PEARLS FOR CORRECT ASSESSMENT OF THE OPTIC DISC

Based on the article by  PROF. BURAK TURGUT and available at the following link:
http://www.touchophthalmology.com/articles/pearls-correct-assessment-optic-disc-glaucoma-diagnosis

Optic nerve head (ONH):

1.       Region where retinal ganglion cells (RGC) leave the eyeball through the scleral canal.

2.       Also contains blood vessels, glial- and connective-tissue.

3.       The terms ONH and optic disc (OD) are often used interchangeably, although the OD signifies the part of ONH visible ophthalmoscopically.

4.       ONH consists of surface retinal nerve fiber layer (RNFL), pre-laminar, laminar (Lamina cribrosa [LC]) and retrolaminar layers.

5.       Structural loss of ONH often precedes functional changes in the visual field (VF). However, the reverse could also be noted.

6.       Glaucomatous VF defects occur when 20-40% of the RGCs are lost.

7.       During examination of the ONH, the following should be correctly evaluated:

a.       Size, color and integrity of neuro-retinal rim (NRR)

b.      Size and shape of cup

c.       Cup to disc ratio (CDR)

d.      Shape and configuration of vessels in the ONH

e.      Laminar dot sign in the cup

f.        Changes in peripapillary region (ONH hemorrhages, peripapillary atrophy)

g.       Loss of lines belonging to RNFL in red-free illumination

8.       ONH examination can be done by:

a.       Direct ophthalmoscopy, indirect ophthalmoscopy and slit-lamp biomicroscopy with a contact lens or non-contact hand-held lens (+66, 78 or 90D) or a Hruby lens. The slit lamp method provides good stereopsis and magnification.

9.       Correction factors commonly used in fundus lenses:

10.   The boundaries of the OD conform to the edges of the scleral canal which appears as a whitish circular band. Thus, the size of the scleral canal determines the optic disc size. Eyes with small canals have small discs and those with large canals have large sized discs.

11.   The size of the OD can be measured on the slit-lamp by using a narrow vertical slit beam and condensing lens. Correction factors are then applied upon the type of lens.

12.   ONH diameter (mm)= (X/H) x D x C [X=Height of the beam, H= Height setting on the beam height indicator; D= diameter of OD measured by the beam height indicator (mm); C= correction factor]

13.   Area of ONH can be calculated by: ONH area (mm²)= r/4 x horizontal disc diameter x vertical disc diameter (r=correction factor of lens)

14.   Using a direct ophthalmoscope with 2 spot sizes, the small light spot which is 5-degrees in size, is approximately the size of the average OD. In an ophthalmoscope with 3 spot sizes, the middle spot can be used for the assessment of the disc size. When the spot is placed over the disc, if the disc lies within the spot the disc is small and if outside the illuminated spot, it is large.

15.   The distance from the temporal edge of the OD to the centre of fovea is approximately 2-3 DD in eyes of normal size and axial length. When the distance between the edge of the disc and centre of fovea is less than normal, the OD is large in size and vice versa.

16.   The diameters of the disc can also be measured using the scale available on the slit-lamp. The beam is focused on the inner margin of Elschnig’s white scleral ring; atleast 3 measurements (in mm) taken and finally, the average multiplied by the correction factor of the lens used. [Elschnig’s scleral ring is a white circular band not belonging to the optic disc. It separates the intrapapillary region of the OD from the peripapillary area]

17.   Average vertical OD diameter = 1.88 mm (Range = 1700-2000µ)

18.   Average horizontal OD diameter = 1.77 mm (Range = 1600-1800 µ)

19.   The thickness or diameter of the central retinal vein (CRV) is approximately 125µ where it crosses the inferior NRR. An OD around 12-14 of the CRV is considered normal.

20.   Optic disc surface area= 2.1 mm² -2.8 mm² (Mean= 2.69 mm² / range= 0.80 mm² – 5.54 mm²) in normal Caucasian population. Patients with hyperopia >+5D have smaller ODs and those with myopia >-8D have larger discs compared to emmetropes.

21.   Large ODs have larger cups and appear to have thinner NRRs. This may give a false impression of glaucomatous optic neuropathy (GON). In a small disc, the crowded NRR may hide the glaucomatous cupping.

22.   Pale cupping (Pallor exceeding the cupping) indicates a non-glaucomatous etiology such as: Anterior Ischemic Optic Neuropathy (AION), compressive lesions (tumors, hemorrhage), secondary optic atrophies (papilledema) and inflammatory or infectious optic neuropathies.

