There are a range of neurological conditions and their disease states that are accompanied by pseudobulbar affect (PBA). Pseudobulbar affect is an emotional disorder, which is characterized by uncontrollable outbreaks of crying and laughing. The causes of pseudobulbar affect are still unclear and might involve lesions in neural circuits that regulate the motor output of emotional expression.

Numerous agents have been used in the treatment of various other psychiatric and neurological disorders and have been applied in the PBA treatment with few success rates but the data available are limited. These agents are associated with disagreeable side effects because of nonspecific activity in diffuse neural networks.

Dextromethorphan (DM), is an extensively used cough suppressant, which acts at receptors in the brainstem and cerebellum along with brain regions involved in the regulation of emotional output. The combination of dextromethorphan and quinidine, which is an enzyme inhibitor that blocks dextromethorphan metabolism, has been recently tested in phase III clinical trials in patients with amyotrophic lateral sclerosis and multiple sclerosis. The combination was safe as well effective in alleviating pseudobulbar affect symptoms. Also, clinical studies related to the safety and efficacy of dextromethorphan and quinidine combination in a variety of neurological disease conditions are still ongoing.

Source: Miller A, Panitch H. Therapeutic use of dextromethorphan: key learnings from treatment of pseudobulbar affect. J Neurol Sci. 2007 Aug 15;259(1-2):67-73. doi: 10.1016/j.jns.2006.06.030. Epub 2007 Apr 16. PMID: 17433820.