**3. Non-arterial anterior ischemic optic neuropathy**

Non-arterial anterior ischemic optic neuropathy (NAION) is the second most common optical neuropathy after glaucoma optic neuropathy (GON) among middleaged and older people caused by hypoperfusion in the posterior short ciliary arteries that supply blood to the ONH [42–44].

The pathogenesis of NATION is not fully known. However, anatomical predisposition, narrow scleral canal, optic disc druses, systemic hypertension, diabetes mellitus, cardiovascular diseases, and hypercholesterolemia are risk factors for NAION [45]. Ischemic damage to the optic nerve head triggers a vicious circle of increasing ischemia. Hypoxia of axons of ganglion cells blocks axoplasmic current and leads to edema of the ONH. The optic nerve is quite rigid, so axons can expand only due to the compression of surrounding tissues (the mechanism of compartment syndrome). Compression of capillaries and smaller vessels increases axonal hypoperfusion, which, in turn, leads to further axoplasmic stagnation and edema [46, 47].

Diagnosis of NAION in the acute phase with painless loss of vision for several hours or days, ONH edema does not cause difficulties. In the chronic stage, it can be

#### **Figure 6.**

*48-year-old man with GON. A—OCTA, radial peripapillary plexus, B—a graph of RFNL thickness, C—grayscale of values, D—ONH b-scan and peripapillary retina.*

#### **Figure 7.**

*42-year-old man with NAION. A—OCTA, radial peripapillary plexus, B—a graph of RFNL thickness, C—grayscale of values, D—ONH b-scan and peripapillary retina.*

difficult to differentiate NAION from GON, since the pattern of RNFL loss and visual field defects may be similar (**Figures 6** and **7**).

Despite the comparable thickness of the nerve fiber layer in both patients, a patient with NAON has a more diffuse decrease in peripapillary capillaries and a more pronounced depression of the visual field, a greater thickness of the peripapillary choroid and the lamina cribrosa.

The importance of differential diagnosis is due to different prognosis and treatment approaches. There are no effective methods of NAION therapy. Treatment is reduced to the elimination of vascular risk factors to prevent the occurrence of the disease in the paired eye [48]. Therefore, a correct diagnosis will prevent unnecessary excessive treatment that is prescribed due to incorrect diagnosis of NAION as glaucoma.

Similar to glaucoma, NAION also leads to the loss of ganglion cells and damage to the RNFL in the superior and inferior quadrants. However, in contrast to GON, in NAION the damage to the RNFL in the superior and inferior quadrants is more diffuse (from the temporal to the nasal edge) (**Figure 7**). Defects of the RNFL in the superior quadrant are formed more often. The superior-nasal segment is the most vulnerable [6, 49, 50].

*Morphofunctional Changes of the Retina and Optic Nerve in Optical Neuropathy of Various… DOI: http://dx.doi.org/10.5772/intechopen.109850*

Different patterns of RNFL and GCC damage reflect different patterns of primary damage to neurons and indicate different mechanisms of vascular disorders in each condition. In GON, changes in the form of arc-shaped defects are consistent with the topography of retinal nerve fibers [51]. In turn, in NAION, the changes are due to the structure of the Zinn-Haller arterial vascular circle, which is formed by the distal branches of the posterior short ciliary arteries and consists of the lower and upper halves [52].

Obviously, the changes in both retinal and choroidal peripapillary blood flow will be different. The decrease in the density of the capillaries of the radial peripapillary plexus in NAION is more pronounced than in GON [53]. A thicker peripapillary choroid is a predisposing factor for the development of NAION [54–56]. Unlike GON, capillary perfusion of the prelaminar tissue induces axonal damage without deformation and damage to lamina cribrosa [57].

Thus, the state of the choroid and lamina cribrosa can be additional differential diagnostic criteria of GON and NAION.

### **4. Optic neuritis associated with multiple sclerosis**

Optic neuritis associated with multiple sclerosis: Multiple sclerosis (MS) is a chronic neurodegenerative disease characterized by demyelination of various parts of the central nervous system caused by (autoimmune) inflammatory processes.

Optic neuritis (ON) is an initial symptom in about 20% of MS patients and occurs in 50% of patients during the course of the disease [58].

The pathological mechanism underlying the decrease in the thickness of RNFL and GC in patients with MS, both with and without previous ON, still remains controversial. However, it is recognized that progressive structural changes are caused by neurodegeneration, not inflammation [59]. There is a correlation between the OCT parameters and the MS manifisattions. RNFL and GCC thinning associated with optical neuropathy in MS correlate not only with visual acuity, and the rate of progression of the disease but also with a lesion of the gray matter of the brain. Therefore, OCT is used to monitor the disease, potentially reducing the need for frequent magnetic resonance imaging [60–64].

The changes in the GCC in MS are ahead of the RNFL changes. Volume and thickness analysis GCC is a more sensitive and reliable indicator of retrobulbar neuroaxonal damage in MS (appears on average after 2 weeks) [3].

