Human Eye Structure and Function

Eye Structure and Function

• The eye is like a ball with most of it sitting in the orbit.
• The eye is cushioned by orbital fats and fibrous tissues, and moved by extraocular muscles.
• The eye has a multi-layered wall, and the space inside contains the lens and fluids (vitreous and aqueous humor).
• The eye detects light and converts it into electro-chemical impulses transmitted to the brain via the optic nerve and visual pathway to the visual cortex.

Structure of the Eye

• The wall of the eye has three layers: outer (cornea and sclera), middle (iris, ciliary body, and choroid), and inner (retina).
• The eye has three types of tissue: refracting (focus light), light-sensitive (detect light), and support (structural).

Clinical Examination of the Eye

• Visual acuity (VA) testing measures the smallest object resolvable by the eye at a given distance.
• VA is expressed as a fraction (e.g., 6/6, 6/18).
• Low-vision testing involves measuring the distance at which a patient can see the largest optotype.

External Head and Face Examination

• Inspect the patient for disturbances in head and face bony structures, skin, lymph nodes, mouth, eyelids, lacrimal system, and globe position.

Pupil Examination

• Examine the pupil's size, shape, and reaction to light.
• A normal response is constricted pupils in response to light.

Anterior Segment Examination (Slitlamp)

• Use a slitlamp to visualize the anterior segment of the eye, including the cornea, conjunctiva, sclera, anterior chamber, iris, lens, and vitreous.

Posterior Segment Examination (Fundoscopy)

• Examine the posterior segment of the eye, including the retina, optic nerve, and retinal blood vessels.

Intraocular Pressure (IOP) Measurement

• Measure IOP using applanation tonometry or indentation tonometry.
• IOP varies in the population, with a mean of 16 mmHg and a standard deviation of 3 mmHg.

Ocular Motility Testing

• Examine ocular motility by asking the patient to follow a target with their eyes.

Refraction and Best-Corrected Visual Acuity (BCVA)

• Measure refraction using automated or manual methods.
• BCVA is the best possible visual acuity with corrective lenses.

Diseases of the Ocular Adnexa

• The ocular adnexa include the eyelids, lacrimal gland, and orbit.
• Diseases of the ocular adnexa can affect the eye's protective structure and function.

Orbital Diseases

• Orbital diseases can cause proptosis, which is an abnormal protrusion of the globe due to increased intraorbital pressure.
• Causes of proptosis include congenital, acquired, endocrine, traumatic, inflammatory, neoplastic, and vascular disorders.

Workup of a Case of Proptosis

• Take a thorough history and examination, including measuring the degree and direction of proptosis.
• Investigations may include lab tests, imaging (CT or MRI), and biopsy.Here are the study notes for the given text:
• Diseases of the Ocular Adnexa*

Thyroid Eye Disease (TED)

• Age/sex: Middle-aged females
• Risk factors: Smoking, history of radioactive iodine treatment for thyroid dysfunction
• Pathogenesis: Autoimmune disease with infiltration of the orbit by plasma cells and lymphocytes, leading to extraocular muscle enlargement and fibrosis
• Manifestations:
  • General: Symptoms and signs of hypo/hyperthyroidism
  • Ocular:
    • Hyperemia along the recti muscles insertions
    • Lid signs: lid retraction (Dalrymple sign), lid lag on downgaze (Von Graefe's sign), infrequent blinking (Stellwag sign)
    • Restrictive stage: Proptosis and limited motility (diplopia)
• Complications:
  • Exposure keratitis
  • Compressive optic neuropathy
  • 2ry glaucoma
• Investigation:
  • Thyroid function test (T3, T4, TSH)
  • CT, MRI: EOM enlargement
• Treatment:
  • Treatment and prophylaxis of exposure keratitis
  • TTT of thyrotoxicosis (may not improve)
  • Infiltrative stage: systemic steroids, immunosuppressive therapy, and radiotherapy
  • Surgical: orbital decompression if severe exposure or compressive optic neuropathy
  • EOM recession if diplopia in primary or reading position (+/- relieving prisms)

