In order to understand the clinical significance of glaucoma-related quality of life (QoL) studies, the term visual ability (VA) must be conceptualized. In fact, VA is a rather subjective term that is supposed to encompass the overall perception of the environment from the visual system. However, it is well-known that the way we perceive and interact with our environment is still not fully understood; furthermore, our visual system interacts with the rest of the systems in a highly individualized manner,1 making difficult to extrapolate research data in larger populations. In simple words, the impact of a vision-threatening disease like glaucoma on VA is highly variable in a multifactorial way.
Because of these facts, in clinical settings, VA is rarely used as an index of the severity of the glaucoma or of the efficacy of the therapy. On the other hand, easily measurable indexes like the mean deviation (MD), the visual function index (VFI) or the retinal nerve fibre layer (RNFL) are considered the ‘gold standards’ for the evaluation both of the current and potentially of the future state of the glaucoma patient.2 Unfortunately, despite the fact that these prevalent indexes quantify certain aspects of the functional status of the patient, they provide only indirect and often misleading information regarding the overall VA for two primary reasons: (i) the majority of them (i.e. MD, VFI) derive from clinical tests that simulate poorly the actual environment; and (ii) the correlation of objective indexes like the RNFL with VA is yet to be fully explored.3
Quality of life questionnaires attempt to construct the variable VA by introducing four major, a priori, assumptions that determine the overall interaction of the patient with the environment. These assumptions are: (i) each glaucoma patient has a certain capacity of VA; (ii) certain activities require certain capacities of VA; (iii) two different glaucoma patients with the same capacity of VA should, ideally, be able to perform the same range of activities; and (iv) two different glaucoma patients with different capacities of visual abilities should, ideally, differ in the range of activities that are able to perform. Thus, if a glaucoma patient is able to perform the whole range of interactions with the environment, then he/she should have a perfect VA score. On the other hand, when the glaucoma patient cannot interact with the environment then the VA score should be zero. Between these extreme values and closer to zero, is the visual disability or impairment (VI) score.
Further to the assessment of the VA, QoL instruments provide valuable information by evaluating limitations of specific activities of daily living (ADL) of the glaucoma patient. These limitations are quantified by the subscale scores and demonstrate those areas of human activities that glaucoma primarily exerts its negative impact. For example, QoL studies suggest that the outdoor mobility and the driving ability is among the most affected activities in glaucoma patients.3
In accordance to the above, the clinical value of the QoL studies in glaucoma is the quantification of the variable VA and of the ADLs in the daily clinical praxis, for a series of reasons: (i) VA reflects the overall functional visual capability of the patient and not certain aspects of it; (ii) ADL scores are directly associated with specific activities of the patient; (iii) ADLs directly quantify specific limitations of the patient; and (iv) they allow the identification of those ADLs that are more important to the patient according to his/her needs/wants. Thus, further to the traditional management of the glaucoma patient, which is primarily based on the performance of the prevalent clinical indexes like the MD, the VFI and the RNFL, VA and ADLs indexes attempt to introduce a contemporary, individualized patient management methodology that is supposed to be more compatible with the patient's specific needs. In simple words, the ‘peripheral vision’ subscale score might provide more information regarding the overall efficacy of the therapy in a young glaucoma patient than the MD index in a specific time-frame within the disease continuum. In fact, QoL studies outcomes are considered so important that in certain countries they actually determine the reimbursement policies of the insurance funds.4
It becomes obvious that the performance of the QoL instruments depends heavily on three fundamental properties that they should have: (i) the capability to explore the whole spectrum of interactions of the glaucoma patient with the environment, efficiently; (ii) the capability to correlate, even, minor difficulties in the interaction with the environment with VA; and (iii) the capability to identify non-glaucoma-related reasons that interfere with the glaucoma patient's activities. Aforementioned capabilities of the QoL instruments are evaluated during their psychometric validation.5
Summarizing, QoL studies introduce a contemporary framework for the assessment of the impact of glaucoma with high clinical significance.