Arriving at the diagnosis
In the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), DI is classified as a “delusional disorder, somatic type 297.1 F22.” The diagnosis requires that the delusion be present for at least 1 month, criteria for schizophrenia are not met, and the condition cannot be attributed to other medical or neuropsychiatric conditions.
“Many of these people are very high-functioning. I have corporate CEOs who fly in to see me in their private jets. At work, they’re king of their domain. At home, their family is falling apart because of their delusion,” said Dirk M. Elston, MD, professor and chair of the department of dermatology and dermatologic surgery at the Medical University of South Carolina, Charleston.
Dr. Dirk M. Elston
“These people suffer, and the people around them suffer,” he emphasized.
Dozens of medical conditions can cause intractable itching or biting sensations. Far and away at the top of the medical differential diagnosis is thyroid disease, given its high incidence and frequent presentation with anxiety and itch. Other possibilities that can readily be ruled out via lab tests include substance use – especially involving amphetamine/methamphetamine, cocaine, or opioids – liver or kidney disease, diabetes and other sources of peripheral neuropathy, polycythemia, dermatitis herpetiformis, and pemphigus, Dr. Elston said.
Scott A. Norton, MD, MPH, MSc, a dermatologist and preventive medicine specialist at the Uniformed Services University of the Health Sciences in Bethesda, Md., noted that a diagnosis of DI requires three elements: The presence of abnormal sensations in the skin, a patient’s tenacious conviction that the sensations are caused by an infestation, and a lack of supporting evidence for that conviction.
Dr. Scott A. Norton
Taking an accurate medical history can be a challenge in these patients because they are often so guarded. They won’t disclose that they’ve already seen other health care providers, or that they’ve been self-treating with OTC veterinary medicine products, such as high-dose topical or oral ivermectin. They’ll often even deny repeated scratching despite clear evidence to the contrary from the skin exam.
As a dermatologist, Dr. Norton considers his first task to be a search for evidence of an infestation. Scabies is usually the first diagnosis proposed to account for the uncomfortable skin sensations. The presentation can be subtle. While the classic teaching is that the telltale signs of infestation by Sarcoptes scabiei are burrows in the skin and a rash in the web spaces between the fingers, he finds these features are often absent or equivocal.
“I think there are two more reliable presentations of scabies: Check to see if there’s symmetric involvement of the volar or palm side of the wrists; if there isn’t, I’m skeptical of the diagnosis. And every male older than 1 year of age with scabies will have scabies nodules on their genitalia. If the penis, the glans, or the scrotum aren’t involved with the nodules, I discard scabies as a possible diagnosis and look for evidence of other skin conditions that can plausibly explain the sensations and skin lesions, like eczema, contact dermatitis, scalp folliculitis, or dry skin,” he said.
If he can’t find evidence of infestation, he next systematically looks for another dermatologic cause of the patient’s sensations. When that proves fruitless, he tries to determine if there might be a biomedical or neuropsychiatric cause, such as depression, anxiety, schizophrenia, or dementia.
Taking a personal hygiene history is helpful. Patients who believe they have an infestation may bathe or shower three to five times daily with harsh soaps, causing dry, inflamed, itchy and uncomfortable skin.
“Many patients are thrilled to hear the good news that the history, physical examination, and lab tests do not show an infestation and that we have another explanation to account for their unwanted sensations. However, there are some patients who vehemently reject that idea and immediately return to their unwavering, unalterable belief that they are in fact infested. At this point, the possible diagnosis of DI looms large,” the dermatologist said.
Clues suggestive of DI include a patient’s obsessive focus on collecting “specimens” of the offending pathogen in Ziplock bags for assessment during the office visit – “usually a mix of unhelpful household debris and environmental detritus” – and eager presentation of a lengthy and detailed infestation diary, Dr. Norton said.
“Among the most distinctive signs that the patient is detached from reality are the biologically implausible descriptions and explanations of the supposed attacking organism. It’s a fanciful amalgamation of mutable features, behaviors, and life cycles composed of a composite of taxonomically unrelated organisms – for example, fungal hyphae with wings – that shapeshift at will to evade detection,” he said.
Dr. Elston observed that DI skin lesions are typically excoriated, sometimes because of a patient’s systematic use of a sharp object in an effort to dig out the infestation.
“One of the clues is the angularity of the lesion,” the dermatologist noted. “We always say round-to-oval lesions suggest an inside job; angulated lesions suggest an outside job, like fingernail work. There’s often a row of good healing border showing there’s really nothing wrong with wound healing, but a fibrinoid base where the excoriations have occurred. And the lesions are often in various stages of healing.”
Don’t forget neuropathic itch in nondelusional individuals as a potential cause of sensations of infestation and self-injury due to relentless scratching, urged Anne Louise Oaklander, MD, PhD, associate professor of neurology, Harvard Medical School, Boston, who is director of the nerve unit and the neurodiagnostic skin biopsy lab at Massachusetts General Hospital, Boston.
Dr. Anne Louise Oaklander
“There’s no one cause of patients’ impressions that they may have insects. Let’s be sympathetic: It is a normal assumption that insects may be present if the skin itches. One problem is that when patients don’t get good medical diagnoses they make up their own explanations, and sometimes these include persistent ideas of infestation. Many of them don’t realize that their scratching is a cause, not a result, of their skin lesions,” said Dr. Oaklander, who has conducted pioneering research on unintentional self-injury due to neuropathic itch accompanied by loss of pain signaling.