Response to Facchin et al.: Use of yoked prisms in patients with acquired brain injury: A retrospective analysis

We have read your ‘Letter to the Editor’ regarding our paper on the clinical application of yoked prisms in specific diagnostic groups of patients with ABI [1]. We would like to address the key concerns.

First, the title of the letter is misleading. Part and parcel of the ‘prism prescription’ is the embedded phenomenon of ‘prism adaptation’, as described fully in our paper [1] and reviewed in detail by one of us elsewhere [2].

Second, this was a clinically-based, retrospective analysis of our experiences with yoked prisms in three specific groups of patients: homonymous hemianopia/quadranopsia, unilateral spatial inattention/visual neglect and abnormal egocentric localization. As with any retrospective (vs. prospective) study, there are unique limitations, such as the present use of multiple, clinician-based evaluations. However, our goal was to share our clinical experiences, which were quite positive in nature, and likely reflect typical testing in the clinic setting as compared with the research environment. Hence, despite any potential limitations, we believe that valid, useful information was provided. Subjective impressions are what the clinician has to deal with when testing such/all patients, as least in part. Thus, while there may be other ways to evaluate yoked prism application and its efficacy [2], the one employed in our study was not invalid and, moreover, represents functioning in the ‘real world’.

Third, we did not mean to ignore the several randomized clinical trials in the neuropsychological/prism adaptation literature domains, which were well-executed and in fact support our positive clinical claims. Rather, several had relatively small samples sizes (e.g. < 40) and, hence, our suggestion.

Fourth, with respect to terminology, this has been a problem historically in this area, as the authors correctly state. While we too used the phrase ‘visual neglect’, as it has been in the literature for decades, we believe it is not accurate—it implies purpose or intent, which is clearly not the case. We believe that the term ‘unilateral spatial inattention’ is better, as it is inattention and not purposeful neglect per se that occurs.

Fifth, we did consider the process of prism adaptation in our Discussion, as well as detailed in some of our references [2]. However, in our study, diagnostic prism application/trial frame testing was first used to assess for the possible need therapeutically for yoked prisms with respect to ambulation, reading and other visuomotor activities. Thus, the prism direction and magnitudes were purposely varied initially for a minute or so each—hence, no/not much prism adaptation would be expected to occur in that short time span. Then, if a specific prism direction/magnitude were found to be positive by the patient, those specific yoked prisms would be placed in a trial frame for another few minutes, with further spatial and visuomotor exploration. Lastly, only once tried again at a separate session would they then be prescribed therapeutically, with prism adaptation occurring afterwards at their home/work environment. We do not understand how ‘spontaneous recovery’ could occur in such a short diagnostic test session.

Sixth, there appears to be a basic misunderstanding of how yoked prisms function—one does not gain visual field, but rather shifts it, based on physics, namely Snell’s Law of optical prismatic deviation [3]. Thus, the spectacle frame has no bearing on the central process/mechanism. Related to this, the authors seem to confuse the yoked prism’s initial effect from the subsequent prism adaptive phenomenon, and this appears to be a basis for some of the disagreement.

Seventh, and lastly, we have proposed here and elsewhere [2] that the term visual ‘midline shift’ is misleading. It is not a shift per se (e.g. lateral with respect to the body midline), but rather a polar-based, rotational deviation from the normal, midline-based reference point, thus reflecting one’s three-dimensional, egocentrically-based, directional map of visual space. Thus, we prefer the more accurate and descriptive term ‘abnormal egocentric localization’. See Ciuffreda and Ludlam [2] for further details and a proposed mechanism for both the initial effect and the subsequent prism adaptation.

In summary, while all investigations may have shortcomings, we believe that our present retrospective study contributes to the clinical literature using valid global approaches and related methodologies.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.