Optic Disc Anatomy and Physiology Assessment

Optic Disc Assessment and Anatomy

• 30% of glaucoma patients have pressure within normal limits, and large numbers of fibers can be lost before a visual field defect can be demonstrated.
• Tools used to assess the optic disc: 78 or 90D lens and slit-lamp, and direct ophthalmoscope.
• The normal optic disc head is slightly vertically oval, and a normal cup often appears slightly horizontally oval.

Blood Supply to the Optic Nerve Head

• Short Posterior Ciliary Arteries
• Circle of Zinn
• Ophthalmic Artery
• Central Retinal Artery
• Lamina Cribrosa

Aqueous Fluid Dynamics and Glaucoma

• An imbalance in anterior fluid dynamics can give rise to increased IOP.
• Examples: angle closure/narrowing, pseudoexfoliation, PDS, anterior chamber inflammation, trauma.

Signs of Glaucoma

• Enlargement of the cup
• Localized loss of neuroretinal rim (NRR)
• Pallor
• Vessel deviations
• PPA (peripapillary atrophy) disc hemorrhages
• Nerve fiber layer defect
• Lamina cribrosa

Defining the Edge of the Cup

• The inner edge of the neuroretinal rim (=cup edge) may be sloped (especially on the temporal side of the disc) or vertical.
• Use small to medium-sized blood vessels to define the cup edge by tracing their path across the scleral ring and then over the rim tissue.

Additional Points

• Larger discs (>2mm) tend to display larger cup-disc ratios.
• Cupping in small discs increases the risk of BRVO (branch retinal vein occlusion).
• Proliferative diabetic retinopathy and carotid artery obstruction are related to CRAO (central retinal artery occlusion).

Angle Closure Glaucoma

• Primary: 0.5% prevalence in age >40, associated with pupil block, causing bowing of the iris forward leading to angle closure.
• Secondary: variety of mechanisms other than pupil block (not relieved by iridotomy/iridectomy).
• Refer all angle closure urgently, and consider referral of all angles of grade 1 (Van Herick) for gonioscopy assessment.

Visual Fields Revision

• Glaucomatous defect: look for corresponding optic nerve head changes.
• Exclude alternative diagnoses.
• NICE guidelines: look for unequivocal, suspicious, and early signs of glaucoma.

NICE Guidelines on Fields

• Threshold or suprathreshold: glaucomatous changes of the visual field that reflect nerve fiber bundle loss.
• Unequivocal signs: arcuate scotomas, nasal steps, altitudinal scotomata, focal defects, and absolute defects.
• Suspicious signs: generalized defect, relative defect, and enlarged blind spot.
• Early stages: mean defect >-6dB, 5% probability level defect for.