Effect of orifice-area reduction on flow characteristics during injection through spinal needles

A numerical investigation of intrathecal isobaric drug dispersion within the cervical subarachnoid space

Intrathecal drug and gene vector delivery is a procedure to release a solute within the cerebrospinal fluid. This procedure is currently used in clinical practice and shows promise for treatment of several central nervous system pathologies. However, intrathecal delivery protocols and systems are not yet optimized. The aim of this study was to investigate the effects of injection parameters on solute distribution within the cervical subarachnoid space using a numerical platform. We developed a numerical model based on a patient-specific three dimensional geometry of the cervical subarachnoid space with idealized dorsal and ventral nerve roots and denticulate ligament anatomy. We considered the drug as massless particles within the flow field and with similar properties as the CSF, and we analyzed the effects of anatomy, catheter position, angle and injection flow rate on solute distribution within the cerebrospinal fluid by performing a series of numerical simulations. Results were compared quantitatively in terms of drug peak concentration, spread, accumulation rate and appearance instant over 15 seconds following the injection. Results indicated that solute distribution within the cervical spine was altered by all parameters investigated within the time range analyzed following the injection. The presence of spinal cord nerve roots and denticulate ligaments increased drug spread by 60% compared to simulations without these anatomical features. Catheter position and angle were both found to alter spread rate up to 86%, and catheter flow rate altered drug peak concentration up to 78%. The presented numerical platform fills a first gap towards the realization of a tool to parametrically assess and optimize intrathecal drug and gene vector delivery protocols and systems. Further investigation is needed to analyze drug spread over a longer clinically relevant time frame.

The frequency and magnitude of cerebrospinal fluid pulsations influence intrathecal drug distribution: key factors for interpatient variability

BACKGROUND

Intrathecal drug delivery is an efficient method to administer therapeutic molecules to the central nervous system. However, even with identical drug dosage and administration mode, the extent of drug distribution in vivo is highly variable and difficult to control. Different cerebrospinal fluid (CSF) pulsatility from patient to patient may lead to different drug distribution. Medical image-based computational fluid dynamics (miCFD) is used to construct a patient-specific model to quantify drug transport as a function of a spectrum of physiological CSF pulsations.

METHODS

Magnetic resonance imaging (MRI) and CINE MRI were performed to capture the patient's central nervous system anatomy and CSF pulsatile flow velocities. An miCFD model was reconstructed from these MRIs and the patient's CSF flow velocities were computed. The effect of CSF pulsatility (frequency and stroke volume) was investigated for a bolus injection of a model drug at the L2 vertebral level. Drug distribution profiles along the entire spine were computed for different heart rates: 43, 60, and 120 bpm, and varied CSF stroke volumes: 1, 2, and 3 mL. To assess toxicity risk for patients with different physiological variables, therapeutic and toxic concentration thresholds for a common anesthetic were derived from experimental studies. Toxicity risk analysis was performed for an injection of a spinal anesthetic for patients with different heart rates and CSF stroke volumes.

RESULTS

Both heart rate and CSF stroke volume of the patient strongly influence drug distribution administered intrathecally. Doubling the heart rate (from 60 to 120 bpm) caused a 26.4% decrease in peak concentration in CSF after injection. Doubling the CSF stroke volume diminished the peak concentration after injection by 38.1%. Computations show that potentially toxic peak concentrations due to injection can be avoided by changing the infusion rate. Using slower infusion rates could avoid high peak concentrations in CSF while maintaining drug concentrations above the therapeutic threshold.

CONCLUSIONS

Our computations identify key variables for patient to patient variability in drug distribution in the spine observed clinically. The speed of drug transport is strongly affected by the frequency and magnitude of CSF pulsations. Toxicity risks associated with an injection can be reduced for a particular patient by adjusting the infusion variables with our rigorous miCFD model.

Effect of anatomical fine structure on the dispersion of solutes in the spinal subarachnoid space

The dispersion of a solute bolus is calculated for cerebrospinal fluid undergoing oscillatory flow in the subarachnoid space of the spine. The fine structure of the subarachnoid space (nerves and trabeculae) enhances both longitudinal and transverse dispersions five to ten times over a simple model with an open annular space. Overall, dispersion is >10(3) times simple molecular diffusion. The result of enhanced dispersion is rapid spread and dilution of the bolus, effectively stirred by fluid movement around the fine structure.

Inflammatory mass lesions associated with intrathecal drug infusion catheters: report and observations on 41 patients

OBJECTIVE

Several reports have described inflammatory mass lesions at the tip of intraspinal drug administration catheters. We evaluated the number of patients reported with this condition and whether data support hypotheses that have been put forth regarding the cause of these lesions.

METHODS

Information that was reported in the medical literature, and by Medtronic, Inc., to the United States Food and Drug Administration as of November 30, 2000, was reviewed.

RESULTS

Forty-one cases were identified, including 16 from the literature and 25 that were not published previously in the literature. Because of voluntary reporting and other methodological limitations, the actual number of cases must be higher than reported. All of the patients had chronic pain. The mean duration of therapy was 24.5 months. Most masses were located in the thoracic region. Intrathecal drugs included morphine or hydromorphone, either alone or mixed with other drugs, in 39 of 41 cases. No masses were reported in patients who received baclofen as the only intrathecal medication. Thirty patients underwent surgery to relieve spinal cord or cauda equina compression. Eleven patients were nonambulatory at last follow-up, and one died of a pulmonary embolus. Surgical specimens revealed noninfectious chronic inflammation, granuloma formation, and fibrosis or necrosis.

DISCUSSION

The most plausible hypothesis with regard to the cause of intrathecal catheter tip mass lesions implicates the administration of relatively high-concentration or high-dose opiate drugs or the use of drugs and admixtures that are not labeled for intrathecal use.

CONCLUSION

Patients who require high-dose intraspinal opioid therapy and those who receive drugs or admixtures that are not approved for intrathecal use should be monitored closely for signs of an extra-axial mass or catheter malfunction. Prompt diagnosis and treatment may preserve neurological function.