Ocular Pathologies - Overview
Ocular Pathologies
Overview
Key Pathologies
- Here, we will learn key pathologies of the eye.
- First, let's organize some of the pathologies as they affect the layers of the eye.
- We'll use a sagittal view of the eye.
Cornea
Anatomy
- The outermost layer of the spherical-shaped eye: the corneoscleral coat as follows
- Anteriorly is the cornea, which has a pronounced curvature and is transparent.
- Where the cornea ends, the outer layer becomes the sclera, which is opaque, so it blocks the transmission of light.
Pathologies
Trachoma
- Stems from chlamydia trachomatis infection, and is the second-most worldwide cause of blindness (second only to cataract).
Corneal Dystrophy
- Corneal dystrophy comprises a group of hereditary disorders that involve an abnormal accumulation of substance in the bilateral corneas, clouding vision.
Uvea
Anatomy
The uvea comprises:
- The pigmented iris.
- Ciliary body.
- And the choroid, which is a thin, brown, highly vascular layer sandwiched between the sclera and retina.
Pathologies
Uveitis
- Inflammation within the uveal layer; we can further sub-categorize it based on its anatomical location: eg, anterior uveitis affects the iris (aka iritis).
Retina
Anatomy
- The retina is internal to the choroid.
- It transitions into optic nerve when it exits the eye, posteriorly, at the lamina cribrosa.
Pathologies
Retinitis
- Retinal inflammation manifests with retinal edema and necrosis; it's commonly due to viral infection (we address additional retinal disorders below).
Optic Refraction
Key pathologies of optic refraction:
Ciliary body anatomy
- Bbiconvex lens.
- Tthe ciliary body anchors suspensory ligaments, collectively called zonule, which stretch the lens and alter its refractive power.
Physiology
- Normally, light rays enter the eye and bend as they pass through the cornea to ultimately merge at the fovea centralis of the macula.
- Remember, that the cornea (not the lens) has the greatest impact on optic refraction of any eye structure.
Pathologies
Hyperopia
- Objects focus behind the retina (the eyeball is too short).
Myopia
- Objects focus in front of the retina (the eyeball is too long).
Astigmatism
- Unequal curvatures in various parts of the cornea produce variability in optic refraction, which manifests with image blurring (irrespective of object distance).
Presbyopia
- Refractive error from loss of lens elasticity (lens hardening). This is an-age related process; individuals use "reading glasses" to help focus on near objects.
Retinal Disorders
See: Retinal Pathologies
Central Retinal Artery Occlusion
- Central retinal artery and vein pierce the optic nerve and run through its center.
- Central retinal artery occlusion = sudden monocular vision loss: a "cherry-red" spot in the macula.
- Retinal vein occlusion = there's retinal hemorrhage and venous engorgement.
Papilledema
Pressure along the optic nerve
- The meningeal layers transmit along the optic nerve. This is critical to understanding how increased intracranial pressure affects the optic nerve.
- The sclera transitions into:
- Dura mater (aka dural sheath)
- Arachnoid mater (aka arachnoid sheath)
- Pia mater lies along the surface of the optic nerve.
- The subarachnoid space lies between the arachnoid mater and pia mater, which allows increased intracranial pressure to translate along the optic nerve and impair its axoplasmic transport.
Papilledema
- Occurs from increased intracranial pressure.
- It refers to disc edema in the setting of increased intracranial pressure.
- We show the retina of the right eye as a reddish circle: this is how we see it on ophthalmoscopy, called "fundoscopy", because we are looking at the posterior aspect of the eye: the fundus.
- On the nasal side, we show the optic disc (aka the optic nerve head), which comprises the neuroretinal rim (which is pink) and the optic cup, a pale hole through which the central retinal vessels emanate.
- On the temporal side, we show the macula: in its center lies the fovea centralis.
- Papilledema: optic disc swelling manifests with an enlarged blind spot (the optic nerve head forms the blind spot in the visual field).
Glaucoma
- Optic disc atrophy manifests with thinning of the optic nerve head, typically from increased intraocular pressure (we address it in detail at the end).
Macular degeneration
- Loss of central vision.
- Divide macular degeneration into:
- Dry (nonexudative) degeneration for the slow deposition of extracellular material (it's yellowish on exam)
- Wet (exudative) degeneration, which occurs from choroidal bleeding with rapid loss of vision.
Retinal detachment
- Manifests with crinkling of the fundus; the patient experiences loss of vision like a "curtain being drawn down."
Diabetic retinopathy
- Proliferative, in which chronic hypoxemia causes new vessel formation and retinal traction.
- Nonproliferative, in which damaged capillaries promote retinal hemorrhage.
Superficial Eye
Anatomy
Superficial structures of the eye:
- The palpebra.
- Iris.
- Pupil.
- The conjunctiva refers is the portion of the sclera that we can see (the "white of the eye"), so show that conjunctivitis refers to inflammation of the "white of the eye".
- It's most often caused from viral infection or allergic reaction (especially when bilateral).
Cataract
- We show a cataract, seen through the pupil, in the lens.
- Cataracts are the worldwide leading cause of blindness; they first produce hazy vision, reduced color intensity, increased glare, and reduced visual acuity and can progress to blindness.
Horner's syndrome
- P.A.M., for:
- Ptosis - eyelid droop
- Anhidrosis - loss of sweating
- Miosis - pupil constriction
We address its pathophysiology in detail elsewhere.
Glaucoma
Finally, let's address the pathophysiology of glaucoma via a diagram of aqueous humor production.
- The space between the lens and iris is key to the pathophysiology of glaucoma.
- The ciliary process actively secretes aqueous humor into the posterior chamber (the space between the lens and iris).
- The aqueous humor then flows through the pupil into the anterior chamber (the space between the iris and cornea) where it is reabsorbed at the the trabecular meshwork and canal of Schlemm, which lies within the sclera at the corneoscleral junction.
Glaucoma
- In narrow (or closed) angle glaucoma, the rarer form, there is apposition of key anterior structures, which trap the flow of aqueous humor.
- For instance, as we show here, there is abutment of the lens and iris and fluid is obstructed (alternatively, the iris could abut the cornea).
- Sudden closed angle glaucoma is painful and cause reddening of the eye – epinephrine will worsen the condition due to its mydriatic effect on the iris.
- In open angle glaucoma, fluid flows freely in between the iris and lens but its reabsorption is blocked via poor uptake at the trabecular meshwork or canal of Schlemm, such as from uveitis.