20 Tonometry, Visual Fields & Biometry Practice Questions & Answers

1. 1. Goldmann applanation tonometry estimates intraocular pressure from the force needed to flatten a standardised area of cornea. Why is that particular flattened area used?

   • A.At that area, the cornea no longer refracts any light from the slit beam
   • B.At that area, corneal rigidity and tear-film surface tension cancel out✓ Answer
   • C.At that area, the reading can be taken without any topical anesthetic
   • D.At that area, the fluorescein ring is too small to touch the lid margin

   The Imbert-Fick relation treats pressure as force divided by flattened area, but the cornea is not an ideal membrane: its own rigidity resists flattening while the tear film's surface tension pulls the prism toward the eye. The standard applanation diameter is the size at which those two opposing forces cancel, so the force read off the dial reflects intraocular pressure alone.

   Source: Imbert-Fick principle as applied in Goldmann applanation tonometry; IJCAHPO Core Criteria Handbook, tonometry content areaReport a problem with this question

2. 2. While measuring with the applanation tonometer, the two fluorescein semicircles look very wide and thick and overlap broadly. What has most likely happened, and how does it affect the reading?

   • A.Too little fluorescein thinned out the mires, biasing the reading high
   • B.Excess fluorescein or a wet lid thickened the mires, so it reads high✓ Answer
   • C.The cobalt filter is out of the beam, so the reading is not affected
   • D.The prism rotated in its holder, so the reading is biased low instead

   Each mire should be about a tenth of its own diameter in width. Excess fluorescein, tearing or a wet lid margin fattens the semicircles so their inner edges meet with more force applied than the eye truly requires, which reads high; too little fluorescein gives faint, thin mires and reads low. The fix is to dry the lids, reapply a minimal amount of fluorescein and remeasure.

   Source: Goldmann applanation technique, mire width and fluorescein quantity; IJCAHPO Core Criteria Handbook, tonometry content areaReport a problem with this question

3. 3. A patient has had laser refractive surgery and now has a cornea thinner than average. Compared with the true intraocular pressure, how does applanation tend to read?

   • A.Falsely low, but only when corneal oedema is present too
   • B.Unchanged, because thickness does not affect applanation
   • C.Falsely high, because a thin cornea resists flattening more
   • D.Falsely low, because a thin cornea resists flattening less✓ Answer

   Applanation assumes a cornea of average thickness and rigidity. A thinner cornea is easier to flatten, so less force is needed to reach the endpoint and the instrument reports a pressure below the true one; a thicker cornea over-reads for the same reason reversed. What matters clinically is the direction of the bias, which the physician weighs against the measured thickness.

   Source: Effect of central corneal thickness on applanation tonometry; IJCAHPO Core Criteria Handbook, tonometry content areaReport a problem with this question

4. 4. What is the correct endpoint for the fluorescein mires during applanation, and what should be done if they pulsate with the ocular pulse?

   • A.Inner edges just separated; if pulsating, press the lids to damp the pulse
   • B.Semicircles fully overlapping; if pulsating, read the narrowest point
   • C.Inner edges just touching; if pulsating, read the middle of the swing✓ Answer
   • D.Outer edges just touching; if pulsating, read the widest point of the swing

   The dial is turned until the inner edges of the upper and lower semicircles just meet, because that position corresponds to the standardised flattened area the instrument is built around. A pulsating mire reflects the ocular pulse, and the average pressure is represented by the midpoint of the excursion; pressing on the lids or globe to stop the movement would itself raise the pressure.

   Source: Goldmann applanation endpoint and ocular pulse; IJCAHPO Core Criteria Handbook, tonometry content areaReport a problem with this question

5. 5. A patient's pressure reads 31 mmHg in the right eye today, compared with 14 mmHg at the last visit. What should the assistant do next?

   • A.Repeat after checking posture, collar and lid handling, then report it✓ Answer
   • B.Instil more anesthetic, remeasure and chart the average of the two
   • C.Chart the value and move on, since a repeat measurement adds nothing
   • D.Tell the patient that this level means glaucoma and book a field test

   A large unexplained jump is often artefactual: breath holding, a tight collar, squeezing the lids, or the examiner's fingers pressing the globe all raise the reading. The assistant confirms the value with a correctly performed repeat and then reports it, with the method, the eye and the time, to the ophthalmologist; telling the patient what the number means is a diagnosis and outside the assistant's role.

