The validity of analyses testing the etiology of comorbidity between two disorders: a review of family studies

The multi-generational familial aggregation of interstitial cystitis, other chronic nociplastic pain disorders, depression, and panic disorder

BACKGROUND

Interstitial cystitis/painful bladder syndrome (IC) is a chronic pelvic pain condition which has high comorbidity with other nociplastic, or unexplained, pain disorders [e.g. fibromyalgia (FM), irritable bowel syndrome (IBS), and myalgic encephalomyelitis/chronic fatigue (ME/CFS)] and some psychiatric conditions [major depressive disorder (MDD) and panic disorder (PD)]. Here we investigated the shared familiality of IC and these other nociplastic and psychiatric conditions.

METHODS

Subjects were identified in the Utah Population Database, which links genealogy data back to the 1800s to medical record diagnosis billing code data back to 1995. We computed the relative risk of each of these disorders among first (FDR), second (SDR), and third-degree relatives (TDR) of six proband groups: IC, FM, IBS, ME/CFS, PD, and MDD. Given the known familial aggregation of each of these disorders, we conducted our analyses to test for heritable interrelationships using proband subgroups whose members did not have the diagnosis assessed in their relatives.

RESULTS

We observed strong evidence for heritable interrelationships among all six disorders. Most analyses indicated significantly increased risk for each of the six disorders in FDR, SDR, and TDR of all or most proband groups. Out of 30 possible bidirectional disorder interrelationships, 26 were significant among FDR, 23 were significant among SDR, and 7 were significant among TDR. Clustering was observed in both close and distant relatives.

CONCLUSIONS

Our results support a common, heritable component to IC and other nociplastic and psychiatric conditions.

Test of Alternative Hypotheses Explaining the Comorbidity Between Attention-Deficit/Hyperactivity Disorder and Conduct Disorder

There is significant comorbidity between attention-deficit/hyperactivity disorder (ADHD) and conduct disorder (CD). The conclusions of studies that examined the causes of comorbidity between ADHD and CD conflict, with some researchers finding support for the three independent disorders model and others finding support for the correlated risk factors model. We tested these models and eleven alternative hypotheses using the same analytical approach. The participants were 110 monozygotic twin pairs and 181 dizygotic twin pairs recruited from the Colorado Learning Disabilities Research Center Twin Study. The three independent disorders model did not fit the data, whereas the correlated risk factors model fit the data well. Several other comorbidity models fit the data as well as or better than the correlated risk factors model. The results suggest that correlated risk factors are a better explanation for the comorbidity between ADHD and CD than a third, independent ADHD+CD subtype.

Comorbidity between alcohol dependence and illicit drug dependence in adolescents with antisocial behavior and matched controls

BACKGROUND

Knowledge regarding the causes of comorbidity among substance use disorders can have significant impact on future research examining the etiology of these disorders. Unfortunately, the conclusions of past studies examining the comorbidity among substance use disorders are conflicting; some studies emphasize familial influences common to multiple substances, while others emphasize substance-specific influences. Discrepancies in results may reflect different analytical approaches or differences in the samples. Here, we examine the causes of comorbidity between alcohol dependence and illicit drug dependence in adolescents.

METHODS

We ascertained a clinical sample of adolescents treated for antisocial behavior and substance use disorders and their siblings and a matched control sample. A model fitting approach was used to test 13 alternative hypotheses for the causes of comorbidity.

RESULTS

The best supported hypothesis for the comorbidity between alcohol dependence and illicit drug dependence was a model hypothesizing that comorbid disorders are alternate forms of a single underlying liability. The next best fitting models were two of the correlated liabilities models (correlated risk factors and reciprocal causation).

DISCUSSION

The results suggest that the best hypotheses explaining the comorbidity between alcohol and illicit drug dependence in adolescents are that alcohol dependence and illicit drug dependence are manifestations of a single general liability to develop substance dependence or that there are separate liabilities that are highly correlated.

Testing hypotheses regarding the causes of comorbidity: examining the underlying deficits of comorbid disorders

The authors examined the validity of a method commonly used to test alternative hypotheses regarding the causes of comorbidity: the examination of underlying deficits of comorbid disorders. The authors simulated data in which the true causes of comorbidity were known, then compared the patterns of underlying deficits of the comorbid disorders found in the simulated data with the predicted results. The method of examining the underlying deficits of comorbid disorders could distinguish between several comorbidity models, including those that could not be distinguished well using other methods. The ability to distinguish the correct model decreased as the sample size and the correlation between the underlying deficits and the symptom scores decreased, suggesting that the issue of power should be considered carefully.