Intraocular pressure (IOP) is the most important parameter used in determining glaucoma development and progression. However, measured IOP values can be influenced by various factors including central corneal thickness (CCT), corneal rigidity, Valsalva’s maneuver, astigmatism, corneal curvature, and inappropriate amount of fluorescein. Among the techniques used to estimate IOP, Goldmann applanation tonometry (GAT) is the most commonly used device worldwide. GAT estimates IOP by flattening the corneal apex to a given area and then assessing the force needed. With this device, a flattened area with a diameter of 3.06 mm is empirically chosen to offset the surface tension of the tear film and both the corneal and ocular rigidity. GAT is designed to estimate IOP using the assumption that the CCT is 500 μm. However, it is has been shown that the CCT actually varies among individuals. The IOP will be overestimated in eyes having a thicker cornea, while it will be underestimated in eyes with a thinner cornea [4–7]. Meanwhile, thinner CCTs have been identified as a risk factor for both the development of primary open-angle glaucoma (POAG) in eyes with ocular hypertension [8] and for the observation of advanced glaucomatous damage at initial examinations [9]. According to the Ocular Hypertension Treatment Study, subjects with a corneal thickness of 555 μm or less had a threefold greater risk of developing POAG compared with subjects who had a corneal thickness of more than 588 μm. It has not been completely determined whether this increased risk of developing POAG is due to underestimating actual IOP in eyes with a thinner cornea or whether thinner corneas are a risk factor independent of IOP measurement. Furthermore, the corneal mechanical properties, such as elasticity, are known to have a greater effect on the tonometric IOP measurement errors than either the corneal curvature or thickness.

Several previous studies have examined the relationships between CCT and AL or myopic refractive error. Al-Mezaine et al. demonstrated that the AL was not correlated with the CCT in myopic eyes [10]. Fam et al. also found that there was no correlation between the degree of myopia and CCT in Singaporean Chinese subjects [11]. On the other hand, a significant correlation was found between CCT and refraction in a normal Japanese population [12].

The Ocular Response Analyzer (ORA) is a non-contact tonometer that is used to measure intraocular pressure and is the only instrument that can measure corneal hysteresis (CH), which is one of the biomechanical properties of cornea and a parameter of the viscoelastic properties of the cornea. The IOP will be overestimated in eyes having a stiffer cornea. The ORA determines the CH during the rapid motion of the cornea that occurs in response to a rapid air impulse. The air impulse causes deformation of the cornea, which is monitored by an electro-optical system. It has been previously reported that CH is lower in keratoconus, Fuchs’ dystrophy, post-laser in situ keratomileusis (LASIK), and glaucoma [13–15].

Shen et al. demonstrated that the CH was significantly lower in high myopic eyes (spherical equivalent (SE) lower than −9.00 D) compared with subjects having a SE between −3.00 and 0 [16]. Moreover, CH was positively correlated with refraction, while the refraction was negatively correlated with the IOP. Thus, mechanical strength in the anterior segment is compromised in high myopia [17]. Congdon et al. showed that the CH measured by the ORA was correlated with the CCT and was an independent risk factor for worsening of the glaucomatous visual field [14]. Therefore, the measurement of CH is important for high myopic eyes with glaucoma. In order to assess glaucoma risk and its clinical course, it will be important that detailed measurements of the physiological properties of the cornea be performed in high myopia.

The anterior chamber depth (ACD) has been reported to be deeper in myopic eyes than emmetropic or hypertropic eyes [18, 19]. Use of anterior segment optical coherence tomography (ASOCT) makes it possible to determine measurements of novel parameters, including the anterior chamber width (ACW) and lens vault. ACW is defined as the horizontal scleral spur-to-spur distance. Nongpiur et al. [20] studied 1465 community-based subjects and 111 subjects with narrow angle in Singapore. They found that ACW and ACD were significantly correlated with the axial length (AL) and that shallow ACD and shorter AL were correlated with narrow angles. Lens vault is another parameter measured from the ASOCT images. Lens vault is defined as the perpendicular distance between the anterior pole of the crystalline lens and the horizontal line joining the two scleral spurs. Tan et al. showed that lens vault was negatively correlated with the axial length [21]. They found that greater lens vault was associated with narrow angles.

Established risk factors for primary angle closure include shallow ACD, thick and anteriorly displaced lens (increased lens vault), and short AL. In spite of these risk factors, some eyes with high myopia have been shown to have angle closure. In one retrospective study, 6 (1.9 %) out of 322 primary angle closure cases occurred in myopic eyes [22]. Barkana et al. additionally examined 17,938 patients with myopia of spherical equivalent (SE) of more than −6.0 diopters and reported finding nine cases of primary pupillary block and three cases of plateau iris configuration and syndrome [23].