   Source: Artefactual elevation of applanation readings and reporting of findings; IJCAHPO Core Criteria Handbook, tonometry content areaReport a problem with this question

6. 6. A patient arrives with a painful red eye and a corneal epithelial defect that stains with fluorescein. How should the pressure be handled?

   • A.Proceed, because the anesthetic drop protects the injured epithelium
   • B.Proceed, but add extra fluorescein to cushion the prism on the cornea
   • C.Defer contact tonometry and let the physician decide how to measure✓ Answer
   • D.Switch to indentation tonometry, which does not touch the cornea at all

   An eye with an epithelial defect, a possible infectious keratitis or recent surgery should not have a prism placed on it: contact can enlarge the defect, seed infection, and readings over an irregular cornea are unreliable anyway. The assistant reports the finding and lets the physician decide whether and how to measure. Indentation tonometry also rests on the cornea, so it is no safer.

   Source: Contraindications to contact tonometry; IJCAHPO Core Criteria Handbook, tonometry content areaReport a problem with this question

7. 7. After a reusable tonometer tip is disinfected, why must it be rinsed with water and dried before it touches the next patient's eye?

   • A.Water removes fluorescein that would otherwise thicken the mires
   • B.Disinfectant left on the tip can injure the corneal epithelium✓ Answer
   • C.Rinsing restores the calibration of the tonometer measuring drum
   • D.Drying is the step that actually inactivates virus on the prism

   The agents used on tonometer tips are epithelial toxins: any film of bleach, peroxide or alcohol carried onto the cornea can produce a chemical keratitis and a painful abrasion. Disinfection kills the organisms, and the rinse-and-dry step removes the agent itself; disposable tips or covers avoid the problem altogether.

   Source: Tonometer tip disinfection and rinsing; IJCAHPO Core Criteria Handbook, tonometry and infection-control contentReport a problem with this question

8. 8. An automated field shows a dense arcuate defect in the superior hemifield that stops sharply at the horizontal midline. What does respecting the horizontal midline point to?

   • A.Compression of the crossing fibres at the optic chiasm
   • B.A lesion of the optic radiations in the temporal lobe
   • C.An infarct of the occipital cortex on the opposite side
   • D.Damage to nerve fibre bundles at the optic nerve head✓ Answer

   Retinal nerve fibres arc around the macula and meet along the temporal raphe, which lies on the horizontal meridian, so bundle damage at the disc produces arcuate defects and nasal steps that stop at the horizontal midline. Lesions at or behind the chiasm follow the vertical midline instead, because that is where the nasal and temporal halves of the retina divide.

   Source: Retinal nerve fibre anatomy and glaucomatous field defects; IJCAHPO Core Criteria Handbook, visual field testing content areaReport a problem with this question

9. 9. A patient's fields show loss of the temporal half of each eye's field, with a sharp edge at the vertical midline. Where does this pattern localise?

   • A.The optic nerves on both sides, in front of the chiasm
   • B.The optic tract on one side, just behind the chiasm
   • C.The maculae of both eyes, from retinal disease
   • D.The optic chiasm, where nasal retinal fibres cross✓ Answer

   Fibres from the nasal half of each retina carry the temporal half of that eye's field and cross in the chiasm, so a lesion pressing on the chiasm knocks out both temporal fields with an edge at the vertical midline. A defect behind the chiasm would be homonymous, affecting the same side of the field in both eyes rather than opposite sides.

   Source: Visual pathway anatomy and chiasmal field defects; IJCAHPO Core Criteria Handbook, visual field testing content areaReport a problem with this question

10. 10. Why does automated static perimetry use the patient's distance correction together with an age-appropriate near add?

    • A.The add offsets the general depression caused by a cataract
    • B.Blur does not change thresholds, so the lens is for comfort
    • C.The bowl is at a near distance, so the eye must focus there✓ Answer
    • D.The lens dilates the pupil so that dim stimuli are easier to see

    The stimuli are projected on a bowl only a short distance from the eye, so a presbyopic patient cannot focus on that surface with a distance correction alone. Defocus blurs the stimulus and lowers measured sensitivity, producing a generalised depression that mimics disease, which is why the lens is chosen for the bowl distance, kept close to the eye and used in a full-aperture holder.