Orbital Cellulitis

• Incidence: Main cause of unilateral proptosis in children
• Source of infection:
  • Sinusitis (especially ethmoidal)
  • Trauma: surgical, penetrating trauma
• Symptoms:
  • General: Fever, headache, anorexia, malaise
  • Ocular: Severe pain, hot and tender skin
• Signs: Signs of orbital disease
• D.D: Preseptal cellulitis: no ophthalmoplegia, no proptosis
• Complications:
  • Spread: eye (panophthalmitis), brain (cavernous sinus thrombosis, meningitis, brain abscess)
  • Optic neuritis and CRVO
  • Exposure keratitis
  • Localization: orbital abscess and fistula
  • Healing by fibrosis: Enophthalmos with restricted motility (frozen orbit)
• Investigation:
  • CBC: leukocytosis
  • CT scan: detect sinusitis and abscess localization
• Treatment:
  • Treatment and prophylaxis of exposure keratitis
  • Hospitalization
  • Antibiotics (IV or IM broad-spectrum G+/G- and anaerobes)
  • Hot fomentations and analgesia
  • Surgical: if abscess—drain

Cavernous Sinus Thrombosis (CST)

• Etiology:
  • Infection in the dangerous area of the face (angular vein connected to ophthalmic vein)
    • Lid: stye
    • Orbit: cellulitis
    • Acute dacryocystitis
    • Globe: panophthalmitis
• CST appears as orbital cellulitis or complicates orbital cellulitis, but differs in:
  • Associated with more general symptoms and drowsiness
  • Severe congestive signs
  • Other eye affection (starting by 6th nerve palsy—diplopia and esotropia)
  • Mastoid edema due to emissary vein affection
  • 25% mortality rate (brain abscess, septicemia, and meningitis)

Mucormycosis

• Epidemiology:
  • Associated with diabetes mellitus (especially with diabetic ketoacidosis)
  • Other associated conditions: neutropenia, hematologic malignancy, chronic steroid or immunosuppressive drug use, history of transplant, and history of multiple blood transfusions
• Diagnosis:
  • Signs and Symptoms:
    • Initial symptoms of sinus-involving mucormycosis are consistent with acute or chronic sinusitis
    • Involvement of the orbit is heralded by findings of chemosis, proptosis, extra-ocular motility deficits, and multiple cranial neuropathies, and loss of vision
    • Direct visualization of the nasal mucosa and para-nasal sinuses reveals dark, necrotic tissue and a characteristic black eschar
  • Investigation:
    • CT of the orbits and/or para-nasal sinuses can show contrast-enhancing hypodense soft-tissue thickening of the involved sinuses
    • Definitive diagnosis of mucormycosis can be made via histopathologic evaluation of tissue via biopsy or scraping
• Management:
  • General treatment:
    • Early recognition and initiation of appropriate treatment is very important
    • Urgent correction of underlying metabolic derangements such as hyperglycemia and metabolic ketoacidosis, and tapering of corticosteroids or immunosuppressive medications
    • The mainstay of medical treatment remains intravenous anti-fungals

Anatomy and Physiology of the Eyelids

• Functions of the eyelids:
  • Maintenance of ocular surface integrity and protection of the eyes from injury and excessive light
  • Mechanical barrier to a variety of insults
  • Sweeping mechanism to remove debris from the cornea (blink reflex) and spread tears over the surface of the eyes
  • Contribution to the production (Meibomian gland function) and drainage (lacrimal pump and capillarity of the lid margin) of the tear film
• Gross anatomy:
  • Position: In the primary position, the upper lid covers the upper 1/6 of the cornea, and the lower lid is at the level of the lower limbus
  • Palpebral fissure: Opening of the two lids being separate apart
  • Canthi: Angles of the palpebral fissure
  • Upper lid crease: Develops as the upper lids rise due to the attachment of the levator muscle
• Minute anatomy:
  • Skin
  • Subcutaneous areolar tissue
  • Striated muscle layer
  • Sub-muscular space
  • Septo-tarsal layer
  • Palpebral conjunctiva
• Blood supply:
  • Arteries: Medial and lateral palpebral arteries from the ophthalmic and lacrimal arteries
• Lymphatic drainage:
  • Lateral: pre-auricular LN
  • Medial: submaxillary LN
  • Then both in deep cervical LN
• Nerve supply:
  • Motor: orbicularis oculi (VII cranial nerve), levator palpebrae superioris (III cranial nerve), Muller's muscle (sympathetic fibers)
  • Sensory: branches of the trigeminal nerve (V)### Lid Hygiene and Treatment
• Apply warm compresses for 30 minutes, twice daily
• Topical treatments: antibiotics, lubrication, and tear substitutes; corticosteroids for severe inflammation
• Oral tetracyclines: doxycycline 100 mg once daily
• Dietary recommendations: high omega-3 fats for ocular surface health

