Lacrimal Flashcards
What are the three components of the lacrimal system?
secretory, distribution, elimination
what does decretory system of lacrimal system do?
produce majority of tear fluid - aqueous
divided into 2 portions/lobes by levator aponeurosis - orbital lobe and palpebral
lobes connected by ducts to open into conjunctival fornix.
What is a tubulo-acinar gland? What is the relationship to intralobular ducts?
Tubulo-acinar gland contain many acini (clusters of secretory cells) which join to form intralobular ducts.
Intralobular ducts then drain the lacrimal gland into the surface of the eye.
What is the purpose of the accessory lacrimal glands and where are they located?
The accessory lacrimal glands produce 10% of the aqueous production
Krausse - fornix
Wolfring – along orbital margin of tarsal plate
How is the lacrimal gland innervated?
Autonomic input
Parasympathetic
Sympathetic
Sensory input
Via lacrimal nerve (branch of trigeminal nerve)
What are anatomical structures surrounding the lacrimal sac?
Horner’s muscle and medial palpebral ligament surround the lacrimal sac. When horner’s muscle contracts, pushes tears out of lacrimal sac past the Hassner’s valve
What is the importance of the hydrophilic nature of mucins?
Helps to wet the ocular surface
What are the functional roles of the tear film?
Osmotic boundary – maintaining corneal hydration
Gas, nutrient, and waste solvent for exchange with avascular cornea
Optical Boundary
Lubricant
Protection - antimicrobial
How do you maintain tear film stability?
temperature, humidity, gland outputs and blink rates
What is a result from lack of tear film stability?
Short tear film break up time
Keratoconjunctivitis Sicca (dry eye)
What are the reasons for tear secretion?
Basic secretion
No obvious external stimuli
Reflex secretion
Tear production when external stimuli lead to c
increased tear production (irritation)
Emotional/psychogenic secretion
Discuss the three components of the tear film and secretions?
Mucin
Goblet cells in conjunctival epithelium – stored in
secretory granules within goblet cells (crypts of
Henle – pockets of mucin secreting cells present)
Maintain hydration and lubrication of cornea and
conjunctiva, protect ocular surface by coating
foreign bodies and helps wet ocular surface
(hydrophilic)
Aqueous
Secreted by lacrimal gland (accessory glands of
krausse and wolfring)
Makes bulk of tear film, maintains pH and
osmolaritiy of tears, provides oxygen and
nutrients and waste removal for cornea and
antibacterial components
Lipid
Secreted by meibomian glands (zeis and moll)
Reduces evaporation stabilising tear film, prevents
overflow of ears, smooth optical surface, prevents
contamination of tear film by skin lipids
What is the difference between basic and reflex secretions?
Reflex Secretions
Due to external stimulus (initiates neural signals)
Increases tear production
Can produce up to 100x tear volume
Main secretors of lacrimal gland – main lobe and
accessory lobe
Basic Secretions
Natural secretory
No obvious external stimuli
What are the microscopic components of the lipid layer of the tear film?
Hydrophobic outer layer protects water from escaping as non-polar
Hydrophilic inner layer bonds to the aqueous component (polar)
What are the microscopic components of the aqueous layer of the tear film?
Water
Electrolytes
Proteins
What are the microscopic components of the mucin layer of the tear film?
No distant division between aqueous and mucin
Two phase – one with more mucin the other with less mucin
Glycocalyx is the glycoprotein layer attached to the surface epithelium cells (stratified squamous cells of cornea and conjunctiva)
More about the glycocalyx
Glycoprotein present in membrane bound vesicles within the epithelium cells that fuse to the cell membrane to release the glycocalyx
Contributes to the wetability of the epithelium surface cells and bonds the mucin layer to the epithelium
Surface area for bonding increased by epithelium microvilli
What is the thickness of the tear film?
2-5um
Decreases rapidly after blinking
How much tear fluid it produced and how fast?
Tear volume – 1ul
Tear prism (meniscus) – 4ul
Rate 1.2ul/min
Turnover 16%/min
How can age effect tear fluid volume?
Lacrimal gland innervation not always complete at birth – 87% of newborns show reflex tearing at blink – 95% show within one week
Infants have increased tear film stability – potentially due to increased thickness of the lipid layer
Secretion decreased in old age – reduced tear film stability
What does pilocarpine do to tear volume?
Increases the flow (parasympathomimetic)
What does atropine do to tear volume?
Decreases flow (parasympatholytic)
What other drugs decrease tear production?
Diuretics
Antihistamines
Antidepressants
Antianxiety medications
What are the electrolytes in the tear film and what are they responsible for?
Na+
Cl-
K+
HCO3-
Ca++
Mg++
Responsible for pH and osmolarity of tears
What are the major proteins in the tear film and their purpose?
Lysozyme
Antibacterial – dissolves bacterial wall
Secretory IgA
Antibody – important in immunological defence
Lactoferin
Antibacterial – iron binding protein
Lipocalin
Thought to contribute to stability of lipid layer
Albumin
Concentration increases in inflammation
Ig
Concentration increases in inflammation
What are the regulated proteins? Which protein is contitutive?
