Teaching case. A two-year-old, constant left esotropia, no alternation. Refraction after cycloplegia OU +2.00 D. Clear media. Fixation preference is for the right eye; the left eye "holds until a blink" and then goes back. The family wants "surgery straight away, while he's little". Surgery without a conversation about amblyopia is a common route to a nice angle and the same suppression.
Strabismic amblyopia arises with constant tropias that do not alternate or alternate unequally, typically with esodeviations. PPP: the mechanism is competitive or inhibitory interaction between neurons processing non-fusable inputs; dominance of the cortical centres of the fixating eye and chronically reduced responsiveness to the non-fixating eye.
The key words here are constant and non-alternating. Intermittent exotropia with good control produces amblyopia less often than constant esotropia: part of the time both channels still get in. This is not permission to "not check acuity in exotropia". It is an explanation of why esotropia takes a central place in the PPP statistics: a third of infants aged 9–14 months with esotropia and more than a quarter of preschool children with esotropia have amblyopia; the odds of amblyopia with strabismus are 2.7–18 times higher (observational data, PPP summary).
Why alternation protects. If the child switches the fixating eye spontaneously, both channels periodically win the competition. Unequal alternation is already a risk: one eye is "in charge" most of the day. Assessment of fixation in a preverbal child (holds through blink, induced tropia test) is not a substitute for acuity, but it is a working surrogate while recognition acuity is not possible. The PPP states honestly: fixation preference tests cannot be considered highly accurate screening on their own; in the clinic, together with other findings, they help decide whether there is amblyopia severe enough to treat.
Optics is not an ornament here either. Even with a residual tropia in glasses, the amblyopic eye can gain substantially with correction alone (the PPP cites the relevant study). The Cochrane review by Taylor and Elliott 2014 (CD006461.pub4) on strabismic amblyopia: occlusion with the necessary refractive correction is more effective than glasses alone; mean difference −0.18 logMAR (95% CI −0.32 to −0.04) comparing part-time occlusion plus glasses with glasses alone. That is, glasses are the base; occlusion is an added effect in this form, not a replacement for the base.
The PPP calls the optimal timing of strabismus surgery in pre-existing strabismic amblyopia unknown. This is an important honest gap. The course does not fill it with someone else's schedule of "operate at 11 months" or "only after 1.0 acuity". The working logic that can be defended by the text of the PPP: support the acuity of the worse eye, correct the refractive error, do not postpone surgery indefinitely because of the myth of "perfect acuity first" — and do not operate instead of treating amblyopia.
Boundary. The course does not teach you to choose between bilateral recession and resection. It teaches you not to miss amblyopia behind the cosmetics of the angle and not to regard surgery as a method of treating acuity.