PC - Tonometry - Week 1 Flashcards
Define indentation.
Measurement of IOP by direct pressure on the eyeball.
Define applanation.
Measurement of IOP by determining the force necessary to flatten a corneal surface of constant size.
What law does applanation rely on?
Imbert-Fick law
Name the four forces that act on a tonometer during measurement, and how they relate to the Imbert-Fick law.
-Capillary attraction of the tear flim to the probe
-Force of the probe
-Fluid pressure behind the cornea
-Corneal resistance
These forces invalidate the law.
What happens to the internal volume of the eye during applanation, and why this is significant.
The volume within decreases, resulting in increased pressure.
Describe the findings of Goldmann and Schmidt on corneal applanation.
They found if the circle of applanation is 3.06mm, the tissue tension forces cancel out.
Describe the two main means of measuring IOP.
Give three examples.
Contact (Goldmann/Perkins)
Non-contact (airpuff)
What is the typical range for IOP? What is the typical variation between eyes?
Typically 10-21mmHg
Variance of ≤ 3-4mmHg between eyes.
Name 6 long-term factors that affect IOP.
Age (increases) Systemic blood pressure Race Body weight Ethnic origin (asians higher) Seasonal variations (higher in winter)
Name 4 short-term factors that affect IOP.
Diurnal variation (higher at AM, lower at PM).
Corneal thickness
Body posture (from prone to standing)
Exercise (endurance decreases, resistance increases)
What over/under-estimation is given by thin/thick corneas?
Thin - underestimates up to 5.9mmHg
Thick - overestimates up to 6.8mmHg
What effect does repeated IOP measurements have on IOP, and what guideline should be followed?
Decreases IOP.
Generally no more than 2-3 attempts.
What kinds of medications can affect IOP (aside from those affecting aqueous humour)?
BP drugs, steroids
List 9 indications for measuring IOP.
- All 1st time patients
- Prior to dilation
- Risk or known diagnosis for glaucoma
- Symptoms/signs associated with glaucoma
- Diagnose/manage/effectiveness of tx for glaucoma
- Ocular conditions
- Recent significant refractive change in one eye
- Systemic conditions (diabetes, HT, migraine)
- Previous ocular injury, surgery, or post-surgery
List 7 contraindications for measuring IOP.
- Corneal or conunctival infections
- Significant central corneal epithelial defects
- Significant epithelial basement membrane dystrophy
- Known sensitivity to anaesthetics
- Perforation of the eye/severe ocular trauma
- Contact lens wear
- Sensitive patients
