lecture 3 opt511

• Lacrimal system


  Structures concerned with the formation of tears & its transport

  
• Lacrimal apparatus


  • Main lacrimal gland
  • Accessory lacrimal glands
  • Lacrimal passages (puncta, canaliculi, lacrimal sac, nasolacrimal duct)

  
• Main lacrimal gland


  About the size of an almond, located under supero-temporal orbit, divided into orbital and palpebral portions, has 10-12 lacrimal ducts that open into superior palpebral conjunctiva

  
• Structures of lacrimal gland

  • Lobules, acini (double layer, irregular arrangement of secretory cells, surrounded by myoepithelial cells), network of ducts

  
• Accessory lacrimal glands


  • Glands of Krause
  • Glands of Wolfring
  • Glands of Zeis
  • Meibomian glands
  • Crypts of Henle
  • Goblet cells
• Blood supply of lacrimal gland

  Lacrimal artery, drain by superior & inferior ophthalmic veins

  
• Nerve supply of lacrimal gland


  Sensory innervation by lacrimal nerve, sympathetic nerves supply vessels, parasympathetic nerves provide secretomotor innervation to myoepithelial cells
• Tear film


  Also referred to as 'pre-corneal tear film', in contact with large ocular surface, tear meniscus in contact with bulbar conjunctiva and lid margin

  
• Tear film structure


  • Approximately 7-10 um thick, composed of 3 discrete layers: lipid, aqueous, mucin
• Lipid layer


  Produced by Meibomean glands, 800-2000 A thickness, prevents evaporation, contamination, lubricates lid action, stabilizes tear film
• Aqueous layer


  90% of total tear thickness, 95% from lacrimal glands, 5% from Krause and Wolfring, can be lost through evaporation or reabsorption
• Mucin layer


  Innermost layer, rests on conjunctival and corneal epithelium, has 2 layers: innermost tightly bound to epithelial surface and secretes glycocalyx, outer loosely attached layer from goblet cells

  
• Tear film is unstable and can rupture if lid action is compromised, known as 'break' and clinically tested as TBUT (tear break up time)
• Normal tears production averages 1.2 ul per minute, decreases under GA or sleep, increases under stimulation
• Tear film abnormalities


  • Aqueous tear production decreased with age (keratoconjunctivitis sicca)
  • Sjogren's syndrome
  • Thyroid eye disease (exophthalmos)
  • Contact lens wear
  • Irregular cornea surface
  • Hormone levels (e.g. estrogen)

  
• Lacrimal drainage system


  • Lacrimal punctum
  • Lacrimal canaliculi
  • Lacrimal sac
  • Nasolacrimal duct
• Elimination of tears


  25% lost to evaporation, 75% pumped into nasal cavity through lacrimal drainage system, lower canaliculi drains 60% of tears
• Tear drainage mechanism


  Tears enter lacrimal puncta, pumped through canaliculi into lacrimal sac by blinking, contraction of orbicularis muscle creates negative pressure to draw tears in, relaxation pushes tears out to nasolacrimal duct

  
• Blinking control


  Autonomic and voluntary neural control, innervated by CNVII and CNIII, reflex blinks occur in response to corneal stimuli, wiper action removes debris and stimulates secretion