Intracranial pressure fluctuations: a potential risk factor for glaucoma?

The authors would like to thank Dr. Wostyn and colleagues for their letter and their interest in our study (Wostyn et al. 2013; Zhang et al. 2013). Dr. Wostyn and colleagues mentioned studies on 1) wind instrument players who showed a rise in intraocular pressure (IOP) related to playing the instruments and in whom a greater frequency of glaucomatous optic nerve damage was associated with a greater number of hours of playing the instruments over their lifetimes; and 2) on studies in which 45% of patients with normal-pressure glaucoma (as compared to 11% with primary open-angle glaucoma) were exposed to conditions causing intrathoracic or intra-abdominal pressure elevation, including playing high resistance wind instruments, weightlifting, chronic asthma and cough, obstruction of the urinary system and constipation (Schuman et al. 2000; Krist et al. 2001). According to our study (Zhang et al. 2013), Valsalva manoeuvres are associated with a rise in the cerebrospinal fluid pressure (CSFP), at least partially compensating for the rise in IOP with respect to the trans-lamina cribrosa pressure difference (TLCPD). Dr. Wostyn wonders whether not only the absolute pressures but also the pressure changes on both sides of the lamina cribrosa and thus the fluctuations in the TLCPD may play a role in the pathogenesis of glaucomatous optic neuropathy. The authors fully agree with this notion. Morgan and colleagues demonstrated in a previous study a difference in the phasing of the IOP curve and the phasing of the CSFP curve with respect to the cardiac cycle, with the ICP curve reaching its height earlier than the CSFP curve (Morgan et al. 2012). It was then wondered whether these physiological short-term changes in the TLCPD, potentially even resulting in short-term reversals of the TLCPD and thus in a swinging TLCPD, may physiologically be needed to allow the retrograde axoplasmic flow entering the eye (Jonas et al. 2012). The thoughts by Dr. Wostyn and colleagues, the observations by Morgan and co-workers and the considerations based on Morgan's finding may show that the dynamics of IOP, ICP and TLCPD, including the physiological time shift between the cardiac cycle-related CSFP curve and IOP curve (‘ocular pulse’), may be of potentially high importance for both the physiological stability and the pathophysiology of the optic nerve.