Expanding the invertebrate medicine toolbox: evaluation of opisthosoma tonometry as a novel diagnostic tool for arachnids.

Objective: The objective of this study was to analyze the use of rebound tonometry, as a noninvasive diagnostic tool, in arachnids.

Methods: 5 juvenile (yearling) female and 1 juvenile male curlyhair tarantulas (Tliltocatl albopilosus, previously Brachypelma albopilosum) were used to track estimated ventral opisthosoma pressures over a 9-month period of time. Younger, growing animals were selected as they are more likely than adults to go through multiple molts throughout the 9 months of the study length. An iCare TONOVET TV01 rebound tonometer was used to measure the estimated ventral opisthosoma pressures of the spiders. Measurements were obtained from the ventral opisthosoma, 1 of the thinnest areas of exocuticle throughout the body. Readings were obtained once per week from the ventral opisthosoma for the first 2 months, then once every 2 weeks for 1 month, then back to once per week for the remainder of the study. Additional measurements were obtained following each ecdysis, after a 2% body weight sampling of hemolymph, and at the end of the study to evaluate readings in response to induction of general anesthesia with 5% isoflurane gas and oxygen flow at 2 L/min for 10 minutes.

Results: The average of all estimated ventral opisthosoma pressure when spiders were not in molt was 26.19 mm Hg (SD, 3.54), with a statistically significant decrease postmolt to an average of 15.31 mm Hg (SD, 3.81), followed by a gradual increase back to premolt pressures over a 3-week period with an average of 22 days (SD, 1.93). Estimated ventral opisthosoma pressures decreased post hemolymph removal. There is not sufficient evidence that estimated ventral opisthosoma pressures changed over time following the anesthesia.

Clinical relevance: This study demonstrates that tonometry can be used to generally assess the estimated ventral opisthosoma pressure, which could correlate with where a spider is in an ecdysis cycle.