The Mont Blanc Study: The effect of altitude on intra ocular pressure and central corneal thickness

The aim of the Mont Blanc Study was to investigate the relationship between intraocular pressure (IOP), central corneal thickness (CCT), and altitude in healthy subjects. Thirty-three eyes of 33 healthy volunteers (mean age: 24.8 years, 17 females) had their IOP measured with Perkins and I-Care tonometers and their CCT using ultrasound pachymetry at three locations in Italy with different altitudes: Pavia, (PV), 77 meters above sea level (a.s.l); Courmayeur (CM), 1300 meters a.s.l; Pointe Helbronner (PH), 3466 meters a.s.l.). The measurements were performed at 9 am, 11 am, 1 pm and 3 pm (+/- 30') in indoor settings (mean temperature of 19 degrees C) in PV and PH. At 9 am, CCT and IOP were measured outdoor (mean temperature of -1.4 degrees C) at PH. The mean values of the IOP curve decreased from PV to PH with the Perkins (p= 0.02) and I-Care tonometers (p= 0.001). Instead, CCT increased upon ascension from PV to PH (p= 0.01), and from CM to PH (p= 0.002). When exposed to sub-zero temperature, the IOP increased (p<0.001), while the CCT did not change (p= 0.30). The results suggest that IOP significantly decreased and CCT significantly increased upon ascension from the sea level to higher altitudes.