Glaucoma: Epidemiology and the Principles of Investigation

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Hello Hivers!
In today's article, we'll talk about Glaucoma. Glaucoma is one of the leading causes of blindness worldwide severely affecting millions yearly especially in developing countries. One thing to note is that blindness caused by Glaucoma is irreversible. This makes it even more dreadful.
Let's go.



By James Heilman, CC BY-SA 3.0, Link


Glaucoma refers to a group of diseases that have in common a characteristic optic neuropathy with associated visual field loss for which elevated intraocular pressure (IOP) is one of the risk factors.

The National Eye Institute defines glaucoma as

a group of eye diseases which result in damage to the optic nerve and cause vision loss.


A normal range of vision. By National Eye Institute, National Institutes of Health, Public Domain, Link
The same view with advanced vision loss from glaucoma. By National Eye Institute, National Institutes of Health, Public Domain, Link

Glaucoma accounts for about 16% of total blindness worldwide. It is the 2nd leading cause of blindness worldwide 2nd only to cataracts and the leading cause of irreversible blindness worldwide.
In Nigeria, it accounts for 16.3% of avoidable blindness. In South-South Nigeria, it accounts for 16.7% of blindness.


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Image made by @gamsam


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Image made by @gamsam


The intraocular pressure (IOP) level = rate of aqueous production by the ciliary body epithelium + resistance to outflow and level of episcleral venous pressure. The aqueous humour is a transparent watery fluid secreted from the Ciliary body, a structure supporting the lens.

The outflow channels for aqueous humour are the trabecular outflow channel (90%) and uveoscleral outflow (10%).

The commonly accepted range of intraocular pressure (IOP) in the general population is 10-22mmHg, the average being 16mmHg with a standard deviation of 3mmHg. However there is no clear line between normal and abnormal IOP. Some people could have damage within the ‘normal’ range while some can tolerate IOPs in the 30s.

Glaucoma results from an imbalance between the rate of aqueous production and its drainage. There is poor drainage resulting in aqueous retention and intraocular pressure elevation. Aqueous humour is produced by the cilliary processes in 3 ways.
First is by active secretion which takes place in the ciliary epithelium and is independent of pressure. Second is by ultrafilteration which is pressure dependent. It can also be produced by simple diffusion which is the passive movement of ions across a membrane.



By Rhcastilhos. CC BY-SA 3.0, Link


When flowing out, aqueous passes through the posterior chamber, through the pupil into the anterior chamber, the trabecular meshwork, the canal of schlemn and finally out through the episcleral veins (90% of the aqueous humour).
Aqueous also drains out through the uveoscleral pathway (about 10%).
Outflow decreases with age and is affected by surgery, trauma, medications and endocrine factors.

Generally, increased IOP is caused by increased resistance to aqueous outflow. In most cases, the IOP is thought to be too high for proper functioning of the optic nerve and so if it is lowered, the image will stabilize. However if there are other mechanisms in play, the damage continues even when the IOP is lowered. In cases of normal tension glaucoma, the IOP remains within normal limits.

Sustained elevation of IOP from glaucoma results in irreversible death of optic nerve fibers. Gradual loss of these nerve fibers leads to the classical glaucomatous changes.

Factors that determine the level of IOP are the rate of production of aqueous humor production by the ciliary body, the resistance to aqueous outflow in the schlemm’s canal (precisely at the juxtacanalicular meshwork) and the level of the epicscleral venous pressure.

Diurnal variation of IOP: Commonly, IOP is elevated in the morning and gradually decreases throughout the day. Wide swings or fluctuation in IOP is a risk factor for optic nerve damage.


Classification of Glaucoma

Glaucoma can be classified as primary (where there is no other ocular pathology leading to the development of glaucoma) or secondary (where there is the presence of an ocular pathology preceding the development of glaucoma). Glaucoma can also be defined as open angle or closed angle based on the appearance of the drainage angles on gonioscopy.
It may also be classified in the following ways: anatomic, gonioscopic, biochemical, molecular and genetic.
The primary glaucoma usually affect both eyes while secondary is usually unilateral.


Primary Glaucoma can further be classified asSecondary Glaucoma can further be classified as
1. True congenital glaucoma
2. Infantile glaucoma
3. Juvenile open angle glaucoma
4. Primary open angle glaucoma
5. Primary angle closure glaucoma
6. Normal tension glaucoma
1. Secondary open angle glaucoma
2. Secondary angle closure glaucoma

Investigations in Glaucoma

Tonometry

Tonometry is the measurement of intraocular pressure. The normal pressure is 10-21mmHg. The principles of measurement are Applanation and Indentation.



A patient in front of a tonometer. By Jason7825 at English Wikipedia, CC BY-SA 3.0, Link


Applanation (flattening) - This is based on the Imbert-Fick principle. The Goldman tonometer measures the force necessary to flatten area of 3.06mm diameter on the cornea. At this diameter the force needed to flatten is counter balanced by the capillary attraction of tears and the tonometer head.
Prisms incorporated in the head of the tonometer enable the examiner to determine the IOP with great accuracy. Also applanation does not displace much fluid as it does not exert pressure on the eye so it is not affected by ocular rigidity which can introduce errors.
This is the principle used in Perkins tonometer, Goldman (slit lamp) Tonometry and air puff Tonometry.

Indentation - Here, the schiotz tonometer is the model. The instrument measures IOP by how much of the cornea it is able to indent. It does this by using different known weights which are calibrated to give certain readings depending on how much weight is able to displace the dial. This is then read off in mmHg on a chart. However it is affected by scleral rigidity and is thus less accurate than the applanation method. However, it is cheaper to procure and so still in use in some developing countries. This is the principle used in schiotz tonometry.

Gonioscopy

This is the examination of the eye using a goniolens. This examination differentiates between open angle glaucoma and angle closure glaucoma.



Gonioscopy of the anterior chamber angle. By Eric Wiessner, CC BY-SA 4.0, Link


Opthalmoscopy

This is the examination of the fundus consisting of the optic nerve head and retina. Features of glaucoma are usually evident on the optic disc. The normal disc on fundoscopy has a central depression(cup) bordered peripherally by a pinkish rim. The ratio of the cup to the rim is described as the cup disc ratio. It is usually not more than 0.3-0.4 in normal eyes.



Opthalmoscopy. By NIH, Public Domain, Link


A progressive abnormal cup disc ratio results from nerve fiber death. Two theories may account for the nerve damage.
Indirect Ischemic theory which states that the raised intra-ocular pressure leads to occlusion of the blood vessels to the optic nerve head(disc), resulting in ischemia and cell death.
The Direct Mechanical theory states that the increased intra-ocular pressure causes direct trauma to the nerve fibers by obstructing axoplasmic flow.
Also, disturbances of vascular auto-regulation may play a role in decreased perfusion and nerve damage.

Examination of the optic nerve head can be done using the direct ophthalmoscope, the indirect ophthalmoscope or the slit lamp with posterior pole lens. The disc is usually recoreded as a decimal (eg. 0.3).



Opthalmoscope. By James Heilman, Public Domain, Link


Perimetry

Glaucomatous damage is associated with peripheral visual field defects. This is examined using the visual field analyzer (Perimeter) or in its absence, confrontational visual field testing.



Perimetry. By Deutsche Fotothek‎, CC BY-SA 3.0 de, Link


Mild to moderate damage are associated with early field defects (constricted peripheral fields) while severe damage may be associated with tunnel vision or complete loss of vision.


Primary Open Angle Glaucoma (POAG)

It is a significant public health problem and is the most common cause of irreversible blindness in blacks worldwide. In the 2008 national blindness survey, glaucoma was responsible for 16.7% of the causes of blindness in the south-south Nigeria.

It has 3-4 times higher in prevalence in blacks and the prevalence increases with age. The risk factors include increased IOP, advanced age, decreased corneal thickness, race, positive family history and adult onset diabetes.
Good central vision is usually preserved until late in the disease but eventually central vision can be lost. Upon examination, the anterior chamber usually deep and open.

The open angle glaucoma genes are the GLC1A gene on chromosome 1 and GLC1C on chromosome 3. Genes on chromosome 2 are responsible for normal tension glaucoma while congenital glaucoma genes are on chromosome 1, 2 and 14.

It is often painless and the patient may not seek help until late in disease. In later stages, there may be complaints of loss of peripheral fields, halos or colored rings around lights.
It is associated with frequent change of glasses.
Visual acuity in early stages is not markedly affected but progressively worsens in late stages.

Upon investigation, gonioscopy will reveal open angles, opthalmoscopy will reveal optic disc defects, thinning of neuroretinal rim and asymmetry of cups in two eyes. Perimetry would show visual field defects.


Primary Angle Closure Glaucoma (PACG)

The mechanism of PACG follows the 'pupillary block mechanisms' e.g following topical mydriatics, accommodation, dim illumination and 'non pupillary block mechanisms' e.g. Plateau iris configuration. It may be lens induced angle closure and it may also be as a reult of retro-lental causes e.g. malignant glaucoma.

In the acute stage, it presents with severe pain, loss of vision, and halos around lights usually one eye. Visual loss is usually profound and sudden and perimetry is not even possible because of poor vision in acute attack.



Photo showing conjunctival vessels dilated at the corneal edge (ciliary flush, circumcorneal flush) and hazy cornea characteristic of acute angle closure glaucoma. By Jonathan Trobe, M.D. CC BY 3.0, Link


The risk factors include age (55-65years), race (Asians and Inuits), gender (women more than men), hypermetropia (short axial length) and hereditary.