    Source: Refractive correction for automated perimetry; IJCAHPO Core Criteria Handbook, visual field testing content areaReport a problem with this question

11. 11. A field printout shows a dense ring of absolute loss around the outermost tested points, with a normal central field. What is the most likely cause?

    • A.A very small pupil depressed sensitivity across the field
    • B.The rim of the trial lens blocked the peripheral stimuli✓ Answer
    • C.The patient stopped responding as fatigue set in mid-test
    • D.A drooping upper lid covered the pupil during the test

    A trial lens that is decentred or held too far from the eye casts the shadow of its rim on the peripheral test points, producing an absolute ring of loss with a normal centre. Keeping a full-aperture lens centred on the pupil and as close to the lashes as possible prevents it; a ptotic lid gives superior depression, fatigue gives a cloverleaf pattern, and a small pupil depresses the whole field.

    Source: Trial lens rim artefact in automated perimetry; IJCAHPO Core Criteria Handbook, visual field testing content areaReport a problem with this question

12. 12. A visual field returns a high false-positive rate. How does that distort the result?

    • A.Only the blind spot is affected, so the rest of the field stands
    • B.Sensitivities read abnormally high and real defects may be hidden✓ Answer
    • C.Fixation alone is measured, so the thresholds stay trustworthy
    • D.Sensitivities read abnormally low and mild defects look severe

    False positives are responses given when no stimulus was presented, so a trigger-happy patient is credited with seeing stimuli dimmer than they can truly detect. Thresholds are pushed upward, unrealistically bright islands can appear inside a damaged area, and the summary indices look better than the eye is, so the test should be repeated after reinstruction.

    Source: Reliability indices in automated perimetry; IJCAHPO Core Criteria Handbook, visual field testing content areaReport a problem with this question

13. 13. A patient viewing an Amsler grid says the lines near the centre look wavy and bent. What is being described, and how must the test be run?

    • A.Metamorphopsia; test one eye at a time with the near correction on✓ Answer
    • B.A ring scotoma; test one eye at a time without any correction on
    • C.A central scotoma; test both eyes together at distance correction
    • D.Metamorphopsia; test both eyes together after instilling dilating drops

    Wavy, bowed or bulging lines are metamorphopsia, distortion that arises when macular photoreceptors are displaced by fluid or membrane. The grid maps only the central field, so it is held at the patient's reading distance with the near correction, one eye at a time with the fellow eye covered, while the patient keeps fixation on the central spot and reports missing or distorted areas.

    Source: Amsler grid technique and metamorphopsia; IJCAHPO Core Criteria Handbook, visual field testing content areaReport a problem with this question

14. 14. Compared with immersion ultrasound, how does contact (applanation) A-scan biometry bias the measurement and the lens power calculated from it?

    • A.It shortens the axial length, so the calculated lens power comes out too strong✓ Answer
    • B.It leaves the axial length alone but flattens the keratometry readings
    • C.It lengthens the axial length, so the calculated power comes out too strong
    • D.It shortens the axial length, so the calculated lens power comes out too weak

    Resting the probe on the cornea indents it, so the sound path measured is shorter than the true axial length. In every lens power formula a shorter eye calls for a stronger lens, so an artificially short measurement yields an implant with too much power and a myopic result. Immersion technique keeps a fluid column between probe and cornea and avoids the compression altogether.

    Source: Contact versus immersion A-scan technique and axial length error; IJCAHPO Core Criteria Handbook, biometry content areaReport a problem with this question

15. 15. Axial length measures 23.4 mm in one eye and 24.6 mm in the fellow eye, and nothing in the refraction explains the difference. What should the assistant do?

    • A.Pass the readings on for the surgeon to adjust
    • B.Average the two lengths and use that one figure
    • C.Remeasure both eyes before the figures are used✓ Answer
    • D.Use the longer eye's length for both calculations

    Axial length is the measurement that contributes most of the error in a lens power calculation, and a fraction of a millimetre shifts the result by dioptres. A large difference between two eyes that the refraction does not support usually means the probe was off the visual axis, fixation was poor, or the cornea was compressed, so both eyes are remeasured until the readings repeat consistently.

    Source: Axial length error and interocular comparison in biometry; IJCAHPO Core Criteria Handbook, biometry content areaReport a problem with this question

16. 16. Before an A-scan the assistant selects the eye-type setting: phakic, aphakic, pseudophakic or silicone-filled. Why does this setting matter?