23.   Greenfield criteria for non-glaucomatous cupping:

a.       Age <50 years

b.      VA <20/40

c.       OD pallor in excess of cupping

d.      Vertically aligned VF defects

24.   Such patients should undergo neuro-imaging studies to look for the cause.

25.   Non-glaucomatous cupping is usually stable and does not worsen with time.

Optic cup and cup:disc ratio:

1.       Optic cup: It is a central pale excavation in the ONH. It includes glial tissue, but not retinal tissue or RGC axons. The pale color of the cup is due to the exposure of the collagenous LC and loss of glial tissue.

2.       The CDR may not be appropriate in patients with tilted, hypoplastic or dysplastic nerves.

3.       The size of the cup appears smaller on monocular than binocular examination.

4.       The direction or point of deviation of small blood vessels on the surface of the ONH is used to determine the size of the cup (contour method). The area of pallor in the OD (color contrast method) is not an accurate measure of the cup size.

5.       Larger discs have larger cups and vice versa.

6.       Notching is only infrequent in temporal/nasal regions.

7.       Temporal rim notching without other glaucomatous changes is most likely associated with an OD insertion anomaly.

8.       Average CDR is 0.4 (Range= 0.0-0.9).

9.       66% of the normal population found to have <0.3 CDR, with bilaterally symmetrical cups.

10.   A difference of more than 0.2 between the 2 eyes might be an early sign of glaucoma.

11.   Vertical elongation of cup is due to preferential loss of axons in the inferior and superior poles.

12.   Saucerization is a shallow diffuse excavation in the cup and it is very hard to detect in some cases.

13.   Estimation of CDR has only limited value in the identification of GON due to wide variation in the size of the optic disc and cup in the normal population.

Neuroretinal rim:

1.       Tissue between outer boundary of cup and optic disc margin.

2.       Circumlinear vessels usually rest on the NRR, so the boundaries of the cup can be identified by following the track and kink of blood vessels.

3.       PPA can cause wrong determination of the OD boundary and misrepresentation of NRR.

4.       NRR thickness follows inferior, superior, nasal and temporal (ISNT) in order of thick to thin.

5.       Thinner NRR or not conforming to ISNT rule may be suspicious of glaucoma.

6.       NRR is pink or orange in normal healthy individuals.

7.       1-1.2 million RGC axons and capillaries pass through the NRR.

8.       A mismatch between pallor and NRR margin can be seen in early glaucoma.

9.       NRR thinning is defined as generalized (diffuse) loss of NRR.

10.   Notching is focal loss of NRR.

11.   Notching usually corresponds to VF loss.

12.   Typically thinning of NRR occurs in the following sequence: infero-temporal > supero-temporal > temporal> inferonasal>superonasal>nasal.

Optic nerve head hemorrhage :

1.       ONHH or RNFL hemorrhage, splinter hemorrhage, Drance hemorrhage or peripapillary hemorrhage, is a splinter or flame shaped hemorrhage located on the disc margin or within 1DD of the margin in the RNFL. Likely from infarction of blood supply to the ONH. Usually on supero- or infero-temporal regions of the OD.

2.       ONHH common in NTG. Rare in advanced glaucoma due to insufficient NRR tissue.

3.       ONHH is a strong predictor of glaucoma progression.

4.       Patients with OHT, who develop an ONHH have higher risk of conversion to POAG.

5.       ONHH typically occurs in the region of NRR notching or RNFL defects.

6.       ONHH can precede notching, localized RNFL defects or VF loss.

7.       Usually transient (resolves in 6-10 weeks).

8.       Can be recurrent and usually in same position of the disc.

9.       ONHH more common in patients with large IOP fluctuations.

10.   Non-glaucomatous ONHH can occur in: Posterior vitreous detachment (PVD), Branch retinal vein occlusion (BRVO), diabetic retinopathy, hypertensive retinopathy, AION and anemic patients.

Laminar dot sign:

1.       LC is a part of sclera, forming the exit zone of RGCs or retinal nerve fibers.

2.       LC maintains the pressure gradient between IOP and the periocular pressure.

3.       Posterior bulging of the LC causes deformation of pores in the mesh like structure. Thus, compressing the nerve fibers and blood vessels traversing the laminar pores.

4.       The superior and inferior poles of the LC have the largest pores so there is less structural support. Thus, damage to the RGC axons is typically greater in the superior and inferior poles.

5.       Laminar dot sign= increased visibility of the pores in the LC due to loss of RGC axons.

6.       It can occur in healthy eyes (30% vs 70% in glaucomatous eyes).

7.       Laminar pores in healthy eyes are round, while in glaucoma are slit-like.

8.       The size and shape of LC pores may predict glaucomatous VF loss.

Parapapillary atrophy (PPA):

1.       Thinning and degeneration of chorioretinal tissue just outside the OD and exposure of the retinal pigment epithelium (RPE), which gives the tissue a moth-eaten appearance.

