Anatomía del ojo para el médico general - Dra. Paula Andrea Cabrera

Anatomy of the Eye: Understanding Ocular Structures

Introduction to Ocular Anatomy

• Paula Andrea Cabrera Laiseca introduces herself as a first-year ophthalmology resident at Pontificia Universidad Javeriana de Cali and outlines the session's focus on ocular anatomy. This knowledge is essential for interpreting clinical findings and understanding when to refer patients to ophthalmology.

General Characteristics of the Eye

• The axial length of the eyeball is approximately 24.5 mm, which can vary due to refractive conditions like hyperopia (shorter) and myopia (longer). Understanding these variations is crucial in clinical practice.

• The eye weighs about 7.55 g and serves as an interface between the external environment and the brain, with a direct connection through the optic nerve. This highlights its importance in sensory perception.

External Structures: Eyelids and Their Functions

• Eyelids serve mechanical, protective, and functional roles; they protect the eye, distribute tears, and contain various tissues including skin, subcutaneous tissue, and muscles such as orbicularis oculi and levator palpebrae.

• Tarsal plates provide shape to eyelids while housing Meibomian glands that are vital for tear film quality; eyelashes act as a barrier against debris entering the eye. These structures collectively enhance ocular protection.

Conjunctiva: Membrane Overview

• The conjunctiva is a transparent membrane covering both eyelids (palpebral conjunctiva) and eyeball surface (bulbar conjunctiva), facilitating smooth movement of eyelids over the eye while protecting it from pathogens. Color-coded diagrams help identify different parts of this structure: bulbar (yellow), fornix (red), palpebral (green).

Lacrimal System Components

Anatomy of the Eye and Its Functions

Glands and Their Functionality

• The normal pathways of the meibomian glands are visible, indicating good functionality. However, a marked tortuosity suggests an obstructive process affecting these structures.

External Structures of the Eye

• The caruncle or lacrimal caruncle is a raised conjunctival tissue located at the inner corner of the eye, sometimes referred to as a "third eyelid," representing an evolutionary remnant found in some species like felines and reptiles.

• While it can undergo inflammatory processes, it does not serve a specific function but is considered an evolutionary conjunctival remnant. The canthi (medial and lateral) provide anatomical reference points for normal symmetry in facial anatomy.

Lacrimal System Overview

• The lacrimal system has two main functions: secretion and drainage, involving specific anatomical structures. The secretory system includes the lacrimal gland and accessory glands such as Wolfring's gland and Krause's gland.

• For drainage, tears flow from the superior and inferior puncta into their respective canaliculi before reaching the common canaliculus, leading to the lacrimal sac and then through the nasolacrimal duct into the nasal cavity.

Detailed Drainage Mechanism

• Tears accumulate in both upper and lower puncta, draining through superior/inferior canaliculi into the common canaliculus before entering the lacrimal sac. They then pass through Rosenmüller's valve into the nasolacrimal duct, which measures approximately 12 to 18 mm in adults. This valve prevents reflux of tears back into canaliculi from the sac.

Extraocular Muscles

• There are six extraocular muscles responsible for eye movement: four rectus muscles (superior, inferior, medial, lateral) control primary movements while two oblique muscles (superior and inferior) assist with diagonal movements of gaze direction. Each muscle has a specific role in controlling eye motion effectively.

Structure of the Eyeball

• The eyeball consists of three tunics:

• Outer Tunic: Fibrous layer including cornea (transparent allowing light entry) and sclera (white protective outer layer).

Eye Anatomy and Function

Structure of the Eye

• The pupil functions as a stinker, regulating light entry into the eye. The inner layer of the eye is known as the nervous tunic, primarily represented by the retina, which contains photoreceptors (cones and rods) that convert light into nerve signals sent to the brain via the optic nerve, enabling vision.

Cornea: Key Features

• The cornea is a prominent, transparent dome at the front of the eye that allows light to pass through and helps focus it. It serves as the primary refractive medium with significant optical power.

• Composed of multiple layers, starting with:

• Corneal Epithelium: Acts as a protective barrier with rapid regeneration capabilities from the limbus inward.

• Bowman's Membrane: Provides support for deeper layers.

• Corneal Stroma: Accounts for about 90% of corneal thickness, crucial for transparency and strength.

• The innermost layer is:

• Descemet's Membrane: A robust membrane supporting the endothelium.

• Corneal Endothelium: Maintains dehydration in the stroma to ensure transparency; unlike epithelium, it does not regenerate throughout life.

Corneal Measurements

• In adults, typical corneal dimensions are approximately:

• Horizontal diameter: 11-12 mm

• Vertical diameter: 10-11 mm

• Central thickness ranges from 500 to 600 micrometers, increasing towards periphery.

• Refractive index is around 1.376, with an average radius of curvature at 7.8 mm, contributing to its total refractive power of approximately 43 diopters. The central cornea is typically three diopters more curved than peripheral areas, giving it its distinctive shape.

Sclera Overview

• The sclera forms part of the fibrous layer along with the cornea; it begins at the limbus (junction between sclera and cornea) and extends to the optic canal. It has both anterior visible parts and posterior sections not easily seen but essential for ocular protection.

Composition and Structure

• Made up mainly of collagen fibers interspersed with glycoproteins and proteoglycans; divided into three regions:

• Episclera: Outermost layer composed of loose connective tissue rich in blood vessels.

• Scleral Stroma: Contains dense elastic fibers arranged irregularly, resulting in an opaque appearance compared to transparent cornea.

