Introduction:

Glaucoma is a progressive optic neuropathy characterized by loss of retinal ganglion cells with resulting progressive visual field loss. It is the leading cause of irreversible blindness worldwide.

Retinal ganglion cell bodies are located in the ganglion cell layer of the inner retina. Their axons constitute the RNFL. The axons extend in an arcuate pattern to converge at the optic nerve head (ONH). Glaucoma is a heterogeneous condition, with variable pathophysiological mechanisms culminating in retinal ganglion cell loss and optic atrophy. The most prevalent type is primary open angle glaucoma. Mechanical factors at the level of the optic nerve head, genetic susceptibility and oxidative stress are hypothesized to be involved.

Clinical Features:

The hallmark of glaucomatous optic neuropathy is a progressive loss of RNFL tissue eventually culminating in visual field loss, with associated changes in the ONH (Fig. 6.1.1). This may be associated with elevated intraocular pressure. The RNFL loss can be noted qualitatively on careful clinical examination, and is best appreciated with red-free illumination of the fundus. Slit-lamp biomicroscopy with a handheld lens is the best method of optic disc examination, since it provides good stereopsis and magnification. Optic disc stereo photographs are complementary and may identify findings missed on slit-lamp examination. Progressive changes in the ONH follow the RNFL loss, including focal or diffuse loss of neuroretinal rim tissue and enlargement of the cup.

OCT Features:

OCT has become a valuable tool in the initial diagnosis of glaucoma and subsequent monitoring of disease progression over time. OCT scans represent non-invasive, reproducible measurements of the structural changes that occur in glaucoma. OCT permits quantitative documentation and analysis of the areas most affected by glaucoma: the ONH, the peripapillary retina, and the macular retina.

The key anatomical features assessed on the ONH scan are:

Additionally, glaucomatous damage to the ganglion cells of the macula and the resultant overall macular retinal thinning can be assessed with GCC mapping. The diagnosis of glaucoma is improved by the analysis of GCC compared to measurement of full retinal thickness.

Most commercially available OCT machines have software permitting comparison of change over time.

Ancillary Testing:

Visual field testing estimates the functional damage in glaucomatous eyes. However, it has been shown that up to 50% of the RNFL may be lost prior to any change in the visual field. Moreover, visual field testing is dependent on patient cooperation and often exhibits large variation between visits. Detection of visual field changes often requires several repetitions of the test, which may delay the diagnosis (Fig. 6.1.1B).

Treatment:

Treatment strategy is directed toward improving the parameters that cause further damage to the nerve fiber layer. Primarily this involves lowering intraocular pressures. Neuroprotection is also a large area of research in glaucoma.