Goldman contact tonometry is the most common method of measuring the IOP. Variation in corneal thickness is a significant source of variation in IOP measurements between individuals. Corneal thickness is significantly reduced in patients with normal pressure glaucoma as compared with patient with primary open angle glaucoma and healthy controls. The reduced corneal thickness may lead to underestimation of IOP readings. Measurement of corneal thickness should be considered when assessing IOP. Goldman tonometry overestimates the IOP in cases with thick cornea, and underestimates the IOP in cases with thinner cornea. Measuring the central corneal thickness may be useful in determining the accuracy and the importance of elevated IOP in glaucoma suspect or the normal IOP in patients diagnosed as low tension glaucoma.

The role of Tono-pen in measuring the IOP, in corneas with abnormal thickness is not known. In some studies the IOP measured by the Tono-Pen was significantly correlated to the central corneal thickness. The instrument seems to overestimate IOP readings in individuals with thicker central corneal. In other studies, it was suggested that the use of Tono-pen tonometer may be more accurate than Goldman tonometer, the instrument appeared to be less affected by changes in the corneal thickness than Goldman tonometry. Pneumotonometry is also thought to be more reliable, than Goldman applanation tonometry, in measuring the IOP after PRK in the peripheral as well as the central cornea. The use of Goldman applanation tonometry, in these eyes, in the central cornea may underestimate the IOP.

IOP measurement may vary with age. Applanation tonometry markedly underestimates IOP readings in young patient's eyes. The pneumotonometer method seems to be the best method for measuring the IOP clinically in all ages. It is thought that applanation tonometry is better reserved for patient of 10 years of age or older. Changes in blood circulation, (e.g. increase in the venous pressure in the Valsalva technique) may affect the IOP. These changes can be observed in measuring the IOP in very obese patients due to the difficulty they encounter in positioning their heads on the slit lamp. Perkins tonometry in these situations may be more accurate than Goldman tonometry.

The sensation of light elicited by a non-photic stimulus is entoptic phenomena called phosphene. Application of pressure on a closed eyelid gives rise to a phosphene sensation described as a glow, and perceived opposite to where the pressure is applied. The source of pressure phosphene is supposed to be the bipolar cells in the retina. A new tonometer (The pressure phosphene tonometer) has been described. The tonometer is applied with the eye close and the patient is instructed to indicate when the pressure phosphene is perceived, the IOP is then determined by reading the pressure on the dial. The IOP measured by the new instrument seems to be comparable to Goldman's tonometry results and may be used for home IOP self-measurement.

Objective computerised image analysis methods with digital videographic image storage and processing techniques have recently bee used to avoid this problem. The main computerised image analysers used at present are:

• The Rodenstock optic nerve head analyser.
• The confocal scanning laser tomograph (The Heidelberg retina tomograph).
• The scanning laser polarimetry for the nerve fibre layer.
• Optical Coherence tomography.

The Rodenstock optic nerve head analyser uses a stereoscopic video camera to produce digitised image of the optic nerve head. The instrument is, however, technically difficult to operate, it also needs pupil dilatation and clear media.

The Heidelberg retina tomograph projects a diode laser beam on the retinal via a confocal system. The instrument has a high spatial resolution due to its confocal system. The depth of scanning ranges from 0.5 to 4.0 mm with 0.5-mm increments. The instrument performs 32 scans within this depth. The computer then forms a three-dimensional image of the disc. The instrument has a better axial resolution than the Rodenstock optic nerve head analyser. The instrument also uses a low light intensity, and images can be obtained through undilated pupil. The main disadvantage of the instrument is that it needs a reference plane.

The Optical Coherence tomography operated on the same principle as ultrasound but by using optical, rather than, ultrasound beam. The instrument provide a very high resolution images without contact with the eye. The instrument is not affected by the refractive status of the eye. Media opacities may, however, affect its results. Testing with this instrument also need pupil dilatation.

It is generally thought that none of these instruments is suitable for routine use in clinical diagnosis of glaucoma.

Normal optic nerve heads in the general population may vary significantly in the disc and neuro-retinal rim area. Age does not appear to be associated with significant disc characteristics.

Different patterns of disc damage have been described in glaucoma patients. Optic disc damage may be diffuse (in the majority of patients), localised, or undetectable. Patients with different disc appearances show differences in their demographic characteristics, prevalence of certain systemic risk factors, IOP levels and the patterns of their visual field loss. The presence of various disc appearances may represent different patient population with different mechanisms for the glaucoma. The main four different patterns of glaucomatous discs are:

1. Focal ischaemic disc.
2. Myopic glaucomatous disc
3. Senile sclerotic disc.
4. Generalised enlargement of the optic cup disc.

 Disc and retinal imaging can be carried out by different imaging systems (e.g. fundus camera, infrared scanning laser ophthalmoscope and the Heidelberg retina tomograph). The size of the disc parameters depends on the magnification by the camera and by the eye. The use of a single magnification correction value for all these methods is not accurate, this may have significant implication in the calculation of disc and retinal lesions sizes.