Lactoferin
Lipocalin
Lysozyme
Secretory IgA
What does hypoxic stress do to the cornea?
Makes it swell
What is another metabolite of the tear film?
Glucose
Source in blood
Given to cornea as avascular
What happens to osmolarity after prolonged eye closure? What about dry eye?
Osmolarity is decreased as there is more water (less evaporation)
Dry eye – osmolarity increases as there is more evaporation – more solutes on the eye
What is the normal range for pH of the tear film and why would it be low in a normal patient?
The normal range is 7.14-7.82 (about 7.45)
Generally lowest after waking due to acidic bi-products of anaerobic respiration
Explain the Holly and Lemp model for stability of the tear film
Based upon the lipid mucin interaction
After blink, hydrophobic lipid layer separated from hydrophilic mucin layer by aqueous layer – tear film thins
At certain critical thickness – hydrophobic lipids contaminate small areas of the mucous phase
Contaminated mucin layer cannot support aqueous resulting in break up
Explain the Sharma and Ruckenstein Model for stability of the tear film
Based on mucin dispersive forces
Influences of short range intermolecular interactions: two steps
After blink, thinner mucus at tips of microvilli begins to thin
Aqueous phase contacts epithelial surface – cannot support aqueous phase
Explain the Fatt (McDonald and Brubaker) model for stability of the tear film
Meniscus induced thinning of tear film
Explains tear film break up occurring near eyelid margins
What additives and substitutes can be used observing the tear film and what do they do?
Fluorescein
Observing tear film
Well stain damaged cells and accumulates in
intercellular spaces resulting from discontinues in
epithelium
Rose Bengal
Stains dead and devitalised muscous
Lissamine Green
Stains dead and devitalised cells
Stains mucous
what is dry eye
Multifactorial disease of the tears and ocular surface that results in
Symptoms of discomfort
Visual disturbance
Tear film instability
Potential damage to the ocular surface
accompanied by increased osmolarity of the tear film and inflammation of the ocular surface
What is aqueous tear deficiency and what are the types and their descriptions?
Lacrimal gland and accessory gland not producing sufficient tears
Sjogren’s syndrome
Progressive auto-immune disease associated with severe aqueous tear deficiency (also affects saliva glands – dry mouth, and associated with Rheumatoid arthritis)
Typically affects older female patients
Non-Sjogren’s
Lacrimal gland disease
Lacrimal duct obstruction
Reflex
What is Evaporative issues with the tear film and what causes it?
Quantity of tears produced is normal, BUT tears produced evaporate excessively
Lipid deficiency
Meibomian gland dysfunction, blepharitis
Surface changes
Mucin deficiency
What other outer anatomical feature can cause dry eye?
Lid anomaly
Blinking
Lid surface incongruity
What is the prevalence of dry eye and the different types?
Dry eye prevalence ~5-30%
Prevalence greater with older age and in females
Evaporative dry eye more common than aqueous deficient
Estimated prevalence of Meibomian gland dysfunction >60% in some older populations
How can tear quantity be tested and what are general values?
Schirmer test
<5mm in 5min sign of dry eye
>10mm normal
Phenol thread test
<10mm in 15sec suspicious of dry eye
Tear meniscus height
Usually >0.3mm
<0.1mm indicative of dry eye
Dye dilution
Tear turnover rate reduced in dry eye
Fluorescein dye instilled into the eye diluted less with dry eye patients
How can tear quality be tested and what are general values?
Fluorescein break up time
Instill fluorescein into eye – patient blinks – observe tearfilm for breakups (dark spots in fluorescein)
Time taken for tearfilm to break up = fluorescein break up time
<10secs indicative of dry eye
Non-invasive break up time
Estimation from observation of reflection from ocular surface – discontinues in reflection show break up
Majority >20sec
Lipid layer observation (some evidence between showing lipid layer thickness and meibomian gland dysfunction)
Less ferning present when tested (mucous ferning)
What is the osmolarity of normal and dry eyes?
Normal <312mOsm/kg
Dry eye >323mOsm/kg
What is conjunctival hyperaemia?
Dry eye will cause inflammation of ocular surface
General sign of ocular surface inflammation
How does dry eye affect goblet cells?
Goblet cell density decreases
How do you examine dry eye?
Order of testing for least invasive to most invasive test
Examine lids for blepharitis
Examine meibomian glands for dysfunction
Evaluate tear meniscus and tear film
Measure break up time
Observe fluorescein or rose bengal staining
How do you manage dry eye?
Tear substitutes
Ointment
Lipid Sprays
Evaporation control
Reduce drainage: punctal occlusion
Meibomian gland care (hygiene, antibiotics, omega 3)
How can you treat meibomian gland disease?
Lipiflow
Applies heat and pressure to the eyelids
Evidence of improvement in signs and symptoms of MGI after a single treatment
Blephasteam
Goggles that warm lids to treat meibomian gland dysfunction
Evidence of improvement in symptoms of dry eye
