OPT 308 Optic Nerve in Glaucoma

Angle closure is most commonly the result of what underlying cause/mechanism?

iris/pupillary block

What are key features to evaluating glaucoma?

-specific glaucomatous changes

-correlations to diagnostic testing

What are components of the optic nerve?

-ganglion cell axons

-glial cells

What is the average disc area in blacks vs whites?

2.94 mm vs 2.63 mm

*caucasian

What are the regions of the optic nerve?

-intra ocular (surface NFL, prelaminar, laminar)

-intra orbital

Approximately how many axons make up the surface RNFL?

1 million

What provides blood supply to the ONH (surface RNFL)?

central retinal artery

What supplies the pre laminar optic nerve?

branches of short posterior ciliary arteries

What is the benefit of the saddle shaped collagen structure of the ONH?

pores allow for axons and vessels to pass (they are larger at S and I poles)

What ultimately drains the ONH?

central retinal vein

What is true about having larger pores?

there is a higher risk of damage

What do glial cells do?

mantle at edges of the ONH (fibrous astrocytes that support blood vessels)

What does the central retinal artery supply?

primarily surface NFL via peripapillary network

What do the short posterior ciliary arteries supply?

most of prelaminar, laminar, and retrolaminar ON

*directly, from circle of zinn haller, via pial vessels, or from peripapillary choroid

What are the unique characteristics of the optic nerve blood supply?

-non fenestrated

-autoregulated

-no nervous innervation of retinal or ON vessels

-for continuous vascular network longitudinally along ON

What is autoregulation?

the intrinsic ability of vascular beds to maintain a constant level of blood flow regardless of changes in perfusion pressure and metabolic demand

What is perfusion pressure?

blood flow pressure to the ONH (balance of BP and IOP)

What is proportional to perfusion pressure?

blood pressure

What is inversely proportional to perfusion pressure?

IOP

In normal patients, what does an IOP rise result in?

auto regulation response; in glaucoma patients, IOP rise shows inadequate auto regulation response

What is auto regulation regulated by?

endothelin-1: vasoconstrictor that down regulates NO

nitric oxide: potent vasodilator

What are challenges to autoregulation?

-perfusion pressure changes exceed its ability

-dysregulation of mediators

-vessel wall damage (pericyte damage, atherosclerosis)

What overlies/obscures the blood vessels?

RNFL

What is the average area of the posterior scleral foramen?

1.6 mm^2

What population generally has larger optic discs?

African Americans (not generally affected by age, refractive error or axial length)

Up to what value of myopia is there not impact on glaucoma?

8 D

What are parameters to keep in mind when assessing optic cup/NRR?

-size

-ISNT rule

-color

-C/D ratio

Why is the size of the optic cup important?

gives us information about the size of the disc and the number of axons

What is the ISNT rule?

the rim is the thickest inferiorly followed by superior, nasal, and temporal

What is indicated by optic nerve pallor?

decreased vessels and gliosis

What are the C/D norms according to the farmingham eye study?

0.28 +/- 0.17

vertically elongated

What are examples of glaucomatous optic nerve changes?

-vertical elongation

-neuroretinal rim notching

-concentric enlargement of cup

-asymmetric C/Ds

-C/D change over time

-disc hemorrhages

-vessel changes

What are examples of findings with enlargement of cup?

-deepening

-bean potting

-vessel baring/bayonetting

What are examples of glaucomatous vessel changes?

-nasalization

-arterial attenuation

-baring

-bayoneting

How much vertical cupping is critical?

97.5%

-VCDR of 0.70 across all ethnic groups

When considering the 99.5%, what VCDR is considered to be critical?

0.80

Since glaucomatous damage occurs preferentially at the ONH, what areas are more impacted?

-inferior NRR

-superior NRR

What factor influences the size of the cup?

-larger optic nerve head means a larger cup (not glaucoma)

-smaller optic nerve head means a smaller cup (this may conceal glaucoma)

What are the norms for mean area of the disc?

2.1-2.8 mm^2

What age does the disc size stabilize?

10

What is the mean vertical size of the ONH in Caucasians?

1.82 mm (+/-0.16)

What is the mean vertical size of the ONH in African Americans?

1.96 mm (+/- 0.15)

What is notching at the NRR?

-focal area of loss within NRR

-appearance that inner portion of NRR has been chipped

-associated with RNFL defects

What is bean potting?

surface of NRR looks intact but it appears that the rim tissue is lost just below the surface of the NRR

*this is the appearance of the NERVE

What is bayonetting?

think of the appearance of a pencil in water; when following the path of a vessel, it appears to change its course as it approached the NRR

*this is the appearance of the VESSELS

What is vessel baring?

the NRR is used to support circumlinear vessels and now there is a space between the vessel and the NRR

Why may C/D ratios be asymmetric between the eyes?

POAG is bilateral but often asymmetric so the C/Ds may not be identical

What is the rule of thumb for C/D ratio asymmetry?

0.2

Where do vessels normally enter the nerve?

center to nasal 1/3 of cup

*vessel nasalization will be present in progressed glaucoma

What is arterial attenuation?

narrowing of the arteries as a result of damaged tissue not requiring as much blood supply

What is a drance heme?

superficial heme at the disc

What techniques should be used to assess the nerve?

-practice binocular view

-small rectangular beam over nerve 78/90

-red free

-welch allyn direct

What are the 5 Rs of evaluating the optic nerve?

-observe the scleral RING

-evaluate the RIM

-examine the RETINAL nerve fiber layer

-evaluate the REGION around the disc

-evaluate for RETINAL/disc hemes

What is the average vertical disc size?

1.8 mm

What is the average horizontal disc size?

1.7 mm