title: Human scleral hydraulic conductivity: age-related changes, topographical variation, and potential scleral outflow facility.
abstract: OBJECTIVE
  To measure the specific hydraulic conductivity (K) of human sclera over a range of ages, to assess topographical variation, and to provide a theoretical estimate of potential scleral outflow facility.
  
  
  METHODS
  Human donor sclera (n = 18; mean age 56.7 +/- 25.9 years; range 4-89) was clamped in a modified Ussing chamber connected to a water column set at 15.7 mm Hg. Column descent was measured over 24 hours at 20 degrees C with a digital micrometer. Scleral thickness of glutaraldehyde-fixed specimens was measured by light microscopy, taking the mean of 15 measurements per donor. Topographical variation in hydraulic conductivity (HC) was determined in an additional 10 donor eyes (mean age, 54.1 +/- 26.4 years; range 12-89), comparing anterior, equatorial, and posterior sclera. The potential transscleral outflow facility was calculated by multiplying HC by total scleral surface area and adjusting water viscosity to core body temperature.
  
  
  RESULTS
  Mean K +/- 1SD in adults (>18 years) was 5.85 +/- 3.89 x 10(-18) m(2). K tended to be higher in pediatric donors, but there was no statistically significant age-related change. However, when all data sets were combined (n = 28), HC showed a significant decline with age. There was no significant topographical variation in HC. The potential transscleral outflow facility was 0.33 microL.min(-1).mm Hg(-1).
  
  
  CONCLUSIONS
  Quantifying HC may help refine ocular pharmacotherapy, as transscleral water movement increases intraocular drug elimination and impedes transscleral drug delivery. The potential scleral outflow is two to three times higher than that which occurs in vivo; hence, medical or surgical interventions that fully exploit this pathway have considerable capacity to lower intraocular pressure.
authors.affiliation: Academic Department of Ophthalmology, The Rayne Institute, GKT Medical School, London, United Kingdom. timljackson@hotmail.com
authors.forename: Timothy L
authors.initials: TL
authors.lastname: Jackson
authors.forename: Ali
authors.initials: A
authors.lastname: Hussain
authors.forename: Andrea
authors.initials: A
authors.lastname: Hodgetts
authors.forename: Ana M S
authors.initials: AM
authors.lastname: Morley
authors.forename: Jost
authors.initials: J
authors.lastname: Hillenkamp
authors.forename: Paul M
authors.initials: PM
authors.lastname: Sullivan
authors.forename: John
authors.initials: J
authors.lastname: Marshall
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D001692:Biological Transport; D002648:Child; D002675:Child, Preschool; D015194:Diffusion Chambers, Culture; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010539:Permeability; D012590:Sclera / Q000378:metabolism*; D014867:Water / Q000378:metabolism*
chemical_list: D014867:Water