The Plesa Skwerer paper started with the observation that various psychiatric symptoms and conditions seem to be over-represented when it comes to a diagnosis of autism (see here for example). They noted that much of the research on this topic tended look at those who could be considered to be at the 'more able end' of the autism spectrum based on skills like self-report ability. They noted that "the population most severely affected, the ~30% of individuals with ASD who remain non- or minimally verbal beyond school-age" are not particularly well-represented in such comorbidity studies. The specific words they use are the "neglected “severe end of the spectrum”."
So: "Sixty-five participants diagnosed with ASD who had limited verbal abilities" were invited to participate in their study. They were already part of a larger research initiative. When I say 'invited' what I really meant is that: "Informed consent was obtained from the parents." All were diagnosed with an autism spectrum disorder (ASD) and the group was fairly equally split between children (5-11 years old) and young adults (12-18 years old). Parents/caregivers had a big role to play in the Plesa Skwerer study as we told that they were asked to complete various questionnaires about their children, including the Child and Adolescent Symptom Inventory (CASI-5) to "examine the frequency and severity of comorbid psychiatric symptoms." Completing the CASI-5 is no mean feat as per it having "173 items, which rate behaviors as occurring never, sometimes, often and very often."
There are some important caveats to the Plesa Skwerer findings, not least that proxy-reporting was the method used to ascertain the presence of not of such psychiatric comorbidity. This point tells us that a lot more needs to be done to help those who are minimally-verbal to be able to communicate much more readily. Yes, it's a tall order but where there's a will, there's a way. Also, researchers admit that they "excluded those with the most severe behavior problems including aggression, self-injury or non-compliance, and therefore our findings must be viewed in the context of whom our participants represent." Personally I see this is being a pretty issue across quite a lot of research on autism. Indeed, in light of legal rulings here in the UK (see here) talking about aggression 'not being a choice for children with autism' I daresay that by excluding those who present with such issues means that many, many children and adults on the autism spectrum are under-represented in autism research as it stands.
Still, the important message from Plesa Skwerer et al stands: those with autism who are described (defined?) as minimally-verbal seem to show a similar profile of psychiatric comorbidity and a "high degree of maladaptive behavior" as that identified in other parts/regions of the autism spectrum. Screening is implied and, so as to ensure that health inequalities are minimised, access to intervention is also indicated.
Bravo to the researchers who look at the under-studied parts of the autism spectrum.
 Plesa Skwerer D. et al. Prevalence and Correlates of Psychiatric Symptoms in Minimally Verbal Children and Adolescents With ASD. Front Psychiatry. 2019 Feb 18;10:43.
 Russell G. et al. Selection bias on intellectual ability in autism research: a cross-sectional review and meta-analysis. Molecular Autism. 2019; 10: 9.