The topography of the optic nerve head seems to be dependent of the level of the IOP. Increases in the IOP are associated with significant enlargement of the optic disc cupping in both emmetropic and myopic eyes. Lowering of the IOP after trabeculectomy surgery may be associated with improved optic nerve morphology (as demonstrated with Heidelberg retinal tomograph). The amount of improvement in the morphology of the optic nerve head seems to be correlated to the degree of IOP reduction. Recent studies, however, showed that there are no apparent differences between high tension and low-tension glaucoma in the morphometric parameters as measured by scanning laser ophthalmoscopy.

Vertical cup/disc ratio has been used in the evaluation of disc cup in glaucoma patients. There is, however, a wide variation in the cup/disc ratio in normal population. The cup/disc ratio relative to the disc size is a new parameter that can be a useful in the evaluation of glaucomatous discs, especially in small discs.

The neuro-retinal rim is one of the main features in the diagnosis of glaucomatous optic neuropathy. Analysing the relation between the disc size and the neuro-retinal rim by the Laser scanning ophthalmoscope is a specific and sensitive method to differentiate between normal and glaucomatous discs. The system provides a high degree of reproducibility and can be used in the evaluation of the optic disc in glaucoma patients.

Glaucomatous neuro-retinal rim loss generally begins in the inferior temporal region and progresses to the superior temporal to temporal, inferior nasal, and finally the superior nasal region. Abnormally low inferior / temporal and superior / temporal ratio can be used to detect glaucomatous disc damage in some eyes with raised IOP. These ratios offer a method of neuro-retinal evaluation independent of disc size and magnification problems.

Measurement of the absolute rim area and the rim / disc area, is significantly larger with confocal laser ophthalmoscope than with planimetry or disc photos. This difference may be due to the fact that confocal laser ophthalmoscope measures the retinal vessel as a part of the retinal rim. It should be taken in consideration when comparing data.

Disc haemorrhage appears to be strongly associated with glaucoma (especially low-tension glaucoma). The presence of disc haemorrhage is associated with localised damage of the nerve fibre layer in both NTG and primary open angle glaucoma and seems to increase the risk of further visual field deterioration. Most disc haemorrhages are present in patients with no other signs of glaucoma. They seem to be associated with larger vertical cupping and also with a history of migraine.

Acquired optic nerve head pits is also one of the significant disc changes associated with the development of glaucoma especially low-tension glaucoma. It is often associated with an increased risk of developing progressive disc and visual field damage. The field changes associated with disc pits are deep and have sharp margin scotomas which approaches or even involve fixation. Visual field loss within 1-3 degrees from fixation occurs in about 96.7% of cases. The presence of these pits constitutes a threat to fixation vision specially if it is located in the inferior pole of the optic disc. 76% of the acquired optic nerve head pits occur in the inferior part while only 11% occur in the upper part.

The value of measuring the peripapillary atrophy in the diagnosis of glaucoma is uncertain. 38% of eyes with primary angle-closure glaucoma and 68% of eyes with primary open-angle glaucoma have peripapillary atrophy. The peripapillary atrophy in primary angle-closure glaucoma has a different relationship to the structural and functional optic disc changes than that in primary open-angle glaucoma. Different mechanisms seem to be involved in the development of the optic disc damage in the two types of glaucoma. Beta zone (central peripapillary atrophy), and alpha zones (peripheral peripapillary atrophy) are found in 49% and 100% eyes with primary open-angle glaucoma, respectively. It must also be remembered that the prevalence of zone beta seems to be higher in high myopic eyes. Both areas, especially beta zone, are associated with glaucomatous visual field loss when tested by standard automated perimetry and short-wavelength automated perimetry. It is suggested that clinical decision should not be based only on the presence or absence of these zones.

Frequency doubling test is a new test that is based on detecting abnormalities in the magnocellular ganglion cells layer (other tests include Motion sensitivity testing, scotopic sensitivity). In this test, alternating light and black bars, that are rapidly reversed between white and dark, appear to have twice the actual number of bars. There is theoretical and clinical evidence that the use of this phenomenon may be useful in the diagnosis of early glaucoma.

Grading of glaucomatous visual field defects, in a clinically significant manner similar to that obtained by conventional visual field testing, is possible with this new psycho-physiological test. The test seems to be a promising method in screening for glaucoma. It is simple, rapid and not affected by the patients refractive errors. Motion impairment may be used as a predictor for visual field loss in glaucoma patients. The period between motion impairment and visual field loss, in not known.

When systemic and topical anti-glaucoma treatment fail to control the high IOP, and when peripheral iridotomy is not possible (e.g. in cases of severe corneal oedema), the technique of Argon Laser Peripheral Iridoplasty may be effective in controlling the IOP and clearing the corneal oedema. The aim of this procedure is to, mechanically open the appositional closed angle. In this technique a ring of low power, long duration, and a large size burns is applied to the iris periphery to contract the iris stroma and open the angle. This technique is often applied after controlling the IOP medically. Argon laser iridoplasty without medical treatment may also be used in cases with primary angle closure of duration less than 48 hours. A full 360 degrees ring is often applied, but a more limited area of treatment may also be effective.