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March 21, 2022
An international study team conducted the first meta-analysis of studies examining differences in time perception between persons with ADHD and normally developing controls. A systematic search of the peer-reviewed medical literature identified 55 studies that could be combined into various subgroups for meta-analysis.
A meta-analysis of 25 studies with a combined 1,633 participants looking at time discrimination found a medium effect size deficit among persons with ADHD in the number of correct comparisons between the length of two signals. There was little between-study heterogeneity and no sign of publication bias.
Turning to time estimation, a meta-analysis of eight studies with a combined 1,024 participants found a small-to-medium effect size increase in absolute errors (i.e., the absolute value of deviation between the specified and the estimated time interval, representing the absolute amount of error regardless of its direction) among persons with ADHD, compared to controls. Again, there was little between-study heterogeneity and no sign of publication bias.
A meta-analysis of seven studies with combined 380 participants looked at differences in time production, in which they had to produce a previously specified time interval by pressing and holding a button. In this case, those with ADHD manifested a small effect size increase in absolute error relative to their normally developing counterparts. There was moderate between-study heterogeneity and no sign of publication bias.
Finally, a meta-analysis of 26 studies with combined 2,364 participants examined differences in time reproduction, in which they had to reproduce the duration of a previously presented stimulus by pressing and holding a button. Here, those with ADHD exhibited a medium effect size increase in absolute error. There was moderate between-study heterogeneity and no indication of publication bias.
An acknowledged limitation of these meta-analyses was the inability to assess the effects of pharmacological treatment. In addition, 84% of the studies did not report the ethnicity of participants.
The team concluded, "We found meta-analytic evidence of significant deficits in individuals with ADHD across all timing paradigms ... individuals with ADHD have difficulties to discriminate stimuli that vary from each other for only several milliseconds, and they are more variable in their time estimates of several seconds irrespective of the paradigm examined, which may both be driven by their lowered alertness levels."
They suggested that this might eventually become a criterion to help diagnose ADHD: "Our findings have possible clinical implications, albeit not currently directly applicable to the clinical practice. As timing has been proposed as an independent neuropsychological pathway to ADHD, timing tasks should be considered in the clinical assessment of ADHD to better characterize the clinical profile of the patient... To characterize further the phenotype of the patient during the diagnostic process that may deserve clinical attention, we suggest developing a tool based on the time estimation paradigm. The time estimation accuracy score not only represents an intuitive score reflecting faster internal clock mechanisms in individuals with ADHD, but the paradigm also shows high internal consistency and test-retest reliability, allowing for a reliable assessment of developmental or interventional changes in timing abilities related to developmental factors or external interventions."
Ivo Marx, Samuele Cortese, Michael G Koelch, Thomas Hacker, "Meta-analysis: Altered Perceptual Timing Abilities in Attention-Deficit/Hyperactivity Disorder," Journal of the American Academy of Child and Adolescent Psychiatry (2021), published online,https://doi.org/10.1016/j.jaac.2021.12.004.
The Background:
Stimulant medications (like methylphenidate and amphetamines) and non-stimulants (like atomoxetine and guanfacine) remain the first-line treatments for ADHD, but their long-term use is often constrained by parental concerns, side effects (reduced appetite, sleep disruption, cardiovascular effects), and the fact that roughly 30% of children respond only partially or not at all. These medications tend to help with attention and behavior, but they do less to address emotional regulation or broader physical health, leaving some parents to seek other safe, complementary treatments.
Nutritional approaches, including omega-3 fatty acids, are one such avenue. Oily fish is the natural source, but clinical trials typically rely on supplements instead, since they allow for precise dosing, avoid the heavy-metal exposure that can come with high fish intake, and sidestep the problem of inconsistent dietary habits. Two omega-3s stand out as particularly relevant: DHA, which supports brain structure and cell-to-cell communication, and EPA, which helps reduce brain inflammation and improve blood flow. Both influence the dopamine and serotonin systems implicated in ADHD.
The Study:
Previous meta-analyses have generally found only modest overall benefits with mixed results. This mixed-results pattern may reflect the fact that children who had low omega-3 levels to begin with stand to benefit more than those whose levels are already adequate. This meta-analysis set out to test that idea directly, asking whether a child's baseline omega-3 status predicts how well they respond to supplementation.
To be included, studies had to be randomized controlled trials (RCTs) involving children diagnosed with ADHD who did not have major coexisting medical conditions. Trials that used broad multinutrient interventions, where the effect of omega-3s specifically couldn't be isolated, were excluded.
The resulting analysis of seven RCTs covering 654 children found small overall improvements in ADHD symptoms with omega-3 supplementation, consistent with earlier meta-analyses. There was little variation across studies, but a clear signal of publication bias, which the authors did not attempt to statistically correct for.
