Corneal hysteresis and intraocular pressure in mucopolysaccharidosis I and VI

Abstract

Editor, Fahnehjelm et al. (2011) have recently presented an interesting case series of patients with mucopolysaccharidosis I (n = 5) and VI (n = 2) (MPS-I and MPS-VI). The authors measured the corneal hysteresis (CH) and corneal resistance factor (CRF) by ocular response analyser (ORA), as well as the intraocular pressure (IOP) by ORA pneumotonometry or Icare-tonometry to investigate the potential impact of the corneal biomechanical profile on IOP. They have documented an increase in CH and CRF in all patients, suggesting that an increase in corneal stiffness is probably responsible for the falsely increased IOP values, which were recorded in these patients. Nevertheless, we would like to emphasize some points which, in our opinion, merit further consideration. All patients with MPS-I had corneal opacification and they were subjected to stem cell transplantation (SCT), which resulted in an improvement in the corneal transparency, according to the authors. Corneal evaluation with the aid of ORA and measurement of IOP was conducted after SCT treatment. As the authors report, the accumulation of glycosaminoglycans (GAGs) in the lysosomes of the corneal keratocytes may not only cause corneal opacity but may also increase the corneal thickness (Kottler et al. 2010). On the other hand, Summers et al. (1989) have reported normal corneal thickness in some patients with MPS-I. However, the authors do not provide any data regarding the central corneal thickness (CCT) of their patients. Was the CCT increased before SCT? What was the CCT at the time of examination (after SCT)? In our opinion, it is important for the authors to specify whether the increase in CH and CRF was an epiphenomenon occurring predominantly because of the increased CCT or a primary phenomenon associated with the altered stromal homeostasis because of the accumulation of GAGs in the lysosomes of corneal keratocytes. We can imagine that improvement in corneal opacification after SCT, as reported by the authors, may have induced a relative normalization of CCT. The evaluation of CCT would enable a valid interpretation of the biomechanical findings providing evidence regarding the nature of the corneal biochemical profile in MPS-I and MPS-VI. Moreover, the authors conclude that increased rigidity of the cornea and the increased thickness of the cornea and sclera may cause an increased stiffness and reduced elasticity in their patients. Glass et al. (2008) have recently described a viscoelastic biomechanical model of the cornea, which illustrates in an extremely convincing manner the impact of viscosity and elasticity upon CH. Corneal hysteresis may increase or decrease with stiffening depending on the behaviour of the viscous material element, which is modified in MPS because of the stromal accumulation of GAGs. High CH may be associated with high elasticity or low elasticity depending on the viscosity; alteration in CH alone has too many undefined degrees of freedom to deliver a valid statement regarding corneal stiffness (Glass et al. 2008). Finally, the authors suggest that all patients in this study had falsely high IOP values, as measured by ORA pneumotonometry, probably due to increased corneal rigidity. The main question which one should address is whether a primary ‘endogenous’ increase in corneal rigidity accounts for the falsely increased IOP or whether the observed false increase in IOP represents the aftermath of a secondary increase in CCT. In our opinion, the consideration of CCT is of paramount importance in the attempt to evaluate in a valid manner the corneal biochemical properties, especially in a corneal model where a metabolic disorder is involved, such as MPS, which increases the complexity of the biomechanical investigation.