Friday, February 20, 2009

Gender, Age and Subtype Effects on ADHD Comorbid Disorders

We have spoken extensively on some of the related or comorbid disorders associated with ADHD ("Comorbid" here refers to an accompanying disorder that frequently occurs alongside ADHD. These may include disorders such as depression, Tourette's Syndrome, allergies, substance abuse problems and the like). The topic of this post is to investigate whether there is a pronounced gender effect on these comorbid disorders; in other words, whether boys and girls are more prone to a particular disorder comorbid to ADHD based on their gender. As we will see later, age and ADHD subtype effects are also important factors with regards to comorbid disorders.

Much of this info was taken from an article titled Gender Differences in ADHD Subtype Comorbidity by Levy and coworkers. Here is a summary of some of the main points in the study:
  • Additionally, ADHD has traditionally been separated into three different forms or subtypes: inattentive, hyperactive/impulsive, or combined (a combination of the other two subtypes). All three subtypes are heavily skewed towards the boys, which outnumber girls from anywhere around 2:1 to 5:1 (some studies skew this gender difference even higher, up around 10:1). Based on the study by Levy and coworkers, here is an approximate distribution (numbers indicate overall percentages among the study population, which includes non-ADHD individuals) among the prevalence of the three subtypes for both genders:
As we can see, all three subtypes are skewed heavily in favor of the boys.
  • Of the three subtypes listed above, it appears that the subtype (again, perhaps not surprisingly) most associated with comorbid disorders (listed in the first point) is the combined subtype.
  • There appears to be a discrepancy between the genders as far as internal/external symptoms of ADHD and related disorders. Some studies have suggested a general trend in which many of the symptoms or problems of girls with ADHD and related disorders are more internalized (i.e., they do not outwardly manifest themselves as readily as boys), which may contribute to the skewed gender differences mentioned above. On the contrary, the same study suggests that external or outward symptoms are more apparent in boys, which may compound this effect.
  • Reading disabilities are, perhaps not surprisingly, more common in children with ADHD. It appears that reading disabilities correlate more to "internal" symptoms in girls and "external" symptoms in boys with ADHD, however, reading disorders appear to have very little overlap with conduct or oppositional behaviors such as aggression or delinquent behavior. Furthermore, reading difficulties appear to be more related to the inattentive side of the disorder of ADHD than the hyperactive/impulsive side of the disorder. In other words, the inattentive and combined ADHD subtypes are significantly more likely to have problems with reading than the exclusive hyperactive/impulsive subtype for both genders. It appears that reading difficulties and inattentive behavior may have an even stronger correlation in girls.
  • Furthermore, with regards to reading and speech disabilities, there is a strong gender difference for non-ADHD individuals. However, once the disorder of ADHD is introduced, the gender difference becomes less of a factor (this holds for all three ADHD subtypes). This may at least suggest, that ADHD symptoms may override or overpower what appears to be more subtle gender differences with regards to speech and reading disorders.
  • There is a significant association between generalized anxiety disorders and ADHD for both genders. Gender differences for the combined ADHD subtype were especially pronounced, with rates among females with the combined ADHD subtype being significantly higher than the combined subtype males. In addition, the combined subtype was more associated with generalized anxiety for both genders (when compared to the inattentive subtype), which suggests that hyperactivity/impulsivity may play some sort of role in generalized anxiety for both genders.
  • With regards to separation anxiety disorders (such as from parents or loved ones), it also appears that there is a higher correlation to girls with ADHD, especially with regards to the inattentive ADHD subtype. For boys, the separation anxiety disorders were highest for the combined ADHD subtype. The study suggested that separation anxiety disorders may be a sign of immaturity for both genders, and may be indicative of later "internalizing" problems in girls. Furthermore, this assertion is in agreement with several studies which associate ADHD with a delay in maturity.
  • Based on the two findings above, in which girls with the inattentive ADHD subtype had higher rates of separation anxiety disorders and girls with the combined subtype having increased rates of generalized anxiety disorders (both of which are considered more "internal" symptoms) than their male peers, it may be suggest that screening for ADHD in girls who exhibit anxiety disorders may be beneficial, in that it may reveal underlying comorbid ADHD and offset some of the skew among gender differences and ADHD.
  • Finally, age has been shown to be an important factor with regards to symptoms and severity of ADHD comorbid disorders. In this study, comparisons were done between the younger (ages and and under) and older (ages 11 and older) children in the study population. For males, the prevalence of most of the comorbid disorders (speech and reading difficulties, oppositional defiance, generalized and separation anxieties) decreased with age, with the notable exception being conduct disorders, which increased with age. For females, age was less of a factor for all of the comorbid disorders listed above with the exception of Separation Anxiety Disorders, which decreased with age (supporting the earlier assertion that this disorder is tied to maturity levels and would naturally decrease as a child gets older). In addition, inattentive symptoms associated with ADHD actually increased with age for the female population of the study. This was the exception to the overall trend of decreasing ADHD symptoms with age, which was seen in the other two subtypes for females and all three subtypes for males.
I would like to conclude with a final note of personal opinion. I firmly believe that when screening, diagnosing and attempting to treat ADHD and comorbid disorders, we employ far too little emphasis on the gender differences surrounding these disorders. This can lead to several potential problems such as stereotyping or pigeon-holing certain behaviors (i.e. attributing hyperactivity/impulsivity as being a "male" characteristic and either intentionally or unintentionally overlooking these symptoms or behaviors in girls).

In addition, it appears that girls may have a higher prevalence of the more "internal" comorbid disorders such as anxiety, which are often more difficult to detect than the more outward comorbid disorders of oppositional defiance and conduct disorders. This may play a major part in the gender discrepancy of ADHD diagnosis, which may leave a number of girls with ADHD undiagnosed and untreated.

Additionally, the more "internalized" nature of female cases may also lead to a lack of diagnosis and treatment for comorbid disorders associated with ADHD as well. The Levy study pointed this out, citing the discrepancy between referrals for ADHD-related reading disabilities. Reading disorders for boys were more likely to be associated with some of these outward characteristics, while girls with reading disorders exhibited more of the aforementioned "inward" traits. As a result, the rates of referral for boys with reading disabilities (based on their overall representation in the population) was almost twice that of girls.

Furthermore, this study by Levy, as well as several others, indicate that there are several (sometimes unusual or counter intuitive) associations between gender, and ADHD subtype and the expression of symptoms of specific comorbid disorders. For example, attributing an increase in Separation Anxiety disorders to younger females with the Inattentive ADHD subtype or Conduct Disorders to the Combined ADHD subtype in males may give us some possible insight as to which subpopulations of ADHD children are most "at risk" for developing some of the aforementioned comorbid disorders.

Since several of these comorbid disorders carry their own lines of medication and other treatments, the subclassification of ADHD children based on age, gender and subtype may be especially beneficial with regards to developing successful individualized treatment plans. I firmly believe that by separating out and subcategorizing ADHD and its comorbid disorders based on factors such as age, gender and subtype whenever possible could lead to a new a wealth of information for diagnosing and treating ADHD and its associated comorbid disorders.