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:

• 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

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

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