Basics of Glaucoma

Basics of Glaucoma

Scanning Computerized Diagnostic Imaging (SCODI):

Confocal scanning laser ophthalmoscopy

creates a three-dimensional image of the optic nerve head. The optical design of instruments using

confocal scanning laser technology allows for a series of tomographic ‘‘slices,” or optical sections, of the

structure. The images acquired by this method are stored as a computer data file and manipulated to

reconstruct the three-dimensional structure, display the image, and perform data analysis. Parameters

such as cup area, cup volume, rim volume, cup-disc ratio, and peripapillary nerve fiber layer thickness are

then calculated.

Techniques such as scanning laser polarimetry and optical coherence tomography have been used to

acquire images of the retinal nerve fiber layer. The scanning laser polarimeter is essentially a scanning

laser ophthalmoscope outfitted with a polarization modulator and detector to take advantage of the

refractive properties of the retinal nerve fiber layer.

Optical coherence tomography (OCT) uses interferometry and low coherence light to obtain a high-

resolution cross section of biological structures. The Heidelberg Retinal Tomograph (HRT) employs a

confocal scanning laser ophthalmoscope to capture its cross sections of the optic nerve fiber layer. This

testing allows for comparison of the optic nerve from one testing session to another and provides

important information regarding the possible progression of optic nerve damage.

Gonioscopy

Gonioscopy is the evaluation of the eye's angle structure usually using a specialized, mirrored contact

lens while the patient is seated at the slit lamp. Common lenses are the Koeppe lens or the three- or four-

mirror lenses. The mirror is necessary to see into the angle between the iris and the cornea. Not only

does gonioscopy provide the physician with a three-dimensional view of the anterior chamber angle, it

can also be helpful in assessing the depth of the angle structure and in differentiating between narrow

angles and angle closure glaucoma. The depth of the angle is usually noted, often along with the

pigmentation within the trabecular meshwork, presence of adhesions (peripheral anterior synechia), and

presence or absence of new blood vessel growth. Compression (indentation) gonioscopy can determine if

an anatomically narrow angle is likely to be openable with iridotomy or if it is permanently closed due to

adhesions (chronic angle closure).

Pachymetry

The pachymeter is a typical instrument in many ophthalmology and optometry practices. The word

pachymeter is a combination of two Greek words:

pachys,

meaning “thick” and

metron

, meaning

“measure.”

The pachymeter measures central corneal thickness; models include those similar to the one below as

well as handheld, portable models. The Ocular Hypertension Treatment Study (OHTS) was released in

2002 and illustrates how corneal thickness impacts measured IOP. This study found that a thinner central

cornea was a strong predictive factor for the development of glaucoma in subjects with ocular

hypertension. Subjects with a corneal thickness of 555 microns or less had a threefold greater risk of

developing POAG compared with participants who had a corneal thickness of more than 588 microns.

The OHTS study found central corneal thickness to be a risk factor for glaucoma, independent of IOP

level. Since the release of the OHTS study, measuring central corneal thickness by pachymetry has

become an important diagnostic tool in the diagnosis and treatment of glaucoma.