eye prophylactically.
  • If PIs are not sufficient, specialist management is required and may involve peripheral iris laser or surgical lens extraction.

Secondary glaucoma

  • Many eye conditions can cause a secondary rise in IOP with optic nerve damage (e.g. trauma, inflammation, complicated cataract surgery and rubeosis).
  • Treat the cause where possible (e.g. PRP for rubeosis or steroid drops for inflammation and reducing the IOP).

Chronic angle-closure glaucoma

Unlike AACG, the speed of IOP rise is slower in CACG. CACG occurs in small eyes where there is asymptomatic adhesion between the peripheral iris and the cornea progressively occluding the drainage angle. Initial treatment usually involves a laser PI.

Congenital glaucoma or infantile glaucoma

Uncommon, either unilateral or bilateral, primary or associated with ocular malformations or systemic syndromes (e.g. Sturge–Weber syndrome).
It should be suspected in an infant or child with persistent watering photophobic eyes and in cases of clouding of the cornea.

Signs
  • Large eye, or buphthalmos.
  • Wider than average corneal diameters.
  • Reduced vision.
  • Clouding of the cornea.
  • Linear tears in Descemet’s membrane (known as Haabs’ striae).
  • Raised IOP.
Treatment

Congenital and juvenile glaucoma requires subspecialist management.

Detecting glaucoma

Aim

Understand how glaucoma is diagnosed.

History taking

Apart from acute angle-closure glaucoma, most forms of glaucoma are asymptomatic until very advanced optic nerve damage has occurred. A positive family history of glaucoma and myopia are important risk factors in the history.

Clinical examination

  • Visual acuity: Reduced by advanced chronic glaucoma damage or acute angle-closure glaucoma.
  • Ocular examination:
    ○ Slit lamp examination for anterior segment abnormalities associated with glaucoma.
    ○ Detailed assessment of optic disc. Ideally, the optic disc is photographed (conventional or with 3D scanning laser ophthalmoscope; Figure 40.1) to enable the detection of progression.
  • Intraocular (IOP) measurement: Using a Goldmann applanation tonometer (Figure 40.2). Measures force needed to flatten a defined area of cornea. This force corresponds to IOP, measured in millimetres of mercury. Note: normal tension glaucoma has normal-range IOP.
  • Visual field testing: Usually assessed with automated perimetry to allow efficient repeat testing to detect progression. Classic defect is arcuate scotoma (Figure 40.3).
  • Gonioscopy: Used to assess the anterior chamber angle (Figure 40.4). This cannot be viewed directly, so a gonioprism lens is used—the optics of which allow visualization of the angle between the cornea and iris.
    Glaucoma is diagnosed if a glaucomatous optic neuropathy (typical optic disc and field changes) is found to be present. Other parts of the examination help classify the glaucoma so that appropriate treatment can be given.
  • Pachymetry: Used to measure the thickness of the cornea. Average corneal thickness is 540–560 μm. Thinner than average corneas in the presence of high IOP are a risk factor for glaucomatous damage.

Monitoring

Once a patient has been diagnosed and started on treatment, the same techniques are used at regular intervals to assess for progression.
Finding the correct treatment to prevent progression depends on these same examination techniques.

Screening

Most forms of glaucoma are asymptomatic until very advanced optic nerve damage has occurred. Treatment at this stage is often too late, so glaucoma patients may be actively sought out from the community (i.e. screened). This is usually done by opticians, who assess IOP, visual fields and optic disc appearance. Two main groups of people are screened:

  • People with a family history of glaucoma, especially primary open-angle glaucoma.
  • All people over 40 will have IOP measured at a sight test.
    If an abnormality suggestive of glaucoma is detected in this screening process, the person is referred to the hospital ophthalmology department.

Problems of glaucoma screening

  • People are missed if they do not attend opticians.
  • False positives are common because the initial assessment of field
    and disc is difficult.

Medical and surgical treatment of glaucoma

Aims

  • Understand the aims of medical, laser and surgical treatment.
  • Be familiar with different classes of drugs used to treat primary open-angle glaucoma.
  • Manage a case of acute angle-closure glaucoma.

