Posterior Capsular Opacification Case Study

4th Year Reflective Blog

• A 77-year-old female patient sought treatment for a noticeable decrease in her vision since she last visited the clinic.
• There were no abnormalities in OCT and Fondus photos.
• The patient didn't have a history of health conditions or medications linked to visual fluctuations.
• The patient had cataract surgery several years prior, and PCO (posterior capsular opacification) was visible on the left IOL.
• The patient's vision deteriorated from BCVA 6/7.5 to BCVA 6/15 in the left eye, specifically, during presentation.
• Until the left IOL exam, all ocular health checks revealed no abnormalities.
• A significant amount of PCO built up on the patient's IOL.

Blog Comments

• It is important to examine the reasons when a patient can't see 6/6, and NB can rule out pathology.
• What if the eye appears healthy, and OCT scans are normal?
• Factors besides pathology could explain a reduction in VA.
• When tested with a projector chart, one person struggled to see 6/6, but in the NOC, they could see 6/4.8 with a Thompson chart.
• The chart or testing room length may have been the issue in the example.
• Some patients don't bother putting effort into subjective refraction and choosing a clearer lens as they don't want to wear glasses.
• When a patient sees worse than expected, many factors need consideration, and it is not always pathology.

Learning Objectives

• To gain a better understanding of factors that influence Visual Acuity measurements, including:
  • Refractive error
  • Test Chart Design
  • Px Criterion
  • Luminance
  • Glare
  • Eccentricity
  • Eye Movement & Fixation

Types of Spatial VA

• Visual Resolution: The ability to distinguish two closely placed point objects as being separate is related (but not the same as Visual Acuity).
• Localisation: The smallest spatial offset or difference in location between targets that can be discriminated, such as hyperacuity (e.g., stereopsis).
• Vernier acuity, which is measured in this way, is a type of hyperacuity.
• Resolution and localisation tasks produce hyperacuity.
• Levels of performance are over and above the limit of recognition or identification (normal visual acuity).
• Hyperacuity tests, such as stereopsis, are less sensitive to optical defocus.
• Identification or recognition acuity involves the details of the smallest letter on the chart; a good value of MAR is 30 seconds of arc.
• Convert 6/6 and 6/3 to MAR or find the equivalent of 30 seconds of arc in Snellen Acuity.

Visual Acuity

• Visual acuity (VA) is the sharpness or clearness of vision.
• VA measures spatial resolution, which is dependent on optical and neural factors:
  • Sharp retinal focus
  • Intactness and functioning of the retina
  • Brain's sensitivity to interpret

Pathology; Clinical Importance

• VA measurement is the only measure of central vision used on every patient.
• VA needs to be measured carefully & under well-controlled settings.
• Changing environments affect VA due to pathology or test conditions.

Refraction; Clinical Importance

• VA measurement helps determine the best sphere in subjective refraction and whether extra minus/less plus sphere improves VA or makes letters blacker.
• Relying on the patient to say if the letters appear blacker can lead to problems.

Normal Visual Acuity

• The norm is 6/6 for patients aged greater than 60 years.
• The norm is better than 6/6 for patients aged less than 50 years.
• Patients may be able to reach 6/4 with extra [-] sph./less [+].
• Patients with 6/6 may have lost 2 lines due to pathology.

Factors that Influence VA

• Retinal cone spacing: spacing of photoreceptors Pupil size
• Diffraction: Raleigh's criteria & Airy discs
• Aberrations: Chromatic and Spherical
• An optimal mid-size pupil between 3-5mm balances diffraction and aberration limits.

Factors that Influence VA

• Refractive Error
• Test chart design
• Px's criterion
• Luminance
• Glare
• Eccentricity
• Moving and/or briefly presented targets
• Eye movements & fixation

Refractive Error

• Refractive errors cause defocus on the retina, affecting visual acuity.
• Defocus blurs fine detail, sharp edges, and contrast sensitivity by affecting its point spread function.

Chart Design: The Snellen Chart (1862)

• Snellen published the first visual chart based on optotypes built on a 5x5 matrix.
• In Snellen notation (e.g., 6/12), the letter height is 5' at 12m.
• Snellen letters are constructed so detail size (stroke or gap width) subtends 1/5th of the overall height.
• Visual acuity (Snellen notation) is determined by the smallest line of letters correctly identified:
  • VA = TD/LS
  • TD=Testing distance (6 M)
  • LS = Letter size - distance at which letter subtends 5 minutes of arc
  • 6 M is the standard test distance
• Defined in visual angle, stroke width of the smallest reliably identified letter (dist at which optotype is 5 min of arc or stroke width is 1min of arc).
• A typical optotype conforms to a 5-unit high by 5-unit wide matrix, where each stroke is 1/5 of the height.

Types of Spatial VA

• MAR: The eye angle of just resolvable bars

Snellen Chart Disadvantages

• Letters are not equally legible.
• Non-uniform progression of sizes.
• Unequal letters per line.
• Irregular spacing.
• Inadequate scoring
• Results can be inaccurate, use a Bailey & Lovie, Sloan chart. -LogMAR alternative to Snellen: Bailey & Lovie (1980); Early Treatment Diabetic Retinopathy Study (EDTRS); Sloan.

