GABA Agonists and Gabapentin for Spastic Hypertonia

Gabapentin as part of a multimodal pain protocol for selective dorsal rhizotomy does not impact percentage of rootlets transected

PURPOSE

We aim to determine whether preoperatively initiated gabapentin for pain control impacts the percentage of rootlets cut during monitored, limited laminectomy selective dorsal rhizotomy (SDR) procedure.

METHODS

This retrospective cohort study includes participants with cerebral palsy who had SDR for treatment of spasticity between 2010 and 2019 at a single-institution tertiary care center. One-level laminectomy SDR aimed to evaluate the cauda equina roots from levels L2-S1 with EMG monitoring. Gabapentin titration began 3 weeks prior to SDR. Data was analyzed using simple linear regression. Thirty-one individuals met inclusion criteria. Mean age was 7 years, 4 months. Eighteen participants (58%) identified as male, 12 (39%) female, and one (3%) non-binary. Thirty (97%) had bilateral CP. Sixteen (52%) were GMFCS II, four (13%) GMFCS III, five (16%) GMFCS IV, and six (19%) GMFCS V.

RESULTS

Mean percentage of rootlets transected was 50.75% (SD 6.00, range 36.36-60.87). There was no relationship between the dose of gabapentin at time of SDR and percentage of rootlets cut with a linear regression slope of - 0.090 and an R2 of 0.012 (P = 0.56).

CONCLUSION

Results indicate that preoperative initiation of gabapentin did not impact the percentage of rootlets transected. Thus, gabapentin can be initiated prior to SDR at moderate dosages without impacting SDR surgical outcomes.

Pain in Multiple Sclerosis: Understanding Pathophysiology, Diagnosis, and Management Through Clinical Vignettes

Neuropathic pain and other pain syndromes occur in the vast majority of patients with multiple sclerosis at some time during their disease course. Pain can become chronic and paroxysmal. In this review, we will utilize clinical vignettes to describe various pain syndromes associated with multiple sclerosis and their pathophysiology. These syndromes vary from central neuropathic pain or Lhermitte's phenomenon associated with central nervous system lesions to trigeminal neuralgia and optic neuritis pain associated with nerve lesions. Muscular pain can also arise due to spasticity. In addition, we will discuss strategies utilized to help patients manage these symptoms.

GABAB Receptor Chemistry and Pharmacology: Agonists, Antagonists, and Allosteric Modulators

The GABA_B receptors are metabotropic G protein-coupled receptors (GPCRs) that mediate the actions of the primary inhibitory neurotransmitter, γ-aminobutyric acid (GABA). In the CNS, GABA plays an important role in behavior, learning and memory, cognition, and stress. GABA is also located throughout the gastrointestinal (GI) tract and is involved in the autonomic control of the intestine and esophageal reflex. Consequently, dysregulated GABA_B receptor signaling is associated with neurological, mental health, and gastrointestinal disorders; hence, these receptors have been identified as key therapeutic targets and are the focus of multiple drug discovery efforts for indications such as muscle spasticity disorders, schizophrenia, pain, addiction, and gastroesophageal reflex disease (GERD). Numerous agonists, antagonists, and allosteric modulators of the GABA_B receptor have been described; however, Lioresal^® (Baclofen; β-(4-chlorophenyl)-γ-aminobutyric acid) is the only FDA-approved drug that selectively targets GABA_B receptors in clinical use; undesirable side effects, such as sedation, muscle weakness, fatigue, cognitive deficits, seizures, tolerance and potential for abuse, limit their therapeutic use. Here, we review GABA_B receptor chemistry and pharmacology, presenting orthosteric agonists, antagonists, and positive and negative allosteric modulators, and highlight the therapeutic potential of targeting GABA_B receptor modulation for the treatment of various CNS and peripheral disorders.

Baclofen Solution for Low-Volume Therapeutic Delivery

OBJECTIVES

Baclofen is a zwitterion molecule where increased ions in the excipient increase the solubility. We developed baclofen in a stable solution similar to cerebrospinal fluid (CSF) without bicarbonate and proteins to improve the solubility of the baclofen and to reduce the potential toxicity to the central nervous system (CNS) and subarachnoid space. The objective is to develop a solution of baclofen wherein baclofen is solubilized in a multivalent physiological ion solution such as artificial cerebrospinal fluid (aCSF) at a concentration from 2 mg/cc to 10 mg/cc.