The temporal quadrant of the RNFL is more sensitive to damage (**Figure 8**). Lower RNFL values correlate with a decrease in visual acuity, contrast sensitivity, average sensitivity of the visual field, and average indicators of color vision [65].

OCTA demonstrates a decrease in the density of parafoveolar and peripapillary capillaries, respectively, structural damage.

The detection of changes in the paired eye without ON reflects subclinical structural damage of the RNFL. Performing OCT is advisable for monitoring the disease, predicting after relapses of ON, and evaluating the effectiveness of treatment.

Despite the fact that the average thickness of the RNFL and GCC on both sides is within the normative values, the indicators on the right are lower. Local thinning of RNFL (sector 9 h) and GCC by the course of the papillomacular bundle, a decrease in the density of the parafoveolar and peripapillary capillaries along the course of the papillomacular bundle. Diffuse depression of the visual field, central perimetric defect.

*Man, 42 years old. The MS debut. BCVA 0.5/1.0. A—3D scanning protocol of the OND. B—3D macular scanning protocol. C—OCTA of the superficial plexus and radial peripapillary plexus of the right eye. E—grayscale of values OD. F—cluster analysis OD.*

*Morphofunctional Changes of the Retina and Optic Nerve in Optical Neuropathy of Various… DOI: http://dx.doi.org/10.5772/intechopen.109850*

## **5. Compression optic neuropathy**

Compression optic neuropathy (CON) develops more often as a result of compression at the level of the optic nerve (meningioma), orbit (thyroid disease), and anterior segment of the visual pathway (more often at the level of chiasm in pituitary lesions).

If the visual pathway is affected, atrophy can go in the antegrade direction. Experimental studies have proved the existence of retrograde trans-synaptic degeneration of the visual pathway in animals [66, 67].

It was believed that with acquired damage to the central neuron of the human visual pathway, atrophy of the nerve fiber ends at the level of synapses in the external cranial body and does not extend to the anterior segment of the visual pathway [68]. Contrary to this opinion, OCT demonstrates the possibility of the existence of acquired trans-synaptic retrograde degeneration of nerve fibers of the human visual pathway [69, 70]. OCT plays a role not only in the diagnosis and monitoring of compression but also in the prediction of visual functions after surgical treatment.

CON may be detected by OCT earlier than with ophthalmoscopy. A manifestation of the compression effect on the visual tract is the RNFL and GCC thinning. GCC thinning, as a rule, is detected earlier than RNFL changes and standard automated perimeter changes. Hemianopia on perimetry can present as a hemi-macular atrophy on the OCT (**Figure 9**). The excavation increase during compression is due to antegrade degeneration of axons and secondary collapse of glial support tissue [71]. The excavation is usually rounded, central, and not vertically oriented, as in glaucoma (**Figure 10**).

#### **Figure 9.**

*A 44-year-old woman complained for the first time of a vision reduction in her right eye. BCVA 0.6/1.0. For half of the year, frequent headaches occur. A hemi-muscular atrophy on the OCT, hemianopia on perimetry. Homonymous hemianopia. A neurosurgeon's examination revealed pituitary craniopharyngioma.*

**Figure 10.**

*A 15-year-old girl. Optic nerve glioma of OS (prechiasmal localization, external and internal hydrocephalus. BCVA 1.0/0.9. Increased excavation, RNFL and GCC thinning, and unilateral hemianopsia OS (enface optical disk, perimetry, thickness of SNVS and ganglion complex OD—A and OS—B).*

When detecting the asymmetry of the excavation, the RNFL and GCC thickness in symmetrical IOP, OCT may be the key to localizing the compression location [25, 72–79].

Thus, the specific features of CON are decreased visual acuity, vertical defects of the visual field, decoloration of the OND, expansion of excavation, and hemi-macular atrophy on the OCT, age younger than 50 years.

The characteristic pattern of RNFL and GCC damage in optical neuropathies makes it easier to understand the various pathological mechanisms of damage and differentiate early glaucoma changes from other optical neuropathies.

Arcuate inferior temporal defects are a sign of early glaucoma ascending atrophy. Diffuse horizontally oriented loss of RNFL and GCC is a sign of ascending atrophy in NAOIN. Centrally oriented lesions of RNFL, GCC, and visual field defects are formed with descending/ascending atrophy in MS. Homonymous, vertically oriented loss of RNFL and GC is characteristic of descending atrophy in cerebral pathology in the chiasm region.

### **6. Conclusion**

The pattern of changes in RNFL and GCC allows for predicting changes in the visual field and can be used to quantify lesions of the visual pathway in patients with brain dysfunction when it is impossible to perform perimeter tests. The high repeatability and reproducibility of objective retinal biomarkers, and ease of visualization explain the attractiveness and prospects of OCT.

*Morphofunctional Changes of the Retina and Optic Nerve in Optical Neuropathy of Various… DOI: http://dx.doi.org/10.5772/intechopen.109850*

Obviously, OCT is not the only tool for the diagnosis of neurological diseases; however, it can be successfully used in complex diagnostics, often replacing expensive invasive studies. This reduces the burden on patients on the one hand and reduces healthcare costs on the other hand.