Visual Acuity

• Visual Acuity results are expressed numerically using Snellen fraction, MAR, or logMAR scale.
  • Testing children involves crowded tests vs single letter recognition. -Note the type of chart used when monitoring a patient. Last line in which Px identifies at least 4 out of 5 letters can be used

Test chart design: Bailey Lovie

• The name LogMAR is derived from the Logarithm of the Minimum Angle of Resolution.
• To convert Snellen to MAR, flip it.
• In the Bailey-Lovie chart, the letters on each line have equal visibility.
• Each line differs from the next by 0.1; the letter size follows a logarithmic progression, increasing in 0.1 logMAR steps.
• The letters on each line are ≈ 26% (0.1 log unit) larger or smaller than those on the line above or below.
  • Just noticeable acuity differences are equal.
  • Difficulty↑ in equal steps in logMAR scale
• Number of letters on each row is = scoring on each line ( 5 letters x 0.02).

Bailey Lovie; Advantages and Disadvantages

• The Baileye Lovie and ETDRS (Early Tx Diabetic Retinopathy Study) are both logMAR charts and are the gold standard.
• Bailey Lovie is easily used at different distances with high and low charts available.
• The log of the min angle of resolution (logMAR) represents VA and is the preferred scale.
• LogMAR charts tend to be larger, therefore can be negative for letter sizes smaller than 6/6.
• LogMAR scoring requires some mental arithmetic.

Bailey-Lovie Chart

• Employs the letter set given in the British Standard. And was selected to give average legibility. Precise scoring:
• Add 0.02 for each missed letter, or subtract 0.02 for extra letters (e.g. 6/6 +2 =.......?)
• Has small letters (≈ = 5, 4 & 3 m letters)

Px Criterion

• Cautious Patients will perform below their true VA.

Patients Memorise the Chart

• Encourage Patients to read the next line by:
  • pointing to a high visibility letter
  • tell Px mistakes don't matter
  • ask if any idea what this letter is
  • Builds confidence
• Problem is more common with elderly Pxs.
• VA is better in LE (OS) compared to RE (OD).
• Patients get better VA with subjective compared to habitual.
• Use different RE/LE charts such as Thomson, or use randomisation of letters in the charts.
• Note monocular acuity before binocular acumen.

Crowding Phenomenon/Factors

• Letter identification depends on size and proximity to nearby letters.
• Crowding: inability to discriminate crowded optotypes and has modern charts standardise letter separation.
• Overestimate VA for an amblyope utilizing single letters.
• Note the type of chart is used especially when testing children.

Retinal Illuminance

• Affects patient VA whereby Scotopic conditions read less difficult under low light.
• Photopic vision is mediated by cones under high luminance
• Levels above cone threshold: As retinal illuminance ↑; VA first improves rapidly & gradually.
• Acuity is clinically measured on the curve's section where luminance has little effect (≈ 100 to 150 cd/m2).
• Fluctuations in room illumination, pupil size, or intraocular scattering do not substantially influence the result.
• VA depends strongly on neural processing and convergence of rods onto bipolar cells at low illuminance Optimal conditions for vision achieved if rods are functioning

Contrast

• Well-focused targets reduce letter acuity.
• Lights in the room decrease projected letter charts providing a "veiling luminance".
• Room illumination should reduce the contrast of projected acuity targets:
  • Contrast =Lt Luminance of target (letter) - Lb Luminance of the background (test chart).

Illuminance; Background illumination

• Ambient room conditions can reduce the projector chart. -Standardise clinical conditions for contrast: Lmax-Lmin/Lmax.

Lighting; conclusions

• Ambient room can decrease chart projector contrast and should be set at dim conditions and should not be greater than 100-150 cd/m².
• Bright light leads to decreased vision caused by cataracts. Total retinal illuminance affects the patient pupil diameter.

Luminance vs Glare & Contrast

• VA is better for targets of high contrast rather than low contrast.
• Standard clinical acuity has high contrast, so small changes do not influence test quality.

Glare,Contrast & Luminance

• Researcher's advocate the use of the high and low charts to pick up/determine ocular and neurological conditions.
• VA is best when imaged onto the fovea by small densely packaged cones, and acuity is systematically presented poorer to other retinal regions.

Eccentricity vs Visual Acuity Task

• Photopic changes linearly expressed as MAR. Letter acuity is reduced when distanced from the patients phobia and depends on grating, letter task. VA declines rapidly

Eccentricity

• Relative acuity in the form of (min. arc) located at the patients fovea.
• Visual ability is acute in the phobia and decreases with rising angle related to : decline in cone size and increase receptive size.

Fovea

• VA sharply declines inside phobia due to fixation, but there may be low causes, such as Micro-strabismus

Micro eye movements

Factors that affect this-

• Involuntary movements affect acuity that allow for great resolution (small jerks).
• Microsaccades that occur, prevent patient image from fading -Stabilise normal vision through flick eye movements.

Phenomenons

• Optical illusion that impacts patients perception.
• In constant for 40 sec = stimulus point and will fade as details blur in vision .
• Neuron adaptation of stimulus , patient may blink to fix vision.

Moving target effect.

• Acuity remains until stimulus exceeds beyond 40 deg/sec, but will deteriorate with high intensity.
• Dynamic VA can be limited due to smooth pursuit movements, and VA reduces from saccades if that rate increases.

Eye movements

• Smooth coordinated eyes require many brain regions in order
• Patients with Schizophrenia may have 80% lower movements compared to normal eye sight.
• Patients with autism and other ailments may lack persuit.
• Horitzontal gaze may be used to determine reaction through eye.

Eye Movement(Saccadic Eye movements)

• The act of during sacrades may blur vision (retnal image).
• VA stops from stability and during.

Eye Movement (Micro)

• When the steady eye is fixated the micro movements start due to jerks, and the movement assists acuity when the disc diameter increases(airys). The eye can move stimulus, and have the act of stabilzation.