The aqueous humor leaves the eye through two major pathways that include the trabecular or conventional pathway (via the trabecular meshwork, Schlemm’s canal, collector channels, and aqueous veins into the episcleral veins) and the uveoscleral or unconventional pathway (via the iris root, uveal meshwork, anterior surface of the ciliary muscle, connective tissue between muscle bundles, suprachoroidal space, and finally through the sclera).

The coefficient of outflow (C) is determined by measuring increases of intraocular pressure caused by indentation with a tonometer, though the increase of intraocular pressure is influenced by ocular volume and rigidity. Tonographical data demonstrated lower outflow facility in high myopic eyes. In high myopic eyes, the increased ocular volume reduces the IOP elevation caused by the tonometer indentation. Thus, the tonographic data in high myopic eyes should be calculated with a correction for ocular rigidity. Only a limited number of studies demonstrated tonographic data in high myopic eyes. Study by Honmura found a decrease in the value of C in myopic eyes and a negative correlation between the value of C and the AL [24]. In the same study, Honmura also found there was lower aqueous production in the myopic eyes. Muto et al. compared hyperopic, emmetropic, and mild and high myopic eyes and found there was a reduced value of C in the high myopic eyes (−6.25 D to −20 D) [25].

The uveoscleral outflow pathway passes from the anterior chamber through the CB to the sclera. Prostaglandin analogues induce the expression of metalloproteinases and may reduce the extracellular matrix within the CB, iris root, and sclera, thereby increasing the uveoscleral outflow. The prostaglandin analogues are also involved in CB muscle relaxation, cell shape changes, and cytoskeletal alterations [26]. The proposed site of action of these prostaglandin analogues demonstrates the importance of the CB in IOP control. However, only a limited number of studies examining ciliary muscle morphology have been carried out in vivo due to its position posterior to the iris.

Several reports have used ultrasound biomicroscopy (UBM) or ASOCT to show that the ciliary muscle is thicker in myopic eyes [27–30]. Oliviera et al. reported that CB thickness measured with UBM was negatively correlated with the refractive error and positively correlated with the AL [30]. Thus, they postulated that a greater CB thickness might lead to a better response to the prostaglandin analogues.

Pigment dispersion syndrome (PDS) is a disorder in which the pigment granules are released from the iris pigment epithelium. The diagnostic triad of clinical features consists of slit-like, mid-peripheral iris transillumination defects; diffuse and dense pigmentation on the trabecular meshwork; and pigment granules on the corneal endothelium (Krukenberg spindle). It is thought that pigmentary glaucoma is triggered by a progressive loading of pigment in the trabecular meshwork. Campbell observed a consistency between iris transillumination defects and the location of the zonular bundles. Therefore, the author proposed that posterior bowing of the iris led to the contact and friction between the posterior pigmented iris epithelium and the zonular bundles [31]. In PDS, posterior insertion of the iris root into the ciliary body (CB) occurs. This anatomical variation predisposes the iridozonular contact and zonular pigment dispersion [32].

While PDS is frequently associated with myopia, pigmentary glaucoma eyes are more myopic than eyes with PDS. It has been reported that a higher degree of myopia is a risk factor for an earlier onset of pigmentary glaucoma [33]. In addition, posterior bowing of the iris can also occur in myopic eyes without PDS.

Development of high-frequency UBM has made it possible for direct visualization of Schlemm’s canal in vivo. Irshad et al. performed an in vivo UBM study of Schlemm’s canal and reported that the average diameter of Schlemm’s canal in 44 myopic eyes (122 ± 45 um) was significantly smaller than that observed in six hyperopic eyes (180 ± 69 um) [34]. They also reported that the location of the Schlemm’s canal in black patients (659 ± 92 um) was posterior from the limbus as compared to white patients (624 ± 73 um), which indicates that there are potential differences in the position of Schlemm’s canal depending upon the race of the patient.

Although limitations of visualization remain, recent advances of ASOCT technology have made it possible to perform more increasingly precise visualizations of the conventional outflow pathway [35–37]. Hong et al. used ASOCT and demonstrated that the area of the Schlemm’s canal of POAG eyes was smaller than that observed in normal eyes [38]. However, the specific structural characteristics of eyes with high myopia, with or without glaucoma, have yet to be elucidated. As this new technology continues to develop, we will be able to collect further knowledge of these various structures that may ultimately deepen our understanding of the pathophysiology of myopic glaucoma.