Abnormalities of Eyelashes

• Trichiasis: abnormal direction of lashes, normal position
• Distichiasis: extra row of lashes behind the grey line, arising from Meibomian gland orifices
• Metaplastic lashes: abnormal position of lashes

Acquired Trichiasis

• Types:
  • Rubbing lashes (4 lashes or less)
  • Trichiasis (more than 4 lashes rubbing against the cornea and conjunctiva)
• Etiology:
  • Trachoma
  • Lid margin inflammation
• Clinical picture:
  • Corneal symptoms: foreign body sensation, lacrimation, photophobia, and blepharospasm
  • Redness of the eye: conjunctival hyperemia
  • Signs: misdirected lashes, conjunctival hyperemia, trichiasis-entropion

Complications

• Chronic conjunctivitis
• Corneal ulceration, opacification, vascularization, keratinization, and epithelial plaque formation

Treatment

• Rubbing lashes (Less than 4 lashes):
  • Epilation (temporary)
  • Destruction of the hair follicle by laser photocoagulation, cryotherapy, thermal coagulation, or electrolysis
• Trichiasis (more than 4 lashes):
  • Trichiasis alone:
    • Upper lid: Van-Millengen's operation (displace rubbing lashes away from the eyeball)
    • Lower lid: Webster's operation (putting a mucous membrane graft in the sulcus subtarsalis)
  • Trichiasis with entropion:
    • Upper lid: Snellen's operation
    • Lower lid: Webster's operation
  • Trans-positioning of the lid skin by Z-plasty

Lid Position Abnormalities

• Ectropion:
  • Definition: Rolling outward of the lid margin
  • Types:
    • In the upper lid:
      • Cicatricial (scarring of the anterior lamella)
    • In the lower lid:
      • Cicatricial (scarring of the anterior lamella)
      • Senile (involutional)
      • Paralytic
      • Congenital
• Clinical picture:
  • Symptoms: watering of the eye, bad cosmetic appearance
  • Signs: exposure of the punctum, palpebral conjunctiva, bulbar and forniceal conjunctiva
• Complications:
  • Epiphora
  • Eczema
  • Exposure keratitis

Entropion

• Definition: Rolling inward of the lid margin
• Types:
  • In the upper lid:
    • Cicatricial (scarring of the posterior lamella)
  • In the lower lid:
    • Cicatricial (scarring of the posterior lamella)
    • Senile (involutional)
    • Spastic
    • Congenital
• Clinical picture:
  • Symptoms: as for trichiasis
  • Signs: rolling inward of the lid margin, signs of the cause of trichiasis
• Complications: as for trichiasis

Blepharoptosis

• Definition: Drooping of the upper lid more than 2 mm
• Etiology and classification:
  • According to the site of primary pathology along the pathway from the nerve supply of the levator muscle to its insertion at the lid
  • Dysgenetic, aponeurotic, neurogenic, myogenic, and mechanical
• Clinical assessment:
  • History: duration, variability, and impact on vision
  • Examination:
    • Facial appearance
    • Degree of ptosis
    • Levator function
    • Upper lid crease height
    • Extraocular movements
    • Pupil
    • Visual acuity
  • Protective mechanisms: Bell's phenomenon, corneal sensation, lid closure for lagophthalmos

Management of Ptosis

• Treatment of acquired ptosis:
  • Traumatic ptosis: wait 6 months for regeneration of 3rd nerve fibers or recovery of lacerated levator muscle and absorption of edema
  • Mechanical ptosis: removal of the cause
  • Paralytic ptosis: control the underlying etiology and squint surgery
  • Involutional aponeurotic ptosis: aponeurotic surgery
  • Myogenic ptosis: medication control (prostigmine)
• Treatment of congenital ptosis:
  • Timing: early (severe ptosis with fear of amblyopia) or late (less severe ptosis)
  • Type of surgical intervention: according to the levator function, either levator resection or frontalis sling