The ATS-HOTV protocol: crowded HOTV acuity testing in children

The ATS-HOTV protocol is the standardized visual acuity test developed for the Amblyopia Treatment Studies run by the Pediatric Eye Disease Investigator Group (PEDIG). It presents a single HOTV optotype surrounded by crowding bars, sized in logMAR steps, and is designed for children roughly 3 to 7 years old — the age group in which amblyopia treatment decisions are made and in which full-chart reading is often not testable. This page explains the protocol educationally; it is not a claim that any particular product implements it.

Last updated: August 20, 2026 · Written for clinicians and technicians; sources are the published PEDIG protocol literature.

Why HOTV, and why only four letters

HOTV uses exactly four optotypes — H, O, T and V — chosen because they are left-right symmetric (no reversals to confuse a young child) and easy to match. A child who cannot or will not name letters can point to the match on a lap card held in front of them, which turns a naming task into a matching task and makes three- and four-year-olds testable who would fail a Snellen line outright. Our pediatric optotypes guide covers how HOTV compares with Tumbling E, Landolt C and picture optotypes.

Why a single letter with crowding bars

This is the clinically interesting part. Amblyopic eyes show pronounced crowding: acuity measured with isolated single letters can look substantially better than acuity measured with letters in a row, because surrounding contours degrade the amblyopic visual system's performance. A single uncrowded letter therefore overestimates an amblyope's functional vision and can mask the very deficit being measured.

The ATS answer is to present one letter at a time — keeping the attention demands low enough for a preschooler — but to surround it with crowding bars at a defined spacing, restoring the crowded condition a full chart would create. One letter for the child's sake; bars for the disease's sake.

How the protocol runs

• Single surrounded optotypes, presented at logMAR intervals, tested one eye at a time.
• A defined presentation sequence that screens down to threshold and then retests around it, so the result is a threshold estimate rather than a "lowest line attempted".
• Matching-card response permitted, which extends testability to children who cannot name letters.
• Electronic presentation. The studies used a calibrated electronic tester so letter size and randomization were controlled by software rather than by which printed chart happened to be on the wall.

ATS-HOTV vs E-ETDRS: which children, which test

PEDIG paired the ATS-HOTV protocol with the E-ETDRS protocol (electronic ETDRS) for older children and adults. Published comparisons of the two protocols in children aged roughly 5 to 12 — including in amblyopic children — examined how the tests agree across the overlap ages. The practical division that emerged: ATS-HOTV for approximately 3 to 7 years, E-ETDRS from about 7 up. For the underlying chart logic — five letters per row, equal spacing, per-letter logMAR scoring — see Snellen vs. ETDRS.

What this means outside a clinical trial

Most practices do not run PEDIG protocols by name. The transferable lessons are simpler and apply to any pediatric lane:

• Never assess amblyopia with isolated, uncrowded letters. Use crowding bars or line presentation, or the deficit will be understated.
• Match the optotype to the child: HOTV with a matching card reaches younger children than naming tasks do.
• Control letter size properly. Threshold estimates mean nothing if optotype size is not calibrated to the actual testing distance — see the setup guide.
• Interocular difference is the signal in amblyopia follow-up, which makes consistent method between visits matter more than any single reading.

Sources

This guide summarizes the PEDIG protocol literature, including the published comparisons of the ATS-HOTV and E-ETDRS visual acuity protocols in children aged 5 to 12 (American Journal of Ophthalmology, 2004) and in amblyopic children aged 5 to 11 (Journal of AAPOS, 2008), and the normative pediatric acuity data collected with single surrounded HOTV optotypes under the ATS protocol. Consult the original papers for protocol specifics before citing them.