Glaucoma Screening

From EyeWiki


The basis for healthcare in many countries is for patients to look for medical care when they are symptomatic, however this approach has several problems: individuals must know what the symptoms are, they must have access an appropriate healthcare facility and the health professional must be able to correctly diagnose the disease. This is a problem that goes beyond the point of this article, but points out the need for an accurate and true primary preventive medicine through correct screening procedures.

Glaucoma is a chronic disease that sometimes may be difficult to establish de exact time of origin of the disease, especially when patients may show incomplete clinical pictures or even appear normal; besides the fact that patients are initially asymptomatic and usually do not notice any change in visual function and so do not seek medical care. It is therefore important to have a screening criteria for this entity, to find patients in their initial stage; because glaucoma damage to the optic nerve is irreversible and well established glaucoma progresses in the majority of cases without the appropriate treatment.

Disease

Glaucoma is the most common optic neuropathy in the adulthood. Glaucoma is defined as an optic neuropathy associated in most cases with elevated intraocular pressure (although pressure may be within the normal range), with or without anatomic predisposing factors in the anterior chamber (open angle vs angle closure). The American Academy of Ophthalmology has defined Glaucoma as " a multifactorial optic neuropathy with a characteristic acquired loss of optic nerve fibers" which is usually (but not necessarily) identified in visual field exam and retinal fiber OCT. This cluster of diseases is progressive without appropriate treatment, and unfortunately the damage is irreversible. Primary Open Angle Glaucoma is the most common form of this cluster of diseases and so is the main focus of this article.

Risk Factors

Glaucoma is a multifactorial disease and there are several risk factors that are associated with the development of this entity.

  • Age: age by itself is a risk factor. In the Baltimore Eye Studi, patients in their 70s had x 3.5 times the risk for developing glaucoma than those patients in their 40's
  • Race: Black individuals have 3-4 times more risk than whites to develop glaucoma; and hispanics have 1-2 times more risk than whites to develop glaucoma. The cause of this race variation is unknown.
  • Intraocular Pressure (IOP): Increased intraocular pressure is a risk factor for the development of glaucoma. It should be taken into account that the IOP has a diurnal variation, and so even if one measurement is within the normal range, high peaks of IOP may still be present causing optic nerve damage. And so it is recommended that the time of IOP measurement be recorded along with IOP. Most subjects have a peak of IOP during the night (because of the body position)
  • Family History: people with first degree relatives with glaucoma are at higher risk for developing glaucoma
  • Corneal thickness: patients with thinner corneas have a greater risk for developing the disease (besides the influence on the IOP measurement). It has been thought that thinner corneas may be related to increased biomechanical susceptibility of the lamina cribrosa and peripapillary sclera.
  • Myopia and Diabetes Mellitus: they are thought to be related but no hard evidences exists yet to show a relation.

Epidemiology

The estimated prevalence of Primary Open Angle Glaucoma in patients older than 40 years of age is 1.86%, which means that almost 2.22 million Americans are diagnosed with this disease. The fact that the data takes into account patients with the diagnosis (with visual field loss and other factors), this number probably underestimates the real prevalence due to the fact that does not contemplate the complete spectrum of the disease.

It is a potentially blinding disease, considered the most frequent cause of non-reversible blindness in blacks in the US. It estimated that it is the third leading cause of blindness worldwide (following cataract and river blindness)

Screening

There has been emphasis on glaucoma screening, since most of the times there is an insidious start of the disease (with no clear start point, POAG), and progression may be slow and unnoticed to the patient. In addition to that, there is a recognized stage of the disease in which patients are apparently in a pre-perimetric (before loss of the visual field is present) stage, bringing a challenge to the diagnosis and screening techniques.

The purpose of glaucoma screening tests is to detect those with early stage disease, so that these patients can be treated to reduce the risk of visual field loss.

For patients with pre-perimetric glaucoma, screening tests are limited to the evaluation of the optic nerve and the NFL. Optic nerve and retinal nerve fiber layer imaging is used to find anatomic alterations. Probably one of the imaging systems most commonly used is the OCT of the optic nerve; the new spectral domain OCT has been used to screen for loss of the retinal fiber layer in glaucoma. However clinical evaluation is paramount, the increase in vertical cup/disc ratio, the appearance of cup notching or hemorrhages in the disc are taken as a positive screening for glaucoma. It is recommended that stereoscopic pictures of the optic nerve be taken with some regularity, and is considered as the most sensitive early detection method. Caution must be taken due to the fact that there is certain variability between observers, and to the fact that there is no gold standard unique test for the diagnosis of glaucoma, but rather a set of factors that all together lead to the diagnosis.

The recommendation for screening is complex, and several issues should be taken into account. Every patient during a regular ophthalmologic visit is checked for visual acuity, intraocular pressure and cup/disc ratio as part of the optic nerve assessment. If any of those key points raises suspicion such as decreased visual acuity (with no other apparent cause), high or borderline intraocular pressure, or a characteristic glaucomatous vertical optic nerve excavation or disc hemorrhages studies are ordered for a more detailed evaluation of the optic nerve fibers and visual function.

The usual studies that are taken are corneal pachymetry, optic nerve head OCT and  a 24:2 visual field. The OCT and visual field are helpful not only to screen or diagnose glaucoma, but to have a starting point to compare the patient through a time lapse. Care must be taken however for the interpretation of the studies, since they are not absolute values or diagnostic tools; especially the visual field, where it may take a patient several times to perform correctly on the exam.

The rate of progression through a time lapse should also be registered since it fundamental for diagnostic and treatment decisions.

There are several countries that are pioneering in the field of ophthalmologic telemedicine. Perhaps in the near future patients that are seen in the ER or in another branch of medicine could be taken IOP with an "easy to use" tonometer (Tonopen or icare ) and have a fundus photograph taken and sent to an ophthalmologist, for screening and early referral if needed.

Additional Resources

References

  1. Yanoff, M; Duker, J (2014).“Ophthalmology. Yanoff and Duker.” Screening for Glaucoma Part 10.2. 4ª Edition. Elsevier.
  2. Glaucoma (2011-2012) Basic and Clinical Science Course . American Academy of Ophthalmology
  3. Heale, P. "Screening for Glaucoma". Section 1. Glaucoma in the World