Amblyopia

The first step in the evidence-based practice process is the formulation of an answerable clinical question. This involves identifying terms that can be used in a search for information (evidence) that will help to answer the question. The acronym “PICO” is widely used as a framework to help clinicians identify those search terms. PICO stands for:

P: Patient, Population or Problem. This is a term that identifies the type of patient and/or the condition or problem of interest.

I: Intervention. This term describes the intervention or treatment of interest.

C: Comparison; indicating any state with which the intervention of interest is compared, e.g. a placebo.

O: Outcome; the outcome measure.

The PICO acronym works well for many clinical scenarios in which we are considering intervention or treatment of a condition. However, not all of these terms apply when we are considering scenarios in which we are interested in; diagnosis, prognosis or prevalence, for example.

For the clinical scenario described below, a number of different clinical questions are considered and derived. In each case, modifications of the standard PICO format are considered:

Your 6-year-old patient has never had his eyes tested before. He was referred from a school screening, but he says he has no problems seeing things. On examination, you find significant anisometropia, no strabismus and no pathology. You diagnose amblyopia. You prescribe refractive correction, and you mention that at the next appointment you may prescribe patching treatment. 
Duration

30 minutes

Level

Beginner

Authors

Elizabeth Murray

Audience

Optometrists
Students
Teachers

    We want to provide reliable information on the risk factors for amblyopia, and these are denoted here by ‘R’. This is a question about aetiology. We have a patient type and we know the problem (P). We have no intervention (I) or comparison (C, but we might in some cases be interested in comparing one risk factor with another).

    Note that we have a bit of an overlap between P and O because amblyopia is both a problem and an outcome in this case. In this aetiology question it is fine to think of the condition as being both a problem and an outcome (of the risk factors we are looking for).

    “What are the risk factors [R] for development of anisometropic amblyopia [P] in children [P]?”

    Not all of the PICO terms apply to a question on prevalence. You will need to specify the patient type (or population, or problem), but no intervention, comparison or outcome applies here. Instead, you will need to specify a ‘N’ (Number, or prevalence) term. Note that the terms in bold are terms related to the PICO, or modified PICO, acronym.

    “What is the prevalence [N] of amblyopia [P] in children [P]?”

    Alternatively, you may want to be more specific and look at anisometropic amblyopia in particular, by inserting this before “amblyopia” in the above question. If you want to ask about prevalence in the general population (since prevalence in children is likely to be relatively high, a proportion being successfully treated) you could exclude “in children”.

    In this particular case, you have diagnosed amblyopia. However, you might wonder about diagnostic accuracy of the method you have used, Unilateral amblyopia is generally diagnosed on the basis of an interocular acuity difference of at least 0.2 logMAR in the absence of pathology.Let's say that you would like to find out whether this is more or less effective than using a test of stereoacuity.

    Using a modified PICO format, we would include P (patient/problem), but there is no intervention, so we have no need for I. For diagnostic accuracy (represented by D), we want to compare (C) two techniques, stereoacuity testing (T) and the gold standard letter acuity criterion.

    Suggested question: What is the sensitivity and specificity of stereoacuity testing (T) compared (C) with letter acuity in the diagnosis (D) of childhood amblyopia (P)? (note: you may prefer to specify "anisometropic amblyopia")

    The question relating to prognosis does not fit the PICO acronym perfectly, because there is generally no specific ‘C’ (Comparison). However, we do need ‘P’ and we can specify ‘I’ since we have a particular intervention, of patching plus refractive correction. We can also specify ‘O’, the outcome of interest, which is visual acuity recovery in this case.

    Suggested question:
    We could ask “Does occlusion [I] treatment for anisometropic amblyopia [P] in children [P] restore normal visual acuity [O]?” but since this is very similar to an intervention question we may want to be more specific, with a question that relates to the extent to which visual acuity remains normal in the longer term following treatment of amblyopia. In this case, our question might be “Is the improvement in visual acuity [O] following occlusion [I] treatment for amblyopia in children [P] sustained over time [O]?”

    This is an intervention/treatment issue, so you could use all of the four PICO terms without modification.
    We need ‘P’ (patient/population/problem), ‘I’ (Intervention or treatment). We may specify a comparison (‘C’), such as patching plus refractive correction versus refractive correction only.
    Alternatively, we could specify no comparison, in which case a search based on these terms could result in reports of studies that have used comparisons (e.g. randomized controlled trials) and those that have observed a group of children undergoing patching (cohort studies without comparison).
    If we want to limit the search results, it would be wise to include a comparison term here. We should also specify ‘O’ (outcome) and in this case the outcome measure is likely to be visual acuity, since amblyopia is widely defined on the basis of a visual acuity reduction (unilaterally compared to the non-amblyopia eye, or bilaterally below normal levels) in the absence of pathology.

    “How effective [O] is patching [I] in the treatment of anisometropic amblyopia [P] in children [P]?”

    Note that “effective” masks the fact that we are interested in visual acuity in this case. In the search for information, we would want to specify that outcome measure, so the term ‘visual acuity’ would be specified. For the term “patching” we should use this and the term ‘occlusion’ in a search, since either term may be used by authors.

  • What are the risk factors [R] for development of anisometropic amblyopia [P] in children [P]?

    A good secondary source is the Cochrane Eyes and Vision database. To search sources of this kind, and for original research (primary sources) via Pubmed appropriate key words are needed, based on the question formulated in step one. Appropriate key words are: anisometropic amblyopia; children; risk factors. A Pubmed search using these terms at the time of writing yields over 200 results. Adding “development” reduces this number to around 50, but it may be necessary to further filter the results [link to information on how to do this in Pubmed, Medline, Google Scholar].