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Ocular System

Open-angle glaucoma (OAG)

Progressive optic neuropathy characterized by acquired loss of retinal ganglion cells and atrophy of the optic nerve.

Progressive optic neuropathy characterized by acquired loss of retinal ganglion cells and atrophy of the optic nerve.


Etiology

Major risk factors:

  • Elevated intraocular pressure (IOP)
  • Relatively thin central cornea
  • Insults to the eye: Trauma, uveitis, and steroid therapy.

Other risk factors:

  • Old age
  • Black race
  • Positive first-degree family history
  • Diabetes mellitus
  • Elevated systolic blood pressure
  • Migraine

Pathophysiology

Aqueous Humor Drainage Pathways of Healthy and Glaucomatous Eyes | Weinreb, R. N., Aung, T., & Medeiros, F. A. (2014). The pathophysiology and treatment of glaucoma: a review. JAMA, 311(18), 1901–1911. https://doi.org/10.1001/jama.2014.3192

Optic nerve atrophy:

Schematic Illustration of Normal Anatomy and Neurodegenerative Changes Associated With Glaucomatous Optic Neuropathy: A, The optic disc is composed of neural, vascular, and connective tissues. The convergence of the axons of retinal ganglion (RG) cells at the optic disc creates the neuroretinal rim; the rim surrounds the cup, a central shallow depression in the optic disc. Retinal ganglion cell axons exit the eye through the lamina cribrosa (LC), forming the optic nerve, and travel to the left and right lateral geniculate nucleus, the thalamic relay nuclei for vision. B, Glaucomatous optic neuropathy involves damage and remodeling of the optic disc tissues and LC that lead to vision loss. With elevated intraocular pressure, the LC is posteriorly displaced and thinned, leading to deepening of the cup and narrowing of the rim. Distortions within the LC may initiate or contribute to the blockade of axonal transport of neurotrophic factors within the RG cell axons followed by apoptotic degeneration of the RG cells. Strain placed on this region also causes molecular and functional changes to the resident cell population in the optic nerve (eg, astrocytes, microglia), remodeling of the extracellular matrix, alterations of the microcirculation and to shrinkage and atrophy of target relay neurons in the lateral geniculate nucleus. | Weinreb, R. N., Aung, T., & Medeiros, F. A. (2014). The pathophysiology and treatment of glaucoma : a review. JAMA, 311(18), 1901–1911. https://doi.org/10.1001/jama.2014.3192

Presentation

  • Progressive loss of peripheral vision (typical visual field changes) followed by central visual field loss (blindness) (due to retinal ganglion cell loss)
  • Frequent changes in presbyopic glasses (d/t increasing accommodative failure as a result of constant pressure on ciliary muscle & nerve supply)

Diagnosis

Direct ophthalmoscopy:

Loss of optic nerve tissue results in excavation or “cupping” of the optic nerve head, which is best viewed by direct ophthalmoscopy.
  • ↑ Cup-to-disc ratio (vertical ratio ≥ 0.6)
  • Substantial cup-to-disc asymmetry between both eyes (≥ 0.2)
  • Thinning & notching of neuroretinal rim
  • Flame-shaped disc haemorrhages
  • Peripapillary atrophy
  • Laminar dot sign: Pores in lamina cribrosa are slit shaped n visible upto margin of disc.
  • Marked cupping “Beanpot cupping”

Perimetry:

