Three Important Tests in Ophthalmology

Usman Raja

Assessment of patients in ophthalmology typically involves taking a history and an examination.  History taking is important in the evaluation of patients as it provides information that can lead to the identification and treatment of the ophthalmic condition. For example, identifying systemic conditions that the patient may suffer from, medications that the patient uses and family history. Ophthalmologists typically ask the patient to describe their vision and the visual symptoms that they may suffer from. Whilst having a subjective understanding of a patient’s vision is important, testing it using objective means as well as assessing the eye is also necessary especially when monitoring in chronic conditions.

This article aims to provide a brief overview of three important tests in ophthalmology: testing visual acuity, visual fields and intraocular pressure.

Visual acuity is defined as the patient’s ability to distinguish between different optotypes (such as letters) at a set distance. The most common way it is assessed is with the use of a Snellen chart. The aim of the visual acuity test is to determine patients’ clarity of vision (1,2).  To assess it, patients are positioned at 6 metres from the Snellen chart. One eye is covered and then the patient is asked to read the letters they see from left to right and starting from the top. They are asked to continue until they reach the smallest line of which they can read. This line represents their visual acuity. This exercise is repeated with the other eye covered. Patients are asked to wear their glasses to assess their corrected visual acuity and if unavailable a pinhole can be used to resolve the refractive error.

The results are then interpreted with good vision being 6/6. + and – are sometimes seen when reporting acuity, with + meaning that patients can see letters on the next line and – indicate how many letters were not read on a certain line. For example, 6/12 -2 indicates that patient can read all the letters except for two on the line that corresponds to 6/12.

If patients are unable to read the chart, then they are checked to see if they can count fingers. If they are unable to then they are tested if they can see hand movements. If they can’t identify hand movements then they are tested to see if they can perceive light from a light source.

Testing visual acuity is important as a deterioration in it can be a feature of ocular or systemic disease and is used with other tests to help formulate a diagnosis. (3)

Testing visual fields is an important test in ophthalmology. Patients may describe not seeing particular portions of their vision, but this information can only be objectively understood with a visual fields test. Visual fields testing can be done using a variety of methods that include confrontation testing, automated static perimetry and kinetic perimetry.

A confrontation visual field tests is where an examiner is sat directly opposite the patient and compares their visual fields with the patient.  The patient is then asked to look straight, typically at the examiner’s eye and asked to cover one eye. The examiner covers their opposite eye and moves their fingers to map out an approximation of the patient’s visual field.  It is a quick and simple of testing visual fields, but confrontation tests have shown to be quite insensitive in identifying visual field loss (4).

Automated static perimetry uses a machine to map out an accurate visual field for the patient.

Intraocular pressure (IOP) is typically measured when assessing patients. A normal reading is typically between 10 to 20 millimetres of mercury (mmHg). It is an important measurement for the diagnosis and management of glaucoma as elevated IOP is a modifiable risk factor for the development of primary open-angle glaucoma. If monitored, it can delay or prevent the development of primary open-angle glaucoma (5,6). The most widely used method of measuring IOP is Goldmann applanation tonometry (GAT). GAT estimates IOP based on the force needed to flatten the corneal apex by a tonometer tip which is pressed onto cornea. It requires local anaesthetic (7). Non-contact tonometers can also be used. They detect the force required to flatten the cornea using air and the technology does not come into contact with the cornea.

In summary, this article provides a brief overview of three useful methods that are used to assess patients. These assessments are important to help diagnose and treat ophthalmic conditions.

References

  1. Daiber HF, Gnugnoli DM. Visual Acuity. 2023 May 29. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan–. PMID: 33085445.
  2. Levenson JH, Kozarsky A. Visual Acuity. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Butterworths; Boston: 1990.
  3. Caltrider D, Gupta A, Tripathy K. Evaluation of Visual Acuity. [Updated 2023 Aug 25]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK564307/
  4. Kerr NM, Chew SS, Eady EK, Gamble GD, Danesh-Meyer HV. Diagnostic accuracy of confrontation visual field tests. Neurology. 2010 Apr 13;74(15):1184-90. doi: 10.1212/WNL.0b013e3181d90017. PMID: 20385890.
  5. Machiele R, Motlagh M, Patel BC. Intraocular Pressure. [Updated 2022 Jul 11]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532237/
  6. Kass MA, Heuer DK, Higginbotham EJ, Johnson CA, Keltner JL, Miller JP, Parrish RK, Wilson MR, Gordon MO. The Ocular Hypertension Treatment Study: a randomized trial determines that topical ocular hypotensive medication delays or prevents the onset of primary open-angle glaucoma. Arch Ophthalmol. 2002 Jun;120(6):701-13; discussion 829-30.
  7. Okafor KC, Brandt JD. Measuring intraocular pressure. Curr Opin Ophthalmol. 2015 Mar;26(2):103-9. doi: 10.1097/ICU.0000000000000129. PMID: 25594767.

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