Ophthalmology: Eye Anatomy and Definition

Ophthalmology Course Overview

• Ophthalmology is the branch of medicine focused on the diagnosis, treatment, and prevention of eye diseases and disorders.
• Ocular anatomy includes the eyelids, lacrimal system, extraocular muscles, orbit and paranasal sinuses, visual pathways, and the eyeball.

The Eyelid

• The eyelid consists of several layers from superficial to deep: skin, subcutaneous tissue, striated muscle fibres of the orbicularis oculi, neurovascular bundle, and orbital septum tarsal plates.
• Meibomian glands are embedded in tarsal plates, with approximately 35 in the upper lid and 25 in the lower lid, producing a sebaceous secretion.
• Glands of Moll are modified sweat glands.
• The conjunctiva becomes continuous with the keratinized skin epithelium along the posterior margins of the tarsal gland openings.
• The grey line is a tissue plane where the eyelid may be split into anterior and posterior lamellae.

Chalazion

• Chalazion is a chronic sterile lipogranulomatous inflammation due to obstruction of Meibomian glands.
• Causes include diabetes mellitus, refractive error, acne rosacea, chronic blepharitis, immune-compromised state, and Down syndrome.
• Outcomes include resolution (30%), recurrent or chronic inflammation, secondary infection, and granuloma formation.
• Treatment includes conservative measures, lid massage with hot fomentation, antibiotics to prevent secondary infection (e.g., oral Doxycycline 100mg BD), weak steroids (topical or intralesional), and surgical removal via incision and curettage.

Trachoma

• Trachoma is a chronic infectious keratoconjunctivitis caused by Chlamydia, that can lead to blindness.
• Chlamydia species A, B, and C cause trachoma, while D and E are associated with genital infections.
• Transmission occurs via the 5 Fs: Flies, Feces, Fingers, Fomites, and Faces.

WHO Trachoma Classification

• TF: Trachoma Follicle
• TI: Trachoma Intense
• TS: Trachoma Scarring
• TT: Trachoma Trichiasis
• TO: Trachoma Opacity
• Trichiasis is the misdirection of eyelashes inward or outward, inward is entropion and outward is ectropion.

Management of Trachoma

• Prophylactic management follows the WHO SAFE Strategy:
  • S: Surgical treatment for trichiasis and entropion
  • A: Antibiotics
  • F: Face and Hand washing
  • E: Environmental eradication of all family
• Curative treatment involves first-line tetracycline or a single oral dose of Azithromycin 1g.

Conjunctiva

• The conjunctiva is a highly vascularized mucus membrane composed of non-keratinized epithelium that protects the anterior surface of the eye.

Conjunctivitis

• Non-infectious conjunctivitis involves inflammation of the conjunctiva without microorganisms and is commonly allergic.
  • Acute allergic conjunctivitis is usually self-limited; antihistamines can be given if persistent.
  • Chronic allergic conjunctivitis lasts more than 2 weeks.
  • Symptoms include itching, lacrimation, foreign body sensation, and redness.
• Infectious conjunctivitis involves inflammation due to microorganisms (bacteria, fungi, viruses).
  • Bacterial conjunctivitis is caused by staph, strep pneumo, H.I, and Niessera Gonhorrea, leading to pus formation.
  • Purulent discharges indicate Gonococcal infection.
  • Viral conjunctivitis is commonly caused by Adenovirus and HSV, with watery discharges as the main symptom, and is often self-limited.

Management of Conjunctivitis

• Empirical antibiotics can be given.
• A sample can be taken by scraping or swapping the conjunctiva for culture and sensitivity testing.

The Lacrimal System

• The lacrimal gland is a bilobed gland without a definitive capsule, located superotemporally in the orbit behind the orbital septum.
• The canaliculi are L-shaped structures with a short (2 mm) vertical and a longer (10 mm) horizontal portion.
• The lacrimal sac lies in the lacrimal fossa formed by the lacrimal bone, frontal process of maxilla, and lacrimal fascia.
• The nasolacrimal duct, a membranous structure, passes downwards, backwards, and laterally to enter the inferior meatus of the nose.

Extraocular Muscles

• Rectus muscles and oblique muscles (superior and inferior) are types of extraocular muscles.
• All rectus muscles originate from the annulus of Zinn, situated at the orbital apex.
• Insertion distances from the corneal limbus vary: medial rectus (5.5 mm), inferior rectus (6.5 mm), lateral rectus (6.9 mm), and superior rectus (7.7 mm).
• These insertions form the spiral of Tillaux.
• Tendon lengths vary: medial rectus (3.7 mm), lateral rectus (8.8 mm).
• The superior oblique muscle originates from the sphenoid bone and passes through the trochlea at the superonasal aspect of the orbit.
• The inferior oblique arises from within the floor of the orbital rim.
• Both oblique muscles pass inferiorly to their corresponding rectus muscle before inserting posterior to the equator of the globe.

Innervation of Extraocular Muscles

• The third cranial (oculomotor) nerve supplies the majority of the extraocular muscles, dividing into superior and inferior divisions just before entering the orbit.
  • The inferior division supplies the medial rectus, inferior rectus, and inferior oblique.
  • The superior division supplies the superior rectus and levator palpebrae superioris.
• The superior oblique muscle is supplied by the fourth cranial nerve.
• The sixth cranial (abducent) nerve supplies the lateral rectus.
• The primary blood supply of the extraocular muscles comes from the muscular branches of the ophthalmic, lacrimal, and infraorbital arteries.

The Orbit and Paranasal Sinuses

• The bony orbit is a pear-shaped cavity formed by seven individual bones.
  • The thin orbital floor is primarily the orbital plate of the maxilla with contributions from the zygomatic and palatine bones.
  • The lateral wall is formed by the zygomatic bone and greater wing of the sphenoid posteriorly.
  • The orbital roof is formed predominantly by the orbital plate of the frontal bone with contributions from the lesser wing of the sphenoid bone.
  • The medial wall is formed of the maxilla, lacrimal, ethmoid, and body of the sphenoid.
• All sinuses are lined by a pseudostratified columnar ciliated epithelium, that aids the passage of mucus into the nasal cavity.