Asenapine is an underutilized antipsychotic. Its mechanism of action spans multiple receptors and is less specific in individual receptor activity than other dopamine blockers. It is administered under the tongue due to poor absorption when swallowed, and its molecule has an anesthetic property that causes mouth and tongue numbness/paresthesia. This function may help patients with orofacial pain. Significant somnolence and weight gain (although less than with olanzapine) limit its use. Some patients cannot tolerate the taste.28
Quetiapine is prescribed rather frequently due to its significant antianxiety effect. It is also reported to be beneficial in pain control.29 Weight gain may be severe. In doses smaller than typically administered to patients with bipolar disorder or schizophrenia, quetiapine is widely prescribed off-label for sleep. In lower doses, it acts primarily as an antihistamine (hence the sedation), but at an increased dose it activates the adrenergic system, which offsets sedation. Quetiapine antagonizes H1 histamine and 5HT2C receptors, which may explain its associated sedation and weight gain. Constipation is common. Due to its relatively low risk for EPS, quetiapine is safer to prescribe in patients with Parkinson’s disease. It can cause withdrawal if abruptly discontinued, so it needs to be tapered. Quetiapine has become a commodity in the prison population because of its ability to diminish anxiety symptoms.30 There are also reports that quetiapine may be associated with pain induction. This is consistent with the above-mentioned phenomenon that pain is associated with both the lack and excess of dopaminergic function.31 Pain perception is reported to be diminished in patients with schizophrenia,32 and quetiapine may increase pain just by improving cognition.
Cariprazine is typically well tolerated because of its benign metabolic profile. It does not increase the QT interval and is not sedating. Cariprazine is a D2 and D3 partial receptor agonist. This allows the medication to inhibit overstimulated dopamine receptors (a desirable effect in pain management) and induces them when the endogenous dopamine level is low (helping with cognition, volition, and attention). Pro-cognitive effects are always beneficial for patients with pain. Cariprazine produces less EPS due to more ventral striatum vs dorsal striatum activity. Mood improvement caused by this medication is attributed to its 5HT2A, 5HT2B, and 5HT2C inverse agonism, which modulates the serotonergic system. Cariprazine will likely have a positive future in pain management because it has shown efficacy in the chronic stress model.33
A complex condition
No single medication or group of medications may be exclusively relied on for treating patients with chronic pain. Identifying alternatives to opioids for treating pain brings more attention to centrally-acting medications that may aid in the stabilization of the nervous system, which can decrease pathological pain perception and help patients cope with chronic painful conditions.
Antipsychotics may be a valuable asset in the treatment of chronic pain, offering a potential alternative to prescribing opioids for pain. More research is needed to identify specific ways of using dopamine blockade or dopamine enhancement to help patients with chronic pain.
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