Computer-based test that provides a printout of the visual fields, is a mainstay of glaucoma diagnosis and management
  • Pattern of visual field loss:
    1. Isopter contraction
    2. Barring of blindspot
    3. Relative paracentral scotoma
    4. Siedel scotoma
    5. Arcuate scotoma “Bjerrum’s scotoma”
    6. Double arcuate or ring scotoma
    7. Tubular/tunnel vision and temporal island of vision
    8. Temporal island of vision
    9. Loss of perception of light
Glaucomatous field defects in left eyes | ICEH
Normal, Glaucomatous, and Severe Glaucomatous Optic Nerve Heads and Visual Field Test Results: A, The pink area of neural tissue forms the neuroretinal rim, whereas the central empty space corresponds to the cup. B, Glaucomatous optic nerve showing loss of superior neural retinal rim (thinning) and excavation with enlargement of the cup. The arrowheads point to an associated retinal nerve fiber layer defect, which appears as a wedge-shaped dark area emanating from the optic nerve head. The superior neural losses correspond to the inferior defect (black scotoma) seen on the visual field. There is also a small retinal nerve fiber layer defect inferiorly, but the corresponding hemifield of the visual field remains within normal limits. C, More extensive neural tissue loss from glaucoma with severe neuroretinal rim loss, excavation, and enlargement of the cup. There is severe loss of visual field both in the superior as well as in the inferior hemifield. | Weinreb, R. N., Aung, T., & Medeiros, F. A. (2014). The pathophysiology and treatment of glaucoma: a review. JAMA, 311(18), 1901–1911. https://doi.org/10.1001/jama.2014.3192

Optical coherence tomography (OCT):

Imaging Assessment of the Optic Nerve and Retinal Nerve Fiber Layer Using Spectral-Domain Optical Coherence Tomography: A, The arrowheads point to a retinal nerve fiber layer (RNFL) defect. B, Areas of thicker RNFL appear in yellow and red. Arrowheads point to the RNFL defect. A deviation map compares the RNFL thickness values with a normative database and highlights the defect. E, Arrowheads point to a visual field defect. | Weinreb, R. N., Aung, T., & Medeiros, F. A. (2014). The pathophysiology and treatment of glaucoma: a review. JAMA, 311(18), 1901–1911. https://doi.org/10.1001/jama.2014.3192

Shaffer system:

Describes the degree to which the anterior chamber angle (ACA) is open
Modified from a figure by T.Tarrant. | Gonioscopy Simulation
Anterior chamber angle (ACA) grading system as described by Shaffer. | Evaluation of Nasal and Temporal Anterior Chamber Angle with Four Different Techniques – Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/ACA-grading-system-as-described-by-Shaffer_tbl2_276494875 [accessed 19 Nov, 2020]

Management

Slowing disease progression and preservation of quality of life are the main goals for glaucoma treatment.

Non-pharmacological managemnet:

  • Regular aerobic exercise (help lower intraocular pressure)

Pharmacological management:

Reduction of intraocular pressure (IOP)
  • β-Adrenergic blockers: Timolol, levobunolol, carteolol, metipranolol, betaxolol
    • Reduction of aqueous humor production
  • Prostaglandin analogues (prostamide): Latanoprost, travoprost, tafluprost, unoprostone, bimatoprost
    • Increase in uveoscleral outflow of aqueous humor
  • α-Adrenergic agonists: Brimonidine, apraclonidine
    • Initial reduction of aqueous humor production with subsequent effect of increase in outflow
  • Carbonic anhydrase inhibitors: Dorzolamide, brinzolamide, acetazolamide (oral)
    • Reduction of aqueous humor production
  • Cholinergic agonists: Pilocarpine, carbachol
    • Increase in aqueous humor outflow

Surgical management

Indicated when target intraocular pressure cannot be reached medically, when optic nerve damage progresses despite achieving intraocular pressure goals with maximal medical therapy, or when patient is unable to comply with or tolerate medical therapy
  • Laser trabeculoplasty
  • Surgical trabeculectomy

Triple surgery procedure:

  • Trabeculotomy
  • Extracapsular cataract extraction (ECCE)
  • Posterior chamber intraocular lens (PC-IOL) implantation

Glaucoma drainage devices (GDDs):

Create alternate aqueous pathways by channeling aqueous from anterior chamber through a long tube to an equatorial plate that promotes bleb formation
  • Indications:
    • Glaucoma not responding to medications or trabeculectomy operations
    • Neovascular glaucoma
    • Iridio-corneal syndrome
    • Penetrating keratoplasty with glaucoma
    • Glaucoma following retinal detachment surgery

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