EPOMEDICINE HEALTH AND MEDICAL MAGAZINE
Home » Medical Students

Glaucoma Basics – Aqueous humor dynamics and Intraocular pressure

Volume of aqueous: 0.25 ml (Anterior chamber) + 0.06 ml (Posterior chamber)

Composition of aqueous: similar to plasma except-

  • higher concentration of ascorbate, pyruvate and lactate
  • lower concentration of protein, urea and glucose

Functions of aqueous:

  1. Maintains Intra-ocular Pressure (IOP)
  2. Provides nutrition to avascular lens and cornea
  3. Optical transparency
  4. Place of lymph that is absent in eyeball

Production of aqueous:

Aqueous humor production

  • Source: ciliary process of ciliary body
  • Rate: 2.3 µl/min
  • Stages:
    1. Convective delivery: of H2O, ions, proteins, metabolic fuel by ciliary circulation.
    2. Ultrafiltration and diffusion: from capillaries into the stroma, driven by oncotic pressure, hydrostatic pressure and concentration gradient.
    3. Active secretion: into the basolateral spaces between nonpigmented epithelium followed by water movement down the resultant osmotic gradient into posterior chamber.

Drainage of aqueous:

Ciliary process → Aqueous in posterior chamber → (though pupil) → Anterior chamber

Aqueous drainage pathway

From anterior chamber, the drainage pathway divides into 3 systems:

1. Trabecular/conventional/pressure dependent (90%):

Trabecular meshwork (innermost uveal, middle corneoscleral, outermost juxtacanalicular) → Schlemm’s canal (endothelial lined and present in scleral sulcus) → Collector channels (Direct – Aqueous vein; Indirect – Intrascleral plexus) → Episcleral veins

2. Uveoscleral/Unconventional/Extracanalicular/Pressure-independent (10%):

Ciliary body → Suprachoroidal space → Venous circulation of ciliary body, choroid and sclera

3. Iris outflow (<1%)

Intraocular Pressure (IOP):

Definition: Pressure exerted by intraocular fluids on the coats of the eyeball.

Normal value: 10-21 mmHg (16 +/- 2.5 mmHg)

Goldmann equation:

Pο = (F/C) + PV

where,
Pο = IOP = 10-21 mmHg
F = Rate of aqueous production = 2 µl/min
C = Facility of outflow = 0.22-0.28 µl/min/mmHg
PV = Episcleral venous pressure = 8-12 mmHg

Influencing factors:

1. Long term: Heredity, age, sex, race, refractive error

2. Short term:

  • General:
    • Posture: increased in supine position, highly increased in head down position
    • Exercise: decreased by aerobic exercise, increased by isometric exercise
  • Systemic:
    • Valsalva: increased
    • Food/drugs:
      • lowers IOP: alcohol, fat-free diet, vasodilators, heroin, miotics
      • increases IOP: water drinking, caffeine, steroids, mydriatics
    • General anesthesia: lowers IOP except Ketamine and Succinylcholine
  • Rhythmic:
    • Cardiovascular: 1-2 mm
    • Respiratory: upto 5 mm
    • Diurnal (higher in morning): < 5 mm

IOP measurement:

Indications:

a. All suspected cases of glaucoma:

  • Significant diurnal variation (>8 mm Hg)
  • Significantly positive water drinking provocative test (>8 mmHg)
  • > 0.2 asymmetry of cup to disc ratio in 2 eyes
  • Splinter hemorrhage over or near disc
  • Positive family history
  • IOP constantly > 30 mmHg
  • Diabetic and high myopic patient

b. Age: > 40 years

Palpation tonometry

Digital tonometry with index fingers

Techniques:

a. Digital palpation tonometry:

  1. The patient is asked to look downward
  2. Fluctuation of eyeball is elicited at upper lid with the help of index finger of 2 hands.
    • Increased IOP: firm and hard
    • Normal IOP: Soft (like a partially filled water bag)

b. Indentation (Schiotz) tonometry

c. Applanation (Goldmann’s) tonometry




Share/Bookmark this!

Leave a reply

Add your comment below, or trackback from your own site. You can also subscribe to these comments via RSS.

Be nice. Keep it clean. Stay on topic. No spam.

You can use these tags:
<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>

This is a Gravatar-enabled weblog. To get your own globally recognized avatar, please register at Gravatar.

Side Notes

This entry was posted by on January 8, 2014 at 11:55 am and filed under Medical Students category.

You can add your comments or trackback from your own site. To keep you updated to the latest discussion, you can subscribe to these comments via RSS.

This is a Gravatar-enabled weblog. To get your own globally recognized avatar, please register at Gravatar.

Categories

Heart and Breath Sounds

Subscribe: Epomedicine weekly