Clinical rating scales that correlate to disease severity appear to be the most effective in identifying PBA. The PRISM study, to date the largest clinic-based study of PBA symptoms, used the Center for Neurologic Study-Liability Scale (CNS-LS) to gauge the presence and severity of PBA symptoms.1 A 7-question, patient self-administered tool, the CNS-LS is graded on a 5-point Likert scale. A score ≥13 has high sensitivity and specificity for diagnosis of PBA, compared with physician diagnosis.
Another option, the 16-question Pathological Laughing and Crying Scale, is a clinician-administered screening tool. Again, a score ≥13 is consistent with symptoms required for a PBA diagnosis.
Treating PBA symptoms
Until recently, most pharmacotherapeutic interventions for PBA were based on off-label use of tricyclic antidepressants (TCAs) or selective serotonin reuptake inhibitors (SSRIs). From 1980 to 2010, only 7 of 22 case reports or trials of TCAs or SSRIs for PBA were randomized, double-blind, and placebo-controlled. Five had 12 to 28 patients, and 2 had 106 and 128 patients, respectively. Only 1 controlled trial included a validated symptom severity scale, and none included a scale validated for PBA.18
In particular, imipramine and nortriptyline were studied for managing PBA in patients with stroke; amitriptyline, in patients with MS; and various SSRIs, in patients with stroke.11 Response of PBA symptoms to antidepressant therapy was greater in all placebo-controlled trials than response to placebo.18 As seen in pharmacotherapy of depression, the lower burden of adverse effects and overall better tolerability of SSRIs resulted in their preferred use over TCAs. In some cases, the side effects of TCAs can be leveraged for therapeutic gain. If insomnia is a problem, a nighttime dose of a TCA could ameliorate this. Similarly, if a patient has sialorrhea, the anticholinergic effect of a TCA may show some benefit.19
Dextromethorphan plus quinidine. Dextromethorphan has long been of interest for a variety of neurodegenerative diseases. Studies of its efficacy were largely unsuccessful, however, because rapid metabolism by cytochrome P450 (CYP) 2D6 prevented CNS penetration.20 Quinidine is an avid inhibitor of CYP2D6, even at very low dosages. Adding quinidine to dextromethorphan limits metabolism, allowing dextromethorphan to accumulate to a plasma concentration sufficient to penetrate the CNS.12 In 2010, the combination agent dextromethorphan hydrobromide (20 mg)/quinidine (10 mg) (DM/Q) became the first treatment to receive FDA approval for managing PBA.11
Mechanism of action. The exact mechanism of DM/Q in PBA remains unknown. Dextromethorphan is an agonist of sigma-1 receptors and a relatively specific noncompetitive antagonist of NMDA receptors. It also has been shown to modulate glutamate and serotonin neurotransmission and ion channel function.20 Sigma-1 receptors are concentrated in the brainstem and parts of the cerebellum that are thought to coordinate motor emotional responses. Agonism of sigma-1 receptors on glutamatergic neurons has been proposed to limit release of glutamate from the presynaptic neuron while also limiting downstream transmission of glutamatergic signal in postsynaptic neurons.
Clinical trials. Two large trials have demonstrated efficacy of DM/Q in PBA. STAR was a 12-week, double-blind, placebo-controlled trial with 326 patients diagnosed with ALS or MS who showed PBA symptoms (CNS-LS score ≥13). Compared with placebo, DM/Q use was associated with significantly reduced (P < .01) daily episodes of PBA at 2, 4, 8, and 12 weeks.20 The effect was rapid, with 30% fewer PBA episodes after the first week (P < .0167). At 12 weeks, 51% of patients on DM/Q had been symptom-free for at least 2 weeks.
The PRISM II study examined the efficacy of DM/Q in managing PBA in 102 individuals with dementia, 92 with stroke, and 67 with TBI. After 30 and 90 days, CNL-LS scores were significantly reduced (P < .001) compared with baseline scores.20
Prescribing information. Dextromethorphan—typically in the form of cough syrup—has been implicated as a substance of abuse. A placebo-controlled trial demonstrated that co-administering quinidine with dextromethorphan limits measures of positive reinforcement, such as euphoria and drug liking. This suggests that quinidine may be used to reduce abuse of dextromethorphan.20 As such, the abuse potential of DM/Q appears to be low.
The most common adverse effects reported with DM/Q are diarrhea, dizziness, and cough.12 Notably, patients who received DM/Q in the STAR trial were more likely to report dizziness than those receiving placebo (10.3% vs 5.5%), but patients receiving placebo were more likely to fall.21,22
Package labeling warns that DM/Q causes dose-dependent QTc prolongation.21 Quinidine can be associated with significant QTc prolongation when dosed at antiarrhythmic levels, although mean plasma concentrations found with the 10 mg of quinidine in the approved DM/Q formulation are 1% to 3% of those associated with typical dosages used in antiarrhythmic therapy. Electrophysiology studies of quinidine 10 mg dosed every 12 hours have demonstrated a mean QTc increase at steady state of 6.8 milliseconds, compared with 9.1 milliseconds for a reference control (moxifloxacin).12,21
Although this would seem to indicate a relatively low risk of clinically significant QTc prolongation at these ultra-low dosages of quinidine, it may be advisable to obtain an initial pre-dose and post-dose ECG and longitudinally monitor the QTc interval in patients with conditions that predispose to cardiac arrhythmias. Because quinidine inhibits CYP2D6, use caution when prescribing and monitoring other medications metabolized by this pathway.