    • A.It sets the probe frequency used for that type of eye
    • B.It supplies the lens constant given by the manufacturer
    • C.Sound travels at a different speed in each of those eyes✓ Answer
    • D.It tells the machine which lens power formula it should apply

    Ultrasound biometry times an echo and converts that time into a distance using an assumed velocity, and sound moves at different speeds through a natural lens, aqueous and vitreous alone, an implant material, or silicone oil. Choosing the wrong eye type applies the wrong velocity and returns an axial length that is systematically wrong, no matter how clean the echoes look.

    Source: Sound velocity settings in A-scan biometry; IJCAHPO Core Criteria Handbook, biometry content areaReport a problem with this question

17. 17. Optical biometry cannot capture an axial length in an eye with a very dense cataract. What is the appropriate next step?

    • A.Press harder with the probe to read through it
    • B.Use the fellow eye's length in the calculation
    • C.Dilate and repeat, since pupil size is the limit
    • D.Measure that eye with immersion ultrasound✓ Answer

    Optical biometry works by sending light to the retina and detecting its return, so an opacity dense enough to block that light returns no reading. Ultrasound uses sound, which passes through the opacity, and the immersion technique is preferred because it measures without compressing the cornea; the fellow eye can never substitute for the eye being operated on.

    Source: Optical versus ultrasound biometry and their limitations; IJCAHPO Core Criteria Handbook, biometry content areaReport a problem with this question

18. 18. Why is a corneal thickness measurement obtained along with the applanation pressure, and where does it fall in the sequence?

    • A.Thickness calibrates the tonometer, so it is measured before tonometry
    • B.Thickness gives a corrected value that replaces the pressure charted
    • C.Thickness biases the applanation reading, so it is measured after tonometry✓ Answer
    • D.It is taken first so that the anesthetic need only be instilled once

    Applanation assumes an average cornea, so knowing the actual thickness tells the physician in which direction the reading is likely to be biased and adds independent information about risk. Pachymetry itself touches the cornea and disturbs the tear film, so it follows tonometry; taking it first would degrade the very pressure measurement it is meant to inform.

    Source: Pachymetry and interpretation of applanation readings; IJCAHPO Core Criteria Handbook, supplemental testing content areaReport a problem with this question

19. 19. When screening colour vision with pseudoisochromatic plates, how is the test given and what does it mainly detect?

    • A.One eye at a time in good light; it screens red-green defects✓ Answer
    • B.Both eyes together in good light; it screens blue-yellow loss
    • C.One eye at a time in a dim room; it screens blue-yellow loss
    • D.Both eyes together in a dim room; it grades red-green severity

    The plates hide their figures in colours that lie along the red-green confusion axis, so that is the deficiency they reveal; blue-yellow and acquired defects need other plate sets or arrangement tests. Testing is monocular under good daylight-balanced lighting with the near correction, because an inherited defect is symmetric while an acquired one may affect one eye only and would be masked binocularly.

    Source: Pseudoisochromatic plate colour vision screening; IJCAHPO Core Criteria Handbook, supplemental testing content areaReport a problem with this question

20. 20. A patient reads the acuity chart well in the exam lane but cannot drive in bright sun. Which supplemental test documents this complaint best?

    • A.Colour plates screened one eye at a time with near correction
    • B.Best-corrected acuity remeasured with a bright light in view✓ Answer
    • C.Proptosis measured in both eyes from a fixed base setting
    • D.Best-corrected acuity remeasured through a pinhole aperture

    A dim examination lane hides the problem: light scattered by a cataract or a clouded posterior capsule reduces contrast only when a bright source is in the field of view. Glare testing repeats best-corrected acuity with that source present, so the drop in lines is documented objectively. A pinhole tests for refractive causes, and proptosis and colour testing answer entirely different questions.

    Source: Glare testing in cataract evaluation; IJCAHPO Core Criteria Handbook, supplemental testing content areaReport a problem with this question

Practice questions based on the IJCAHPO core certification content areas for the Certified Ophthalmic Assistant and on standard ophthalmic clinical practice references. COA, COT, and COMT are marks of the International Joint Commission on Allied Health Personnel in Ophthalmology; this site is not affiliated with or endorsed by IJCAHPO. Ophthalmic assistants work under the delegation of a supervising ophthalmologist and permitted tasks vary by state and by practice — confirm your own scope, and check the current exam requirements and content outline with IJCAHPO before testing. About IJCAHPO certification →