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Editors Selection IGR 14-2

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Comment by Kuldev Singh on:

20979 Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma, Berdahl JP; Allingham RR; Johnson DH, Ophthalmology, 2008; 115: 763-768

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The authors are to be congratulated on a novel and thoughtful study. This retrospective case-control comparison demonstrated significantly lower CSF pressure in OAG patients compared to non-glaucoma controls. Despite best efforts, the study is limited by the retrospective design, which makes it vulnerable to selection bias and confounding variables.
Specific limitations and areas that can be addressed further are listed below:

1. The number of POAG and control patients included in the analysis represented only a small proportion of the total number. In the glaucoma's, 88/116 were excluded from the analysis and 507 of the 556 controls were excluded. The possibility of introducing selection bias in such a highly selected cohort is large and it would have been interesting to include a comparison of CSF pressures in all glaucoma and control patients where readings were available.
2. All patients in this study had a neurological complaint that warranted a lumbar puncture. Despite the best efforts of the authors to exclude individuals with CNS disease known to alter CSF pressure there remains the possibility that confounding variables that contributed toward the need for an LP account for the ob-served difference in CSF pressures. There was no major difference in the indications for LP between the two groups, but the actual diagnoses were in many cases not detailed.
3. Most glaucoma patients were taking topical medications, whereas the controls were not. Although it is unlikely that this had a significant impact on CSF pressure, this possibility cannot be excluded. Efforts were taken to reduce the potential impact of glaucoma medication, by excluding patients on systemic CAIs. In addition, it is reported that there was no significant difference in CSF pressure between groups taking different classes of medication.
4. Although a negative correlation between CDR and CSF pres-sure was observed, there was no correlation between CSF pressure and disease stage when determined by visual field score. The authors suggest that this may be related to a lack of study power to demonstrate a difference, which is some-what surprising as field scores were divided into only four categories whereas CDR presumably was divided into ten. Is data available to determine an association between disc size and CSF pressure?
5. It is not clear what the temporal relationship was between the time when IOP and disease stage was measured and the LP performed. Although the closest time was used, what were the intervals?

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