Treatment tips
Dr. Koo’s first-line medication for DI is pimozide (Orap), which in the United States has the advantage of being approved only for Tourette syndrome; it’s an antipsychotic without the perceived stigma of a psychiatric indication.
“Many of these patients will not consider taking any medication that has any psychiatric indication,” he noted.
Low-dose pimozide is highly effective, according to Dr. Koo, who recommends starting at 0.5 mg to 1 mg/day, increasing by 0.5 mg/day every 2-4 weeks. The drug is usually effective at a dose of 3 mg/day or less. Once a patient’s symptoms become clear or almost clear, the patient is maintained on that dose for another 3-4 months, then tapered by 0.5 mg/day every 2-4 weeks.
“In 35 years of seeing a new patient on average every week or two, I’ve had only five patients with one recurrence and one patient with two recurrences. All six responded to repeat therapy,” Dr. Koo said.
Side effects at these low doses are “very rare,” he added. Diphenhydramine (Benadryl) at 25 mg up to four times daily is effective for complaints of stiffness or restlessness. Prolongation of the QT interval is a potential concern, but Dr. Koo has never encountered it despite routinely ordering ECGs for patients on pimozide with known heart disease or who are over age 50.
When a patient can’t tolerate pimozide, Dr. Koo’s second-line antipsychotic for DI is low-dose risperidone (Risperdal), which is also highly effective.
Dr. Lepping noted that the European situation is different. There, unlike in the United States, pimozide has regulatory approval as an antipsychotic, so it loses the advantage of being an under-the-radar neuroleptic. His go-to medication is the first-generation antipsychotic sulpiride (Dogmatil), which he finds has a more favorable side effect profile than pimozide, particularly in the elderly. (Sulpiride is not approved in the United States.)
In treating DI, he prefers more dopaminergic-focused antipsychotics over those covering a broader spectrum of receptors. His alternatives to sulpiride include risperidone and olanzapine, atypical antipsychotics. He explains to patients that just as aspirin is used in low doses for its antiplatelet effect and in higher doses for pain relief, these medications can help them feel better at much lower doses than for schizophrenia.
“Once we get some rapport and a trusting relationship going, we normally try to persuade people to basically try something against their better judgment. We know that they don’t believe in it, but you try to get them to at least try something because everything else has failed,” Dr. Lepping explained. “We tell them it’s a condition we have seen before, and we have seen these medications to be useful because they are good for their distress, they help with making them calmer, and they might help with their symptoms. We say, ‘What do you have to lose if you trust us?’
“About 60% of our patients take the medication and almost invariably they all get better,” the psychiatrist said. “The others we either lose to follow-up or they just refuse to take the medication.”
A patient’s first visit to the Liverpool multispecialty DI referral clinic is 1 hour long. “They know that in advance, and we very much stick to that hour. We say to people up front, ‘We have an hour – that’s a lot, but we don’t have more,’ ” he said.
The initial visit is typically followed by two to four 30-minute follow-up visits. Dr. Lepping recommends that when possible, patients with DI should be seen jointly by a psychiatrist and a nonpsychiatrist physician. He finds this approach leads to substantially better clinical outcomes than with a single health care provider.
“If you have two people in the clinic with the patient, when you get really annoyed and your amygdala really starts going, that’s the time when you can then turn to your colleague and say, ‘Oh yes, and Professor Squire, what do you have to say to that?’ So as you see the red mist rising in yourself because you’re getting so exasperated, you have the other person there to take over so you can calm down. And then the other person does the same. That can be really important to deescalate a heated situation,” Dr. Lepping explained.
Roughly 10% of patients with DI have what is termed folie à deux, where the delusion of infestation is shared by another person.
“Anecdotally, I would say those are much more difficult to treat,” said Jason S. Reichenberg, MD, MBA, professor of medicine (dermatology) at the University of Texas at Austin and president of the Ascension Medical Group Texas.
Dr. Jason S. Reichenberg
Dr. Jason S. Reichenberg
“It’s like getting somebody to quit smoking when everybody else in the house is still smoking. It’s very hard to convince a single family member that they’re wrong when everybody else in their family keeps telling them they’re right,” he said.
Recent advances in DI research
Dr. Lepping and coinvestigators at multispecialist DI clinics in London, Italy, and Moscow reported in an unusually large observational study of 236 affected patients that longer duration of untreated psychosis was associated with significantly worse clinical outcome. It’s a finding consistent with Dr. Koo’s construct of progressive stages of delusionality, and it underscores the need for early treatment.
“Having said that, improvement is still possible, even if people have had quite a long time of untreated psychosis,” Dr. Lepping said. The same study also showed that older age at illness onset was inversely associated with good outcome.
In another study, Dr. Lepping and colleagues reported that substance use involving amphetamines, cocaine, opioids, and other drugs that can cause itch was roughly twice as common in a group of patients with DI compared to the general population. “I highly recommend, if at all possible, a drug screen in suspected DI,” he said.
In a large survey of U.S. and Canadian veterinarians, Dr. Lepping and coinvestigators found that these practitioners not infrequently encountered delusional infestation among pet owners who claimed their dog or cat is infested when it’s not. This is called “delusion by proxy,” and it often leads to unwarranted animal euthanasia. Some of these pet owners claim they, too, are infested, which the investigators termed “double delusional infestation.”