# Goldmann Tonometry and Corneal Biomechanics
## 1 Introduction  
Glaucoma is a multifactorial optic neuropathy characterized by progressive loss of retinal ganglion cells, the neurodegeneration can also involve the neuronal pathways up to the geniculate body and occipital cortex, resulting in changes in optic disk morphology and visual field defects [1, 2, 3]. It is the leading cause of irreversible blindness worldwide, accounting for 8% of vision loss [4, 5]. Intraocular pressure (IOP) is recognized as the most important risk factor for the development or progression of glaucomatous damage, and is controlled by the balance between aqueous humor secretion and the eye's outflow facility [6]. An association between increased IOP and the loss of sight in glaucoma has been noted for many centuries, from observations of eye stiffness for cases of continued impairedvision following cataract surgery, through the establishment of the relationship between the IOP and loss of sight. In particular, elevated IOP increases the likelihood of visual filed alterations or scotoma and even complete blindness. For this reason, the IOP evaluation remains the primary measurement in the diagnosis of glaucoma [7]. Ocular hypertension studies also investigated and addressed whether the treatment of elevated IOP prevented or delayed the onset of glaucomatous damage, showing that a decrease of IOP reduced the risk of progression to glaucoma [8]. Furthermore, in recent epidemiological studies it has been verified that a reduction in IOP of only 1 mmHg from baseline leads to a 10% reduction of damage progression, and of the conversion from ocular hypertension (i.e., without damage) to manifest glaucoma [9; 10]. Therefore, obtaining a correct measurement of IOP during the treatment of the disease or as screening test for the identification of subjects at risk, appears very important. Together with visual field analysis, IOP is the gold standard for the diagnosis and the correct evaluation of the progression of the disease. Recently, useful software has been developed to analyze optical fibers at the peripapillary and macular levels using optical coherence tomography (OCT) to monitor disease progression [11; 12; 13]. Unfortunately, in some glaucoma patients there is a progression of the disease, regardless of an apparently normal IOP. Known as normal tension glaucoma, it is an optic disease in which the IOP before ocular hypotonizing therapy is less than 21 mmHg, apparently normal values [14; 15; 16]. Many factors, such as the central corneal thickness (CCT) and biomechanical properties of the cornea, may affect IOP measurement, particularly in patients affected by glaucoma [17; 18; 19]. The most accurate method to evaluate the IOP is the direct one, which measures the real (actual) pressure in the eye through the cannulation of the eye, using a pressure transducer placed in direct communication with the anterior chamber. This approach, because of its high invasiveness, is not valid for routine measurements of IOP, and has to be considered only for experimental use or during surgery [20; 21; 22]. All other methods of measuring IOP are indirect; the cornea is deformed after application of an external force to the corneal surface. The amount of force necessary to obtain changes in corneal normal conformation is proportional to the pressure inside the bulb. In clinical practice IOP is then calculated--not measured directly--by indirect techniques. The tool that allows to make this measurement is called a tonometer (applanation or non-contact tonometry). In addition to the Goldmann explanation tonometer (GAT), which will be further discussed below, several types of tonometers are currently available, each with advantages and disadvantages [23]. The non-contact tonometry (NCT) is influenced by tear meniscus height (TMH), as the measured IOP increases with increasing tear film, and by central corneal thickness (CCT) [24; 25; 26].