11.2 Core Workflows and Decision Points

11.2 Core Workflows and Decision Points

Each supplemental test has the same skeleton: confirm the indication, identify the patient and eye, prepare and calibrate the device, acquire the image or measurement with correct technique, run a quality check, and hand defensible data to the physician. The COA exam tests the points where assistants commonly go wrong.

A-scan biometry (axial length)

A-scan ultrasonography measures axial length, the front-to-back length of the eye, which feeds the IOL power formula. Two methods exist:

• Contact (applanation) A-scan touches the cornea after topical anesthetic; excessive pressure indents the cornea and falsely shortens axial length, yielding a too-high IOL power and a hyperopic surprise. Use the lightest touch that still gives clean echo spikes.
• Immersion A-scan floats the probe in a fluid-filled shell, avoiding compression and giving more reproducible results. Optical biometry (IOLMaster, Lenstar) is non-contact and now preferred when the media are clear.

A valid A-scan shows tall, steeply rising echo spikes from the cornea, anterior and posterior lens, and retina, each perpendicular to the baseline and of expected height. Velocity setting must match the eye type (phakic, aphakic, pseudophakic, or silicone-oil filled) or the calculated length is wrong. A measurement that drifts more than a few hundredths of a millimeter between readings, or an axial length that disagrees sharply between the two eyes without a clinical reason, should be repeated rather than averaged blindly, since a single bad scan can leave a patient several diopters off target after surgery.

B-scan ultrasonography

B-scan produces a two-dimensional cross-section of the posterior segment and orbit. It is the go-to test when the doctor cannot see the retina: dense cataract, vitreous hemorrhage, or hyphema. It detects retinal detachment, tumors, and intraocular foreign bodies. The probe is applied over the closed lid or directly on the globe with coupling gel; never apply pressure to an eye with a suspected open globe or ruptured wall.

Corneal topography and pachymetry

Corneal topography maps surface curvature as a color map (warm colors = steep, cool = flat). It screens for keratoconus (inferior steepening, skewed axes), fits specialty contact lenses, plans toric and refractive procedures, and clears refractive-surgery candidates. Have the patient blink just before capture so a smooth tear film covers the cornea, then center on the fixation target; a dry spot or a blink line creates a false irregularity that can be mistaken for disease.

Pachymetry measures corneal thickness in microns by ultrasound or optical means; a value under about 500 microns plus topographic steepening raises a red flag for surgery and also explains why an applanation tonometry reading is artificially low in that eye.

Workflow decision table

Specular microscopy and OCT acquisition

Specular microscopy photographs the corneal endothelium, the single layer of pump cells on the back of the cornea, and reports cell density in cells per square millimeter plus morphology (pleomorphism, polymegethism). Normal density is 2000-3000; the count falls with age, surgery, and dystrophies such as Fuchs. The patient fixates on a target while the camera captures a small region; blinks and poor fixation blur the image. The physician uses a low count to predict whether the cornea will tolerate intraocular surgery.

Optical coherence tomography (OCT) uses light interferometry to produce micron-resolution cross-sections of the retina, optic nerve head, and anterior segment. It is non-contact and quick, but it needs a clear optical path and a steady, well-lubricated eye. Every OCT prints a signal strength or quality score; a low score from dry eye, a blink, decentration, or a dense cataract distorts thickness values and must trigger a rescan, not interpretation.

Automated perimetry workflow

For visual fields, seat the patient comfortably, correct for near with the proper trial lens, occlude the non-tested eye, explain the task, and monitor fixation throughout. The machine tracks fixation losses, false positives, and false negatives as reliability indices. Coach a tiring or trigger-happy patient and pause if needed, because a field is only as good as its reliability numbers.

Universal controls

Before any scan: verify patient identity and the correct eye, calibrate per the manufacturer schedule, disinfect contact probes (B-scan, contact A-scan, applanation tonometer tips) or use single-use covers, and document the test ordered. Apply topical anesthetic only for contact methods and only when appropriate. During acquisition, center the device, ask the patient to blink immediately before capture to smooth the tear film, and confirm the echo spikes, color map, or scan look clean in real time.

After acquisition, reject and repeat scans with motion blur, decentration, corneal compression, or low OCT signal strength rather than passing flawed data to the physician. Label every result with the correct eye, date, and device so the surgeon acts on the right number.