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. Consult the original papers for protocol specifics before citing them.

1. Holmes JM, Beck RW, Repka MX, et al.; Pediatric Eye Disease Investigator Group. The amblyopia treatment study visual acuity testing protocol. Arch Ophthalmol. 2001;119(9):1345–1353. doi:10.1001/archopht.119.9.1345
2. Moke PS, Turpin AH, Beck RW, et al. Computerized method of visual acuity testing: adaptation of the amblyopia treatment study visual acuity testing protocol. Am J Ophthalmol. 2001;132(6):903–909. doi:10.1016/s0002-9394(01)01256-9
3. Cotter SA, Chu RH, Chandler DL, et al. Reliability of the electronic early treatment diabetic retinopathy study testing protocol in children 7 to <13 years old. Am J Ophthalmol. 2003;136(4):655–661. doi:10.1016/s0002-9394(03)00388-x
4. Rice ML, Leske DA, Holmes JM. Comparison of the amblyopia treatment study HOTV and electronic-early treatment of diabetic retinopathy study visual acuity protocols in children aged 5 to 12 years. Am J Ophthalmol. 2004;137(2):278–282. doi:10.1016/j.ajo.2003.08.016
5. Drover JR, Felius J, Cheng CS, et al. Normative pediatric visual acuity using single surrounded HOTV optotypes on the Electronic Visual Acuity Tester following the Amblyopia Treatment Study protocol. J AAPOS. 2008;12(2):145–149. doi:10.1016/j.jaapos.2007.08.014
6. Birch EE, Strauber SF, Beck RW, Holmes JM; Pediatric Eye Disease Investigator Group. Comparison of the amblyopia treatment study HOTV and the electronic-early treatment of diabetic retinopathy study visual acuity protocols in amblyopic children aged 5 to 11 years. J AAPOS. 2009;13(1):75–78. doi:10.1016/j.jaapos.2008.07.007