Lateral Sleep Position and Asymmetry in Glaucoma

I read with interest the article by Kaplowitz et al.¹ “Time spent in lateral sleep position and asymmetry in glaucoma.” With an aim to explore sleep position in asymmetric POAG, they examined sleep laboratory videos of 54 POAG patients for lateral sleep, continuously recorded sleep position of 29 asymmetric low-pressure glaucoma (LPG) patients at home for 2 nights by using a portable device, and assessed self-reported sleep position in 178 POAG (63 LPG and 115 high-pressure glaucoma [HPG]) patients in a survey. This study demonstrated that (1) patients spent 19% more time sleeping on one side than on the other, (2) right-sided sleep was preferred and correlated with a lower visual field index (VFI) on the left, and (3) LPG patients had a greater degree of self-reported sleep position asymmetry than HPG patients, and right-sided sleepers with POAG had a lower right RNFL thickness and a lower RNFL symmetry score. I appreciate the authors' time and efforts for their extensive work on the sleep posture of glaucoma patients. I have some suggestions and questions to the authors' work: 

1.  
   Video sleep laboratory analysis: the authors assessed the time spent for each of the cardinal body postures (supine, lateral decubitus, and prone). I recommend that the authors replay the videos of the participants and also assess the time spent for different head positions: (1) the side faced by the patient's head (right, left, up, or down) and (2) on versus off the pillow. Previously, my research team studied the effect of head position on IOP in lateral sleep position,^2,3 and also the effect on IOP of head rotation in prone position.⁴ We found that low-head position supported by no or low pillow further elevates the IOP of the lower-sided eye in the lateral decubitus posture,^2,3 and that head rotation in the prone posture raises IOP in the lower-sided eye compared with the IOP measured in the lateral or supine body position.⁴ I also suggest that the habit (or duration) of burying one's eye into the pillow should also be sought. Kim et al.⁵ reported that posture-induced IOP rise may become greater with eyeball compression by a pillow. Because there is few publication on how the nocturnal head position of glaucoma patients affects IOP in their real sleeping hours, reassessing the video-recorded data for head position may provide some insight on the potential impact of sleep posture on IOP or glaucoma damage.
2.  
   Continuous sleep position monitoring and expanded sleep position survey: the portable monitor captured the position of the body continuously and revealed that the time spent in lateral sleep position was correlated with lower VFI on the opposite side. This finding disagrees with those of the previous studies where the predominant sleep position was determined by a questionnaire.^6,7 Although further studies with a larger number of patients should be conducted to verify this issue, some potential confounders may have been included in the authors' work. First, patients with systemic disease such as diabetes mellitus, hypertension, or cardiac disease may have asymmetric insufficiency in ocular blood flow, which may play an important role in the pathogenesis of LPG. Also, patients with sleep apnea may have poor systemic blood flow during the night time, and they benefit from sleeping on sides. Demographic data on such potential confounders are lacking. Moreover, some studies have associated lower corneal hysteresis with progression or severity of glaucoma. According to Table 3, the left eye had lower corneal hysteresis, albeit of borderline significance. Second, monitoring only the body position may have missed the potential effect of head position and/or neck flexion on IOP during sleep as already mentioned above.^2,3 Third, although VFI is a well-established indicator of functional damage of glaucoma, it is subject to fluctuation or learning effect. Furthermore, the structural damage of glaucoma may precede functional loss at earlier stage, and there may also be mismatches between structural and functional losses. Given that a small interocular VFI difference (over 5%) was used as an inclusion criterion, recruitment of more patients with greater VFI asymmetry and assessment of structural damage might have led to different results. Recently, we compared the VF indices and peripapillary RNFL thickness in our 154 Korean patients with OAG, and found side-sleeping POAG patients had worse VF indices and peripapillary RNFL thickness on the ipsilateral side where they preferred to sleep on.⁷ This finding is in line with the authors' observation of thinner right RNFL in the presumed right-sided sleepers when LPG and HPG were combined to a POAG continuum (expanded sleep position survey).¹ Fourth, the study patients had very large differences in IOP readings measured in the seated position (19.7 vs. 10.6 mm Hg, right eye; 19.9 vs. 11.0 mm Hg, left eye) between pneumatonometry and ORA tonometry. Such differences in IOPs were larger than those reported in the previous studies where IOPs were compared using different tonometers in LPG patients.^8,9 Moreover, it is not clear whether the patients were already on IOP-lowering medication at the time of IOP measurements. If so, different numbers or types of IOP-lowering eyedrops used in fellow eyes might have affected the IOP alterations induced by postural changes.