Upon examination, tonometry reveals markedly elevated pressure (50-60mmHg) but gonioscopy is difficult in acute stage because of corneal oedema*. Angle structures cannot be seen because peripheral iris occludes angle. Ophthalmoscopy is not done in acute attack.
* 0.5% NaCl drops or topical glycerin may reduce oedema.


Childhood Glaucomas

It is primary or congenital (from birth). It accounts for 50-70% of cases, 65% of which affects males. The involvement is bilateral in 70% of cases.

The clinical features in the new born include epiphora (excessive watering of the eyes), photophobia and blepharospasm (involuntary tight closure of the eyelids). Parent usually complain of tearing, enlargement of eyes, corneal clouding or apparent loss of vision. There is also redused vision from corneal cloudiness from raised IOP and if the cornea is stretched, it may cause Haab's striae (or Descemet's tears which are horizontal breaks in the Descemet membrane).

Diagnosis depends on clinical evaluation which includes IOP measurement, corenal diameter measurement, gonioscopy, and axial length measurement by ultrasonography and ophthalmoscopy. On examination there is evidence of buphthalmos (cow or ox eye - refers to enlargement of the globe in glaucoma and usually develops if glaucoma is seen before age 3 years); examination under anaesthesia is done to measure IOP, corneal diameter (which is usually > 11.5mm) and ophthalmoscopy. The treatment is surgical.

The differential diagnosis include birth trauma, kertaomalacia and intrauterine inflammation from conditions like congenital rubella and syphilis.

Medications have limited long term value for congenital and infantile glaucoma so the preferred therapy is surgical. The surgery done is called goniotomy or trabeculotomy. A trabeculectomy if the above has failed.
Drugs like beta-adrenergic blockers and carbonic anhydrase inhibitors can be used on a temporary.basis to lower IOP and to help clear the cornea.

The complications include amblyopia, corneal scarring, strabismus, anisometropia, cataract, lens subluxation and recurrent glaucoma.


Secondary Glaucomas

They are usually suspected if there is positive history of injury to the eye associated with history of redness or inflammation, diabetes, especially associated with neovascularization and a history of previous intra-ocular surgery.


Management of Glaucoma

The current accepted management is to reduce the intraocular pressure to levels at which damage to the optic nerve and retina is stopped. The treatment can be medical or surgical. The ultimate goal of glaucoma management is to preserve the patients visual function and quality of life.

POAG can be managed medically or surgically while PACG is managed surgically after reducing IOP by medical means. Childhood glaucoma is treated surgically as already stated above and secondary glaucomas are treated medically & surgically depending on the underlying cause.

Medical treatment is done with:

  • Drugs that increase outflow of aqueous like miotics (pilocarpine) and prostaglandin analogues (Latanoprost and Travanoprost)
  • Drugs that reduce production of aqueous like sympathomimetics (Epinephrine), beta-adrenergic blockers (Timolol), carbonic anhydrase inhibitors (Acetazolamide) and alpha adrenergic agonist (Apraclonidine).



Latanoprost. By メルビル CC BY-SA 4.0, Link


Surgical treatment varies for the different types of glaucoma. Trabeculectomy (Filtration surgery), laser trabeculoplasty and glaucoma drainage shunts is done for POAG. For PACG, iridectomy and laser iridotomy can be performed while goniotomy and trabeculectomy is done for childhood glaucomas.



Trabulectomy, conventional surgery to treat glaucoma makes a new opening in the trabecular meshwork, which helps fluid to leave the eye and lowers intraocular pressure. By http://www.nei.nih.gov/photo/eyedis/index.asp, Public Domain, Link


Prevention of Blindness from Glaucoma

POAG is especially difficult to prevent because patients present late so there is need for regular screening which aids early diagnosis. Tonometry should be done routinely on all adults especially high risk patients and treatment should be complied with.
Early surgery is also key in prevention.


Do you have any questions, concerns or anything to add? Tell me what you think below.

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Resources

  • Kyari F, Abdull MM, Bastawrous A, Gilbert CE, Faal H. Epidemiology of Glaucoma in Sub-Saharan Africa: Prevalence, Incidence and Risk Factors. Middle East African Journal of Ophthalmology [Internet]. 2013 [cited 2020 Aug 9];20(2):111–125. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3669488/
  • "Facts About Glaucoma". National Eye Institute. Archived from the original on 28 March 2016.
  • Mantravadi, AV; Vadhar, N (September 2015). "Glaucoma". Primary Care.
  • Rhee, Douglas J. (2012). Glaucoma (2 ed.). Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.
  • "Eye Anatomy". WebMD.
  • Human Physiology. An Integrate approach. 5th edition. Dee Unglaub Silverthorn
  • Tao, Le (2017-11-13). First aid for the USMLE Step 2 CS. Bhushan, Vikas., Lee, Kachiu., Deol, Maniver. (Sixth ed.). New York.




Written by @gamsam - a Medical Student
All images used are copyright free
Vancouver Style was used for References.




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