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A-scan ultrasound biometry is a foundational preoperative measurement for cataract surgery and the primary method for determining the power of the intraocular lens (IOL) to be implanted. As an ophthalmic assistant, performing accurate A-scan measurements is one of the highest-stakes technical skills in the clinical workflow — an error of just 1mm in axial length will result in approximately a 2.50D postoperative refractive surprise for the patient.
The "A" in A-scan stands for amplitude modulation — the instrument displays ultrasound echoes as vertical spikes on a time base, with each spike representing a tissue interface within the eye. By measuring the transit time of sound pulses between these interfaces and applying known sound velocities for different tissues, the instrument calculates distances: the depths of the anterior chamber, lens, and vitreous, which together add up to the total axial length.
This guide covers the A-scan procedure in full: contact vs. immersion technique, sound velocities, normal axial length values, the IOL calculation formulas you need to know for the COA exam, and your role in the cataract surgery preoperative workflow.
A valid A-scan waveform must display specific spikes at predictable anatomical locations. Learning to identify a good-quality scan is essential — accepting a poor-quality scan will produce inaccurate IOL power.
| Spike Number | Anatomical Interface | Expected Height | Notes |
|---|---|---|---|
| Spike 1 | Initial corneal spike | 100% (reference) | Must be tall and steep |
| Spike 2 | Anterior lens capsule | High (>75%) | Should be perpendicular to beam |
| Spike 3 | Posterior lens capsule | High (>75%) | Should be tall and steep like spike 2 |
| Spike 4 | Retinal/scleral spike | Very high (100%) | Must be steep; low spike indicates off-axis scan |
| Spike 5 | Orbital fat (behind eye) | Low, multiple | Confirms scan passed through entire eye |
Quality Control: Accepting vs. Rejecting Scans
A valid scan requires: (1) All major spikes are perpendicular and tall, (2) Retinal spike reaches >95% height, (3) Orbital fat spikes present, (4) Scan reproducibility — take 5–10 measurements and reject outliers. The standard deviation among readings should be <0.10mm. Most instruments automatically flag outlier measurements.
Contact A-Scan
Method: Probe tip touches cornea directly
Accuracy: Lower — corneal indentation shortens AL by 0.1–0.3mm
Speed: Faster setup, commonly used in busy practices
Anesthesia: Topical anesthetic drops required
Error risk: Operator-dependent; excessive pressure = shorter AL = higher IOL power
Use: Screening, dense cataracts where optical biometry fails
Immersion A-Scan (Gold Standard)
Method: Fluid-filled scleral shell between probe and cornea
Accuracy: Higher — no corneal compression artifact
Speed: Slower setup; patient must be supine
Anesthesia: Topical drops recommended for comfort
Error risk: Reduced; probe is not in contact with cornea
Use: Pre-op IOL calculation, research, verification of contact scans
Instill topical anesthetic (proparacaine or tetracaine) in both eyes.
Position the patient supine on the exam chair or table. Confirm they can fixate on the ceiling.
Insert the scleral shell (Prager shell or similar) gently under the eyelids so it rests on the sclera, not the cornea.
Fill the shell with sterile saline or methylcellulose coupling fluid. Ensure no air bubbles are present.
Lower the A-scan probe into the fluid in the shell — the probe should not touch the cornea.
Align the probe to the visual axis (patient fixates on a ceiling target or the probe's internal fixation light). Adjust position until all spikes are perpendicular and tall.
Acquire 5–10 measurements. Review for consistency (SD <0.10mm). Accept the scan set and confirm the average axial length value.
