Aqueous Outflow Facility Changes With Posture
Condition: Healthy
Study type: Observational

Summary: 
      Intraocular pressure (IOP) varies with body position, with a significant increase occurring
      from the seated to the supine position. Previous research has indicated that the IOP increase
      cannot be explained by changes in episcleral venous pressure(blood flow leading away from the
      eye) alone. As well, previous research has indicated that aqueous flow rate is independent of
      body position. The purpose of this project was to determine if aqueous humor outflow facility
      varied with changes in posture.
    

Detailed description: 
      On the day of the experiment, patients will be asked to maintain a regular schedule and
      normal activities. This will include avoiding excess caffeine intake, large deviations from
      normal sleep cycle, or exposure to pharmacologic IOP modifying agents.

      Subjects will be seated in standard ophthalmologic examining chairs and reclined into the
      supine position in a quiet room with subdued lighting for approximately 5 minutes. Anesthesia
      will be achieved by instilling 3-4 drops of 0.5% proparacaine. The contralateral eye will be
      patched in an air-tight fashion with an adhesive eye patch to reduce evaporative losses and
      drying of the cornea. The patient will be asked to keep eyes closed prior to start of
      measurements. When ready, the patient will be asked to open their eyes and to observe a dim
      fixation target that will be placed on the ceiling of the room, roughly 6 feet from the
      patient's eyes. Baseline IOP (Po) will be measured using a pneumatonometer (Model 30 Classic
      Mentor). Tonography will then be performed using the electronic Schiotz tonometer with a 10g
      weight in order to obtain a four minute tracing. The procedure will then be repeated on the
      contralateral eye, with patching of the already tested eye. After this, the patient's chair
      will the be raised 70 degrees and patient will be asked to hyperextend their neck until their
      cornea is parallel to the floor. A minimum of 30 minutes will be allowed to elapse, allowing
      for autonomic, postural and hormonal changes to stabilize. Then a repeat baseline IOP for the
      sitting position will be taken and another 4 minute tracing will be collected successively in
      each eye as described above.

      Data from the tonometer tracing will be collected electronically and will be exported to a
      standard Excel file. A smooth curve of best fit will be established through each tonogram
      once graphed, correcting for oscillations and drift. This curve will help determine the
      adequacy of the collected sample. Tracings that have a poorly fitted curve might occur for a
      variety of reasons including patient tenseness and anxiety, uncontrollable tendency to fall
      asleep and incidental interruptions such as a sudden wave of intestinal peristalsis.
      Corresponding "Po" values for the initial steady-state IOP and the "C" value for facility of
      aqueous outflow will be gathered by referring to standard tables and normograms.

      The main potential risk of participating in the study is the possibility of corneal abrasion.
      Previous studies at the Mayo Clinic using the same techniques did not result in any corneal
      abrasions in over 300 eyes of more than 150 patients. Based on this result, we estimate the
      risk of a corneal abrasion to be minimal. If a corneal abrasion occurs, the patient will be
      removed from the study. The patient will be treated with antibiotic drops or ointment and
      followed until the abrasion has completely healed.

      Other minor risks include possible allergic reaction to the anesthetic drops or the adhesive
      glue. As with any medication, allergic reactions are a possibility. However, these will be
      the same drops used in routine clinical practice, and allergic reaction is extremely
      uncommon.
    

Observational model: Case-Only
Time perspective: Prospective
Sampling method: Non-Probability Sample
Study population: 
        Healthy young volunteers aged 18-45 will be recruited from Mayo Clinic employees and
        students, and local area residents.
      
Keywords: glaucoma; outflow facility; Healthy eye function