METHODS

First, to determine the solubility of Baclofen in aCSF, solubility was determined at six different pH levels at 37°C, by the addition of aCSF to a known amount of Baclofen. The final concentrations were confirmed by high performance liquid chromatography (HPLC) analysis. Second, the stability of Baclofen at 4 mg/cc investigated in a test manufacturing batch utilizing standard methods of production of 1500 20 cc vials inverted for 18 months at 25°C at 60% humidity. The stability and purity of the baclofen was verified at 18 months by HPLC analysis.

RESULTS

Baclofen was initially soluble between pH of 6-8 above 7 mg/cc but fell back to 6.3-5.8 mg/cc level with time. Baclofen produced in vials with inversion were noted to be stable at 4 mg/cc at 18 months with less than 2% breakdown of the baclofen in solution.

CONCLUSION

Baclofen is much more soluble in artificial CSF than normal saline. The artificial CSF may also be less toxic to the subarachnoid space than saline.

The diagnosis and management of adults with spasticity

This chapter aims to address the questions of the definition and effective management of spasticity, in order to assist the reader to recognize, assess, and treat people with this impairment. Spasticity is a physiological consequence of an insult to the brain or spinal cord, which can lead to life-threatening, disabling, and costly consequences. It is a common but not inevitable outcome of the upper motor neuron (UMN) syndrome and is characterized by muscle overactivity and high tone spasms, which, if left untreated, will lead to muscle and soft tissue contracture and limb deformity. There have been several attempts to define spasticity. The difficulty reflects the complex features of the syndrome. The most cited definition is by Lance, but does not fulfil all the clinical scenarios seen in clinical practice. The term "spasticity" in the therapeutic world covers the several other features of the UMN syndrome and, therefore, an all embracing definition is probably required as well. Rates for the prevalence of spasticity in different clinical conditions are variable. This may be due to the presence of many patients with mild spasticity, for whom little or no treatment is required for their condition. However, it is estimated that 38% of patients following stroke develop a degree of spasticity and about 19% require pharmacological treatment. Of these about one-third (5% of the total) will benefit from botulinum toxin injections for focal problems. This chapter will inform the reader about the pathophysiology of spasticity, but also includes the practicalities and principles of management, the delivery of its longer term treatments, and the utilization and measurement of relevant outcomes.

Orally Delivered Baclofen to Control Spastic Hypertonia in Acquired Brain Injury

To determine if oral/systemic delivery of baclofen can effectively decrease spastic hypertonia due to acquired brain injury (traumatic brain injury, stroke, anoxia, or encephalopathy). Tertiary care outpatient rehabilitation center directly attached to a university hospital. Patients were a convenience sample recruited consecutively who had been referred for treatment of their spastic hypertonia to our spasticity clinic over a 5-year period. The spastic hypertonia was due to an acquired brain injury by either traumatic brain injury (TBI), stroke, or anoxic brain injury. All patients were more than 6 months postinjury or illness. Retrospective review of patients before and after initiation of treatment with oral baclofen, per standardized clinical data sheets. Thirty-five patients (22 TBI patients) were started on oral baclofen and were reevaluated between 1 to 3 months after initiation of treatment. Data for motor tone (Ashworth scores), spasm scores (Penn spasm frequency score), and deep tendon reflex scores were collected on the affected upper extremity (UE) and lower extremity (LE) side(s). Normal extremities were not assessed. Differences over time were assessed via descriptive statistics and Wilcoxon signed-rank. After 1 to 3 months of treatment when subjects had reached their maximal tolerated dosage, the average LE Ashworth score in the affected lower extremities (LEs) decreased from 3.5 to 3.2 (P =.0003), the reflex score decreased from 2.5 to 2.2 (P =.0274), and there was no statistical difference in the spasm score (P >.05). When the 22 TBI patients are analyzed separately, the average LE Ashworth score decreased from 3.5 to 3.2 (P =.0044) and the reflex score decreased from 2.7 to 2.0 (P =.0003). There was no statistically significant change in UE tone, spasm frequency, or reflexes after 1 to 3 months of treatment (P >.05). The average dosage at follow-up was 57 mg/day of baclofen (range 15-120 mg/day). There was a 17% incidence of somnolence that limited the maximum daily dosage of the medication. The oral delivery of baclofen is capable of reducing LE spastic hypertonia resulting from acquired brain injury. The lack of effect upon the upper extremities may be due to receptor specificity issues. GABA-B receptors may be less involved in the modulation of UE spastic hypertonia.