Repeat for the fellow eye. Remove shell, rinse eye with saline.
| Tissue | Velocity (m/s) |
|---|---|
| Aqueous humor | 1532 |
| Crystalline lens | 1641 |
| Vitreous humor | 1532 |
| Phakic average | 1550 |
| Aphakic eye | 1532 |
| Pseudophakic (PMMA IOL) | 2718 |
| Silicone oil (vitreous) | 987 |
| Category | AL (mm) | Refraction |
|---|---|---|
| Short / Hyperopic | < 22.0 | High hyperopia |
| Emmetropic | 22.0–24.5 | Near plano |
| Average normal | ~23.5 | Emmetropia |
| Mildly myopic | 24.5–26.0 | Low–moderate myopia |
| Highly myopic | > 26.0 | High myopia (>-6.00D) |
1mm Rule — Critical for Exam
A 1mm error in axial length produces approximately a 2.50D error in IOL power. For a long eye (axial myope), a 1mm error produces a smaller IOL error (~1.75D) because the formula extrapolation is more linear. For very short eyes (<22mm), the same 1mm error produces a larger IOL error (~3.50D+). This is why immersion technique and optical biometry are critical for accuracy.
IOL power formulas use axial length (AL), average keratometry (K), and anterior chamber depth (ACD) to calculate the lens power needed to achieve a target postoperative refraction (usually plano or a slight myopic target).
| Formula | Generation | Best For | Key Variables |
|---|---|---|---|
| SRK/T | 3rd gen | Normal AL (22–26mm) | AL, K, A-constant |
| Holladay 1 | 3rd gen | Normal–long eyes | AL, K, Surgeon Factor |
| Haigis | 3rd gen | All AL; good for short eyes | AL, K, ACD, a0/a1/a2 constants |
| Barrett Universal II | 4th gen | All AL — current gold standard | AL, K, ACD, LT, WTW |
| Holladay 2 | 4th gen | Complex cases | 7 variables including lens thickness |
A-Constant
Each IOL model has an A-constant (or equivalent: Surgeon Factor, pACD) that accounts for the specific optical design and placement characteristics of that lens. A higher A-constant = more anterior IOL position = requires lower IOL power. A-constants are provided by manufacturers and fine-tuned by surgeons based on postoperative outcomes. The COA technician enters the correct A-constant for the planned IOL into the biometry software.
Pre-Operative
Day of Surgery
Post-Operative
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What is A-scan biometry and why is it performed?
A-scan biometry (A-scan ultrasonography) measures the axial length of the eye — the distance from the front of the cornea to the retina — using high-frequency ultrasound. It is the most critical measurement for intraocular lens (IOL) power calculation before cataract surgery. An error of just 1mm in axial length measurement translates to approximately 2.50D of IOL power error, highlighting how important accurate technique is.
What is the normal axial length of the human eye?
The average adult axial length is approximately 23.5mm (range: 22–25mm for emmetropes). Myopic eyes are longer than average (axial length >24mm), while hyperopic eyes are shorter (<22mm). High myopes may have axial lengths exceeding 26mm. The axial length is the single most important determinant of a person's refractive error.
What is the difference between contact and immersion A-scan?
In contact A-scan, the probe tip is placed directly on the cornea, which can cause indentation and artificially shorten the axial length measurement. Immersion A-scan uses a fluid-filled shell (scleral shell) between the probe and the cornea, eliminating corneal compression. Immersion technique is considered the gold standard for accuracy and typically measures 0.1–0.3mm longer than contact A-scan on the same eye. Most modern optical biometers (IOLMaster, Lenstar) have largely replaced immersion A-scan in cataract pre-op workups.
What IOL calculation formulas are used in clinical practice?
Common IOL power formulas include: SRK/T (most widely used for normal axial lengths 22–26mm), Haigis (incorporates anterior chamber depth for better accuracy across all axial lengths), Holladay 1 (good for normal eyes), Holladay 2 (adds more variables for complex cases), and Barrett Universal II (current benchmark for accuracy across all axial lengths including extremes). For very long or very short eyes, specialized formulas like Barrett or Olsen are preferred.
What sound velocities are used in A-scan biometry?
Ultrasound travels at different speeds through different ocular structures. Key velocities: Aqueous/vitreous humor: 1532 m/s, Crystalline lens: 1641 m/s, Silicone oil (vitreous substitute): 987 m/s (dramatically slower — requires special setting), Phakic (normal lens) average: 1550 m/s, Aphakic (no lens): 1532 m/s, Pseudophakic (IOL in place): varies by lens type (~1474–1532 m/s). Using the wrong velocity setting is a significant source of measurement error.
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