2.       PPA occurs due to ischemia in the peripapillary choroidal circulation and/or vascular deficiency in the ONH.

3.       PPA is irregular and patchy compared to the normal peripapillary choroidal crescents in high myopia and the inferior scleral crescent in eyes with tilted optic discs, where it is uniform in color and shape.

4.       PPA can also occur in normal eyes or those with OHT.

5.       Usually adjacent to areas of NRR thinning.

6.       PPA consists of= peripheral zone alpha (α) and central zone beta (β). In glaucoma patients zone β is more frequent and larger compared to healthy controls.

7.       Zone α is a non-specific finding.

8.       Zone β is characterized by visible underlying sclera and large choroidal vessels due to complete atrophy of choriocapillaries and RPE. Localized zone β is associated with focal VF defects as well as glaucomatous progression.

9.       Savatorsky et al reported PPA does not always correlate with glaucoma progression and zone α and zone β are not associated with progression in OHT patients.

Evaluation of RNFL and detection of RNFL defects:

1.       RNFL can be observed by red free light. Green light is absorbed by RPE and choroid, while it is reflected by the RNFL. Thus, a better visualization of the RNFL on a dark background is possible.

2.       Brightness and striations of the RNFL should be studied. In normal eyes parapapillary vessels are obscured by overlying healthy RNFL, but become visible in glaucomatous eyes.

3.       RNFL defects appear as loss of normal striations in the retina or darker zones in the areas of expected brightness, characteristic of healthy RNFL.

4.       In RNFL loss= Walls of peripapillary vessels appear clearer, redder and darker.

5.       RNFL defects often precede VF changes.

6.       RNFL defects caused by selective damage to superior and inferior arcuate bundles. Earliest to be seen in infero-temporal (IT) sector, followed by ST, SN and IN.

7.       Localized defects are easier to observe.

8.       Wedge defects more common than slit defects.

9.       RNFL defects follow arcuate pattern.

10.   Pseudo defects are thin and do not reach upto the OD.

11.   Diffuse RNFL defects can be identified by comparison of striations in the superior and inferior poles as well as nasal and temporal regions; increased prominence of peripapillary vessels; and clearer visualization of the underlying choroid.

Bean potting:

1.       Also called “shelving”.

2.       Occurs late in glaucoma (can occur in normal eyes)

3.       Vessels visibly enter the optic cup over the NRR, disappearing behind the excavated tissue and re-emerging at the bottom of the cup.

Baring of circumlinear vessels:

1.       Normally circumlinear vessels rest on NRR tissue within the disc.

2.       Baring of circumlinear vessels can occur normally and not pathognomonic of glaucoma.

3.       Possibly represents focal loss of NRR or notching.

4.       Clinically characterized as “suspending” or “hanging in mid air” of the vessels crossing over the optic cup due to lack of adjacent support of NRR in direct contact with them.

Nasalization of vessels:

1.       More obvious in advanced glaucoma.

2.       It is seen because the supporting NRR is only left in the nasal region after loss of NRR in the superior, inferior and temporal regions.

3.       Blood vessels commonly enter and leave the eye along the nasal border of the cup and any enlargement of the cup will further nasalize the vessels.

Retinal arteriolar attenuation:

1.       A focal constriction of retinal arterioles in the OD likely results from RGC loss and consequent reduced metabolic demand.

2.       Usually seen after development of a VF defect.

3.       It can also occur in other nerve pathologies like Non-Arteritic-AION (NA-AION).

Bayoneting sign:

1.       Retinal vessels deviate from their normal course with a sharp angulation of 90⁰ or more due to focal notching and significant NRR loss. Thus they travel along the bottom of the cup. They re-emerge at the edge of the rim from the deeply excavated cup with a sharp angle over the lip of the cup to pass over the disc margin.

2.       This gives the vessels a ‘z’ or ‘double angulation’.

3.       Usually associated with advanced cupping or localized NRR notching.

The 5 rules for evaluation of the optic disc in glaucoma diagnosis (Fingeret):

Observe the=

1.       Scleral ring to detect the borders and size of OD.

2.       Detect rim size.

3.       Evaluate RNFL.

4.       Evaluate PPA.

5.       Observe retinal and OD hemorrhages.

Hallmarks of correct evaluation of ONH in suspected glaucoma:

1.       Measurement of vertical ONH size at the slit lamp through dilated pupils.

2.       Assess the integrity of NRR and review ISNT rule.

3.       Explore for zone β or extensive zone α.

4.       Assess the ONH for vascular changes: ONHH, baring or nasalization of vessels, and arteriolar narrowing.

5.       Assess the peripapillary and retinal RNFL integrity and exploration of focal or diffuse RNFL defects with red-free filter.