Treatment goal: To preserve the patient’s sight throughout his or her lifetime with minimum side effects

  • Treat to lower intraocular pressure (IOP). This is currently the only modifiable risk factor and is the main aim of glaucoma therapy. Lowering IOP can be achieved by reducing aqueous production (inflow) or enhancing aqueous drainage (outflow.) There is no single target IOP that will be appropriate for all patients. Treatment and target IOP are tailored to each patient’s findings—the patient’s presenting IOP, visual field, optic disc appearance and type of glaucoma.
  • Ensure good ocular perfusion. Prevent arteriosclerosis, treat vasospasm and minimize low blood pressure (nocturnal dips).

Medical therapy

Most patients with primary open-angle glaucoma (POAG) can be managed with topical drop treatment. The aim of medical treatment is to prevent the progression of disc cupping or visual field defect. Pharmacological modulation of carbonic anhydrase, adenosine triphosphatases and adrenoreceptors located in the nonpigmented ciliary epithelium can reduce aqueous production and thus lower IOP. Similarly, pharmacological modulation of adrenoreceptors and prostanoid receptors located in the trabecular meshwork or ciliary body can increase aqueous outflow through both pathways and lower IOP. Pharmacological treatments to lower IOP have improved dramatically. The improved efficacy and tolerability of these drugs are in part responsible for the reduction in the number of glaucoma operations carried out since the 1990s.

Laser treatment

  • Bypass pupil block—peripheral iridectomy (Figure 41.1): this is the treatment of choice to prevent angle-closure glaucoma or after an acute attack has been broken with medical treatment. A YAG (yttrium–aluminium–garnet) laser hole in the peripheral iris allows aqueous flow from the posterior to anterior chamber.
  • Enhance aqueous outflow—trabeculoplasty (Figure 41.2): laser to the trabecular meshwork enhances aqueous outflow. Several different lasers can be used to apply this treatment, including argon laser trabeculoplasty (ALT), selective laser trabeculoplasty (SLT) and micropulse diode laser trabeculoplasty (MDLT).
  • Decrease aqueous production—cyclodestruction: laser to the ciliary body reduces aqueous production. This can be done through the sclera (trans-scleral cyclodiode; Figure 41.3) or endoscopically (endocyclophotocoagulation).

Surgical treatment

  • Peripheral iridectomy. Used in acute angle-closure glaucoma (AACG) when laser is not possible.
  • Trabeculectomy (Figure 41.4). A trabeculectomy creates a fistula between the anterior chamber of the eye and the subconjunctival space to allow controlled release of aqueous. It is still the most commonly performed surgery for POAG worldwide. A small full-thickness scleral hole is covered by a half-thickness scleral flap. Fluid flows from the anterior chamber, passes under the flap and drains under the conjunctiva.
    The scleral flap and conjunctiva can scar, thus reducing the flow and limiting the effectiveness of the operation. Antimetabolites such as mitomycin C and 5-fluorouracil are used to modulate the healing response and improve success rates. Risks include cataract, hypotony, infection and bleb leakage (Figure 41.5).
  • Drainage devices or tubes (Figure 41.6). Silicone tube inserted into the anterior chamber and connected to a scleral sutured plate to drain aqueous.
  • Deep sclerectomy. A section of the sclera and the roof of Schlemm’s canal are excised. No penetration into the anterior chamber is required.
  • Viscocanalostomy and canaloplasty. This is a deep sclerectomy with additional mechanical opening or stretching of Schlemm’s canal.
  • Drainage angle implants. Small, snorkel-like tube implants placed directly into the drainage angle of the eye (iStent®). This is usually combined with cataract surgery.

Treatment of AACG

AACG requires medical, laser and surgical treatment. Immediate treatment is medical to reduce the IOP and to break the attack of angle closure.

  • Intravenous carbonic anhydrase inhibitor (acetazolamide, 500 mg) is used to reduce the production of aqueous fluid.
  • Oral acetazolamide is often required for ongoing aqueous suppression for 1–2 days.
  • Pilocarpine is usually instilled in both eyes. This helps to break the acute attack and reduce the risk of further attacks, particularly in the fellow eye.
  • Topical agents help reduce IOP (e.g. timolol and iopidine).
  • Perform bilateral peripheral iridotomies as soon as possible.
  • Rarely, lens extraction or trabeculectomy is required.