Among four RCTs involving 369 children with low baseline omega-3 levels, supplementation produced a moderate improvement in ADHD symptoms, with consistent results across studies. By contrast, the six RCTs covering 559 children with normal or high baseline omega-3 levels showed no improvement at all, again with consistent results across studies.
The Results:
The authors concluded: this systematic review and meta-analysis offers preliminary evidence that children’s response to long-chain omega-3 supplementation for ADHD may depend on their starting omega-3 nutritional status. Larger effects emerged in the subgroups defined by individual studies as having low baseline levels, while normal, high, or unstratified groups showed little evidence of benefit. In other words, there may be a benefit to children with ADHD taking omega-3 supplements, but only if they are already deficient.
Even still, a sensitivity analysis limited to ADHD symptom-rating scales found only a numerically higher effect in the low-baseline group. This means that these results, though positive, are not considered statistically significant.
The Take-Away:
Given that the evidence rests on just seven trials, that “low baseline” was defined differently across studies (using different biological markers and thresholds), and that several subgroup comparisons were not planned in advance, the authors caution that these findings should be treated as hypothesis-generating rather than conclusive.
The Background:
Previous research has suggested that people with ADHD have higher rates of death than those without the condition. Until now, many of those studies lacked the detail needed to untangle why this might be the case. A recent meta-analysis of eight separate studies reported that ADHD roughly doubled the risk of death, and it hinted that factors like socioeconomic status and other mental illnesses might help explain the link.
The Study:
To explore this in more detail, researchers used the US National Health Interview Survey, a nationally representative prospective cohort of just over 52,000 adults. Survey records were linked with the National Death Index, and participants were followed from the date of interview until death or December 31, 2019, whichever came first. The main outcome was death from any cause. With a median follow-up of just under eight years, there were 6,024 deaths recorded.
When the team adjusted their analysis for age, sex, ethnicity, and the year of the survey, having ADHD was associated with a 58% higher risk of death. Controlling further for family income and educational level reduced that increased risk slightly, to 50%.
Controlling for health conditions (like high blood pressure, diabetes, stroke, heart disease, and cancer) and lifestyle factors (smoking, alcohol use, physical activity, and body weight) also reduced the association to about 46%.
By contrast, when the researchers adjusted for two psychiatric conditions (anxiety and depression) commonly found alongside ADHD, the link between ADHD and higher mortality disappeared statistically. In other words, once anxiety and depression were taken into account, ADHD itself was no longer associated with a clearly measurable increase in the risk of death.
What Does This Mean?
The authors conclude that the apparent effect of ADHD on mortality seems to operate largely through accompanying mental illness, and to a lesser extent through lifestyle and physical health problems. They suggest that treating and managing these co-occurring issues (through medications, therapy, and behavioral interventions) might help reduce the risk of premature death in people with ADHD, although they note that further evidence is needed.
The Background:
Dementia is a rapidly growing public health problem affecting more than 50 million people worldwide. Nearly 10 million new cases are diagnosed each year, and that number is expected to almost triple by 2050. The most common form is Alzheimer’s disease, accounting for roughly 60–70% of cases, followed by vascular dementia at around 20%. Other types include dementia with Lewy bodies, Parkinson’s disease dementia, and frontotemporal dementia. While no cure exists, managing modifiable risk factors (such as high blood pressure, smoking, physical inactivity, and poorly controlled diabetes) can delay or slow both the onset and progression of the condition.
Several studies have reported an elevated dementia risk among people with ADHD, and a number of mechanisms may explain this association. These include shared genetic or neurobiological vulnerabilities, reduced cognitive reserve (leaving the brain less resilient to age-related damage), and greater lifetime exposure to other established dementia risk factors.
Earlier reviews explored the possible link between ADHD and dementia, but tended toward a broader scope (incorporating other neurodegenerative disorders or mixing study designs), which introduced substantial variability and precluded quantitative synthesis. The relevant research literature has also expanded considerably since those reviews appeared.
The Study:
This meta-analysis restricted inclusion to case-control and cohort studies, yielding five that met the criteria. One was a case-control study with 400 participants; the other four were cohort studies collectively covering 3.7 million individuals, two retrospective and two prospective.
The Results:
Pooling the four cohort studies, individuals with ADHD showed approximately two-and-a-half times the dementia risk of those without the diagnosis. There was no indication of publication bias.
The Take-Away:
The authors concluded that ADHD is associated with a significantly increased risk of all-cause dementia in later life, while noting that its relationship to specific dementia subtypes remains less clear and warrants attention in future research.
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