Summary
In this article we summarise the key investigations used in the diagnosis and monitoring of glaucoma.
Tonometry
Tonometry is a technique used to measure intraocular pressure (IOP).
The most widely used method is Goldmann applanation tonometry, which is based on the Imbert-Fick law.
The Imbert-Fick law is used to calculate the amount of force required to flatten an area of cornea that is 3.06mm in diameter, assuming central corneal thickness (CCT) is 520 micrometers.
Technique:
1) Topical anaesthetic and fluorescein are instilled into the conjunctival sac
2) The prism of the tonometer is advanced until it touches the apex of the cornea
3) A pattern of two green semi circles is seen, one above and one below the horizontal midline, which represent the flourescein stained tear film touching the upper and lower outer halves of the prism
4) As the dial on the tonometer is rotated, the force applied varies, and the inner margins of the semi circles align with a circular area of precesiely 3.06mm is flattened
5) The reading on the dial is multiplied by 10 to give the IOP (often shown digitally)
Sources of error:
- Excessive fluorescein (overestimates the IOP)
- Low CCT (underestimates IOP) or raised CCT (overestimates IOP)
- Corneal oedema
- Astigmatism
- Incorrect calibration of the tonometer
- Breath-holding (overestimates IOP)
- Globe pressure/wincing (overestimates IOP)
- Tonographic effect - when repeated measurements are taken of the same eye, the eye adapts (leading to underestimation of IOP)
(lightbulb) If the examiner accidentally applies pressure from their fingers onto the patient’s globe when performing applanation tonometry, or if the patient squeezes their eyelids, this can cause an overestimation of IOP.
Photograph demonstrating applanation tonometry. Image courtesy of Stevens et al.
Gonioscopy
Gonioscopy is a method used to determine whether the iridocorneal angle in the anterior chamber is open or closed.
Optical principle:
The angle of the anterior chamber can’t be directly visualised by the eye, as the light undergoes total internal reflection (because the critical angle of the cornea-air interface is exceeded). When light passes from a material of higher refractive index to lower refractive index, it reflects at the interface of the two, unless the angle of incidence is less than the critical angle. The goniolens has a refractive index similar to the cornea, which overcomes the total internal reflection effect by replacing the tear film-air interface with a tear film-goniolens interface, enabling the iridocorneal angle to be visualised.
The following structures are found at the iridocorneal angle, posteriorly to anteriorly:
- Ciliary body
- Scleral spur
- Pigmented trabecular meshwork
- Non-pigmented trabecular meshwork
- Schwalbe line
(lightbulb) The structures at the iridocorneal angle, from posterior to anterior, can be remembered with the mnemonic: I Can See Til Schwalbe line
- I = Iris
- Can = Ciliary body
- See = Scleral spur
- Til = Trabecular meshwork (pigmented then non-pigmented)
- Schwalbe line
Inability to visualise these structures indicates a closed angle, and visualisation of all the above structures means the angle is wide and open.
Indirect gonioscopy:
These use a mirror to reflect rays from the angle, such that they exit the goniolens at a much lower angle than the critical angle. Must be used in conjunction with a slit lamp.
Direct goniscopy:
Direct goniolenses work by constructing the viewing surface of the lens in a domed or slanted configuration, meaning that exiting light rays strike the contact lens/air interface at a steeper level than the critical angle. This is known as direct gonioscopy as the light rays from the angle are viewed directly, without reflection inside the lens. This does not require a slit lamp.
Imaging in glaucoma
Corneal pachymetry
Pachymetry is a technique used to measure CCT (which is typically 540-560 micrometers). The most widely used technique is a handheld ultrasonic device such as the ‘Pachmate’. In the context of glaucoma, it is important to be able to assess CCT to ensure that IOP measurements using tonometry are accurate (as described above, a low or high CCT can under- or overestimate IOP, respectively).
Ocular coherence tomography (OCT)
OCT is an imaging technique that is used to image the layers of the retina. It uses near-infrared waves to construct a cross-sectional 3D image of the retina. The appearance of the different tissues are described in terms of their density: brighter structures are hyper dense; and darker structures are hypodense.
OCT is typically used to diagnose and monitor macular and retinal pathology and disorders of the optic nerve including glaucoma.
Anterior segment OCT has also been developed which is particularly useful for investigating angle-closure glaucoma. It can demonstrate the relationship between the iridocorneal angle and the peripheral iris.
Confocal scanning laser ophthalmoscopy
This uses a scanning laser ophthalmoscope to construct a 3D image of the optic nerve head and retina.
Perimetry
A visual field can be represented as a 3D structure, like a hill on a map. Visual acuity is sharpest at the very top of the hill: the fovea, and then declines progressively towards the periphery, with the nasal slope being steeper than the temporal. Perimetry is a technique used to detect visual field defects that is commonly used in the subspecialties of Glaucoma and Neuro-ophthalmology.
There are different types of perimetry, such as Goldmann visual field testing (a kinetic test) or Humphrey visual field analysis (a static test). In kinetic perimetry, a stimulus is moved from a non-seeing area to a seeing area, and the location where the stimulus is first seen is recorded. In static perimetry, stationary stimuli are presented at different points in the visual field. Stimuli present for longer time periods may be seen better due to temporal summation of impulses.
The type visual field defects in glaucoma are:
- Tunnel vision with temporal sparing of the visual field
- Nasal step
- Paracentral depressions
- Arcuate defects
- Ring scotoma
Humphrey visual field testing in a 61-year-old patient with severe glaucoma. This patient was diagnosed with POAG 15 years before this visual field test. Both optic discs were terminally cupped and IOP was poorly controlled in both eyes. Image courtesy of Yaqub.
References
- Salmon, John F., and Jack J. Kanski. Kanski’s Clinical Ophthalmology: A Systematic Approach. Ninth Edition, Elsevier, 2020.
- James, Bruce and Bron, Anthony. Ophthalmology Lecture Notes. Eleventh Edition, Wiley-Blackwell, 2011.
- Stevens, Sue, et al. ‘How to Measure Intraocular Pressure: Applanation Tonometry’. Community Eye Health, vol. 25, no. 79–80, 2012, p. 60. PubMed Central, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3588125/.
- Yaqub, Moustafa. ‘Visual Fields Interpretation in Glaucoma: A Focus on Static Automated Perimetry’. Community Eye Health, vol. 25, no. 79–80, 2012, p. 1. www.ncbi.nlm.nih.gov, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3678209/.Authors
Author(s)
Dr Sara Memon
Sara is the Co-Founder of Ophtnotes. She is a doctor who graduated from UCL Medical School in London. She won the Allen Goldsmith Prize in Ophthalmology. Sara is also the co-founder of PAMSA: an organisation linking doctors and medical students of Pakistani origin. She’s especially passionate about teaching and education, having presented a workshop she designed herself at the 2019 Annual GMC Conference.