Infusing the Alligator: Infusion Studies in a Low Compliance System

An infusion study is a neurological procedure in which a volume of fluid is added to the existing cerebrospinal fluid. The additional fluid volume increases the intracranial pressure; by monitoring how the system responds to this challenge, the clinician gains insight into the compliance of the dura and nervous tissue. Though commonly used clinically, the invasive nature of infusion studies means that they have rarely been applied in non-clinical studies, and appear to have only been used on mammalian subjects. Infusing a bolus of artificial cerebrospinal fluid into the cranial compartment of the American alligator (Alligator mississippiensis), produces pressure/volume curves with most of the attributes seen during infusion studies of humans or other mammals. Two consistent, unusual findings were noted: the compliance in the cranial compartment of Alligator is low (around 1.0) likely due to the small size of both the compartment and the dural sinuses; and the peak pressure drops off much faster than in a typical infusion study. A second round of bolus infusions was performed, these had a bidirectional design with infusions conducted at the midpoint of the spinal compartment as well as the cranial compartment. Similar results were obtained: the spinal compartment compliance was low (around 1.0), and the peak infusion pressures dropped off quickly with minimal propagation to the other compartment. The spinal dura of Alligator is ensheathed by a large venous sinus, which contributes to the low compliance of the spinal compartment. A final round of bidirectional infusions tested the influence of the spinal venous sinus; a bolus of Ringer’s solution was injected into the sinus immediately before the infusion. As expected, the pressurization of the spinal venous sinus lowered the compliance of the system, raising peak infusion pressures; however, the pressures still showed rapid decay with little propagation to the other compartment. Herein it is proposed that the paradox of low compliance coupled with rapid pressure loss and minimal pressure propagation is present because the spinal dura of Alligator mississippiensis functions as a pressure modulated relief system for the cerebrospinal fluid.