Researchers Explore the Use of H-reflex Testing to Monitor Effects of Intrathecal Bacolfen Therapy
By Susan Christensen
Health and Research News Service
While intrathecal baclofen therapy (ITB) has given many patients relief from spasticity, the process of finding the right ITB prescription has been fraught with frustrations.
If the dose is too high, serious side effects can occur. If it’s too low, the patient may receive little relief from painful spasms and abnormal postures.
Adding to the challenge is the fact that ITB is dispensed via a surgically implanted pump, making dosage adjustments considerably more complicated than a trip to the corner drug store.
Still, there’s reason for optimism among ITB users. Methodist Rehabilitation Center researchers are exploring a promising new approach for evaluating the effectiveness of ITB at varying levels.
In earlier studies, scientists at Methodist Rehab discovered that a neurophysiological technique known as H-reflex could be used to monitor the spinal cord’s responsiveness to the drug. Their latest research further examines the impact of different doses of continual ITB on H-reflex results.
“We hope to give physicians an understanding of how to use laboratory measures of spasticity to determine continual ITB dosages,” said Dr. Dobrivoje Stokic, a senior scientist at the Center for Neuroscience and Neurological Recovery and administrative director of research at Methodist Rehab.
Described as the electrical equivalent of the tendon tap, H-reflex is commonly evoked by stimulating the tibial nerve behind the knee and recording electrical signals from calf muscles. “The amplitude ratio between maximum H-reflex and maximum M-wave (H/M ratio) has been considered an index of spasticity because, in the majority of patients with spasticity, the H/M ratio is increased above normative limits,” Dr. Stokic said.
“We use the H/M ratio as an early marker of response that hastens reaching a continual ITB dose range where hypertonia relief can proceed in earnest towards achievement of targeted functional goals. Conversely, a lack of change in the H/M ratio despite serial continual ITB dose increases should raise suspicion of a possible early system malfunction.”
Monitoring the H/M ratio can also indicate late system malfunction.This is usually the case when there’s a progressive or substantial increase in the H/M ratio, coupled with loss of previously achieved clinical response despite aggressive continual ITB dose increases.
“We feel that H-reflex has proved clinically useful in a variety of ways,” said Dr. Stuart Yablon, medical director of Methodist Rehab’s brain injury program. “We can use it to objectively confirm ITB bolus trial responses, to help adjust ITB during the early post-implant phase, to evaluate suspected system malfunctions, such as catheter problems and ‘low reservoir syndrome,’ and to confirm favorable response to a change in mode of ITB administration.”
Dr. Yablon said H-reflex sensitivity is particularly useful during the early titration phase since changes in Hreflex often herald a dose when clinical response is about to be observed.
Dr. Stokic said the H-reflex technique has an advantage over clinical assessment in terms of control of stimulation parameters and objective evaluation of responses. “The H-reflex recording is simple and easily performed, requiring minimal patient participation and causing almost no discomfort. In addition, the measurements are stable and reproducible under controlled conditions, which facilitates objective quantification and comparison. In our opinion, neurophysiological evaluation is useful for assessing spinal cord responsiveness, and if available, this information should be considered when judging the overall clinical effectiveness of ITB administration.”