Development and Calibration of Rectal Tonometer for Neurological Studies
Authors: Osorio Miguel , Enikov Eniko T. 
Published: February 2024

Abstract
Neural tube defects (NTD) are a heterogeneous group of congenital malformations of the central nervous system (CNS) that result from incomplete closure of the neural tube during the developmental stage in the embryo. Consequences vary in severity, ranging from incontinence and chronic back pain to acute paralysis below the deformation and stillbirth. From this constellation of defects, spina bifida, sometimes accompanied with occulta tethered cord (OTC), is the mildest but most prevalent due to going undiagnosed until adulthood when the complex symptoms start to appear. The accuracy of operations to close or decompress the spinal cord after childhood are inconclusive as the damage can be irreparable. In order to treat their onset and avoid further neurological deterioration, methods have been developed to monitor the neurophysiological status of the spinal cord with varying rates of success. Currently, diagnosis of OTC is complex and require invasive urodynamics studies which are difficult to carry out in a pediatric setting. An alternative test based on anorectal tone could offer a simpler and reliable assessment. This paper presents a rectal tonometer to be used in diagnosis of various spinal neuropathies including OTC, CES known as Cauda Equina Syndrome, and NTD. The developed transducer measures external sphincter pressure in real-time allowing for the detection of voluntary contractions as well as baseline tone. Unlike existing fluid-based anorectal tonometers, the proposed device consists of a hollow flexible cylinder filled with air. Finite element analysis was used to optimize the geometry of the transducer to achieve high axial and low transversal stiffness. An iterative solution was developed to predict the response via a calibration chamber with a maximum pressure rating of 450 PSI. The apparatus was connected to a nitrogen gas tank to provide the pressure and a secondary regulator with bleeder valve was used for decreasing the pressure range to an acceptable value for the pressure sensor specifications. The preliminary calibration test consisted of applying increments of 2 PSI from the nitrogen gas tank to the transducer and record the sensor’s steady response. The secondary calibration procedure comprised of a step-rate test model where the pressure applied to the transducer was continuously increased by 1 PSI every 50 seconds until reaching the limit of 7 PSI. The pressure was increased manually by rotating the regulator approximately every 50 seconds. Results show that the transducer’s pressure increased rapidly and started to stabilize after approximately 20 seconds. However, as the values were incremented, it took more time to for the sensor to provide constant results. This could be attributed to the time it takes the nitrogen gas to fill the volume of the calibration chamber. For the step-rate calibration test, the peaks showed consistency until arriving at about 80 mmHg where they started to diverge from their constant values. By taking the ratio of sensor pressure over the regulator, it was found that the calibration equation is Y = 2.5018X + 43.744. The final results showed that the rectal tonometer is capable of measuring the anorectal tone with an accuracy of +/−11.43mmHg and a time response of 15 milliseconds.