• Lamina Fusca: Innermost layer connecting with vascular structures containing nerves and ciliary vessels; thicker anteriorly (1 mm) than posteriorly (0.3–0.45 mm).

Vascular Supply

• Two vascular plexuses exist within scleral structure:

• Superficial plexus provides blood supply near surface.

Understanding Epiescleritis and Scleritis

Vascular Structures in Eye Conditions

• The superficial vascular plexus has a radial configuration in epiescleritis, leading to maximum congestion at this level. This is crucial for diagnosis.

• A test using 2.5% or 10% phenylephrine evaluates the dilation of the superficial plexus; improvement in ocular redness indicates epiescleritis, while no improvement suggests deep vascular plexus involvement, indicating scleritis.

Characteristics of Scleral Congestion

• Deep vascular plexus dilation results in a more pronounced violaceous tone in patients with scleritis, especially under light exposure. The sclera itself is avascular and relies on diffusion from the episclera and choroid for metabolic needs.

• The collagen organization within the sclera contributes to delayed antigen elimination, making scleritis and epiescleritis potential indicators of autoimmune diseases due to their inflammatory nature.

Posterior Pole Anatomy

• The posterior pole merges with the dura mater and arachnoid membranes of the optic nerve, explaining why optic nerve edema and visual impairment can complicate posterior scleritis cases.

• Important structures such as blood vessels and nerves are located at various levels around the sclera's posterior face, which is vital for understanding ocular anatomy.

Anterior vs Posterior Chamber

• The anterior chamber lies between the back of the cornea and front of the iris (peripherally) or lens (centrally), with an average depth of about 3.1 mm; narrower chambers may increase risks for conditions like acute glaucoma due to aqueous humor drainage issues.

• Aqueous humor fills this chamber, providing eye tone and nourishment; it is primarily composed of water (99%) produced at a rate of 2 to 3 microliters per minute. Proper drainage through trabecular structures is essential for preventing glaucoma pathology.

Understanding Vitreous Body and Lens Structure

• The posterior chamber contains vitreous body gel that occupies most space behind the iris but before the lens; this gel maintains eye shape as a hollow spherical structure.

Understanding Cataracts and Eye Anatomy

The Role of the Lens in Vision

• The lens, known as the cataract, is part of the natural aging process. Its primary function is accommodation, allowing focus on distant and near objects.

• When focusing on distant objects, the lens flattens; conversely, it bulges to focus on nearby objects. This ability diminishes with age due to lens sclerosis.

Vascular Structures of the Eye

• The eye's vascular components include three structures: iris, ciliary body, and choroid. Each plays a crucial role in vision.

• The iris regulates light entry into the eye like a muscle sphincter. The ciliary body produces aqueous humor and aids in lens accommodation.

Vitreous Humor and Its Implications

• The vitreous humor is a gel-like substance that adheres strongly to key ocular structures. Changes in its composition can lead to posterior vitreous detachment.

• Posterior vitreous detachment is a common cause of floaters (myodesopsias), which often prompt ophthalmology consultations.

Risks Associated with Floaters

• While floaters are usually benign, they may indicate risks such as retinal tears or detachments if accompanied by flashes of light or changes over time.

• Once removed during procedures like vitrectomy, vitreous humor does not regenerate; artificial substances are used temporarily to maintain eye shape.

Retina Structure and Function

• The retina contains photoreceptors (rods and cones), essential for converting light into electrical signals for visual processing.

• Key areas within the retina include the macula and fovea, which have high concentrations of cones for sharp vision.

Importance of Retinal Anatomy

• The central area with many cones (the fovea) enhances visual acuity. Mitochondria support their metabolic needs due to their demanding function.

• Anatomical references include the optic nerve and vascular arcades; understanding these structures is vital for surgical approaches.

Evaluating Optic Nerve Health

• A critical structure during evaluation is the optic nerve head; normal excavation measures around 0.45 mm. Larger excavations may suggest nerve cell loss or conditions like glaucoma.

Visual Processing and Oculomotor Nerves

Visual Information Processing

• The occipital cortex is responsible for processing visual information, generating images that allow us to see.

Oculomotor Nerves Overview

• Oculomotor nerves are crucial for the innervation of extraocular muscles, enabling eye movement such as opening, closing, and shifting gaze. There are three main cranial nerve pairs involved in ocular motility.

Specific Cranial Nerves Involved

• The third cranial nerve (oculomotor) controls several eye movements by innervating the superior rectus, inferior rectus, medial rectus, and inferior oblique muscles.

• The fourth cranial nerve (trochlear) innervates the superior oblique muscle, allowing downward and inward gaze. The sixth cranial nerve (abducens) facilitates lateral eye movement through the lateral rectus muscle.

Blood Supply to the Eye

• The primary arterial supply to the eye comes from branches of the internal carotid artery, specifically the ophthalmic artery which divides into two significant branches: central retinal artery and ciliary arteries.

Central Retinal Artery Importance

• The central retinal artery supplies blood to the inner retina; its occlusion can lead to sudden severe vision loss due to lack of blood supply to this critical area. This is particularly relevant in patients with risk factors for vascular occlusions.

Ciliary Arteries Functionality

• Posterior ciliary arteries nourish structures like choroids and ciliary body while anterior ciliary arteries contribute to muscle function and peripheral circulation around the iris. These vessels play a role in conditions like posterior uveitis linked with autoimmune diseases.

Venous Drainage of the Eye

• Venous drainage occurs via the central retinal vein alongside its arterial counterpart; it drains into either superior ophthalmic vein or directly into cavernous sinus which has implications for intracranial communication and potential complications like fistulas.

Additional Venous Structures