10.3 Scenario Practice for Diagnostic Ultrasound
Key Takeaways
- Sudden floaters and flashes with a painless 'curtain' over vision suggest retinal detachment, evaluated with B-scan when the view is obscured.
- An after-coming, freely mobile membrane on dynamic B-scan suggests a posterior vitreous detachment, not a fixed retinal detachment.
- Posterior vitreous hemorrhage from diabetic retinopathy is a classic indication for B-scan to rule out underlying detachment.
- Read each scenario for the indication, then match the correct ultrasound mode, technique, and safety step.
10.3 Scenario Practice for Diagnostic Ultrasound
Scenario questions combine ultrasound knowledge with clinical reasoning. Use a four-step read: identify the clinical question, pick the correct mode, choose the technique, and predict the finding.
Scenario 1: opaque media, suspected detachment
A patient reports a week of new floaters and flashing lights, now with a dark curtain descending over the temporal field. The ophthalmologist cannot see the posterior pole because of a dense vitreous hemorrhage. The COA prepares a B-scan, not an A-scan, because the question is "what does the retina look like behind the blood?" On B-scan a rhegmatogenous retinal detachment appears as a bright, folded membrane tethered to the optic disc; in a funnel configuration both leaves meet at the disc forming a characteristic shape. The next step is urgent referral for surgical repair.
Scenario 2: telling detachment from PVD
The trace shows a thin membrane in the vitreous cavity. To decide whether it is a fixed retinal detachment or a mobile posterior vitreous detachment (PVD), the examiner performs dynamic scanning: the patient moves the eye and the examiner watches for aftermovement.
| Finding | Retinal detachment | Posterior vitreous detachment |
|---|---|---|
| Attachment | Inserts at the optic disc | Not anchored to the disc |
| Mobility on dynamic scan | Stiff, restricted, little aftermovement | Freely mobile, undulating, marked aftermovement |
| Reflectivity on A-scan | Tall, ~100% spike | Lower, variable spike |
The freely mobile, undulating membrane with marked aftermovement points to a PVD; the stiff, disc-tethered membrane points to a true detachment.
Scenario 3: biometry before cataract surgery
A patient with a dense brunescent cataract is scheduled for surgery. The COA must obtain accurate axial length. Because precision drives IOL power, the assistant selects immersion A-scan to avoid corneal compression, confirms the phakic velocity (1555 m/s average), instills topical anesthetic, and accepts only traces with a tall steep retinal spike. If the axial length comes back unexpectedly short, the assistant suspects probe compression or off-axis alignment and repeats the scan rather than trusting one bad reading.
Scenario 4: intraocular foreign body
A metal worker reports a high-velocity injury. Suspected intraocular foreign body is a classic B-scan indication; metallic objects produce a very high-reflectivity echo with shadowing or reverberation artifact behind them. Note one absolute safety rule: ultrasound is contraindicated on an eye with a suspected open globe (ruptured) injury until the globe is protected, because probe pressure could extrude intraocular contents. When trauma is involved, the assistant confirms with the physician that the globe is intact before any contact technique, and otherwise defers to imaging that requires no pressure.
Scenario 5: dense cataract before surgery, no view of the back
A patient has a white, mature cataract; the surgeon cannot see the retina at all and wants reassurance the posterior segment is healthy before committing to surgery. Here the assistant prepares both studies: an A-scan to obtain the axial length for the IOL, and a screening B-scan to rule out a hidden detachment, tumor, or staphyloma. This pairing is common and the exam may present it as a single order for "ultrasound" that actually requires two modes for two different questions.
Quick scenario-to-action map
| Stem clue | Mode | Key action |
|---|---|---|
| Need IOL power | A-scan | Immersion, confirm velocity, repeat readings |
| Curtain over vision, no view | B-scan | Image retina, look for disc-tethered membrane |
| Mobile vs fixed membrane | B-scan dynamic | Watch aftermovement |
| Trauma, possible rupture | Defer contact | No pressure until globe confirmed intact |
| Pigmented fundus mass | A-scan + B-scan | Assess reflectivity and size |
How this appears on the exam
The stem gives a symptom or surgical need; you match the mode (B-scan for imaging pathology, A-scan for length), the technique (immersion for precision), and the safety constraint (no pressure on a possible open globe). Pick the answer that fits the specific clinical question described in the stem, not the one that merely names a familiar device or sounds technically impressive.
Scenario 6: the unexpected biometry result
A patient's right-eye axial length reads 22.8 mm and the left reads 26.4 mm, yet the patient has no history of high myopia or prior surgery and the eyes look symmetric. A 3.6 mm difference is a red flag. The competent assistant does not simply report both numbers; she suspects an artifact — probe compression on the short eye, off-axis aim, a missed staphyloma, or a wrong velocity setting — and repeats the measurement, ideally by immersion. The teaching point is that ultrasound data are interpreted in context: an implausible value is treated as suspect until reproduced.
Reporting a 3.6 mm asymmetry without rechecking could lead the surgeon to implant a badly miscalculated lens.
Practicing the read
For every scenario, narrate four things aloud: (1) the clinical question, (2) the mode that answers it, (3) the technique and velocity that make it accurate, and (4) the expected finding or the next safe step. Most COA ultrasound items can be solved by that sequence. The wrong answers usually fail step 1 — they answer a different question than the stem asks — or step 3, by offering a faster technique that sacrifices accuracy or violates a safety rule such as pressing on a possible ruptured globe.
A patient with a dense vitreous hemorrhage from proliferative diabetic retinopathy needs assessment of the retina, which cannot be seen. Which study is most appropriate and why?
On dynamic B-scan a thin vitreous-cavity membrane shows marked aftermovement, undulates freely, and does not insert at the optic disc. This pattern most strongly suggests: