1. The Attention-Deficit Hyperactivity Disorder Paradox: 2
The Attention-Deficit Hyperactivity Disorder Paradox: 2. Phenotypic Variability in Prevalence and Cost of Comorbidity
Larry Burd, PhD; Marilyn G. Klug, PhD; Matthew J. Coumbe, PhD; Jacob Kerbeshian, MD

2. Abstract
The objective of this study was to use population-based data to estimate the cost and phenotypic variation of conditions comorbid with attention-deficit hyperactivity disorder (ADHD). The case population was 7745 children. We then examined the 10 most frequent comorbidities for a defined condition from the ICD-9. The observed prevalence of general health conditions was 12 to 70% less than expected. The presence of a comorbid condition resulted in increases in costs of care of $381 to $731 per case per year. The observed prevalence of comorbid mental health conditions was 97 to 5286% higher than expected. We identified an ADHD paradox: decreased rates of comorbid general health conditions and increased rates of comorbid mental disorders. Further research on comorbidity in ADHD is required. (J Child Neurol 2003;18: ).

3. Study Population
The original data set consisted of 1,509,980 visits made by children ages 0 to 21 years in North Dakota in 1996 and This included both inpatient and outpatient visits. For the 2 years combined, a total of 1,473,611 visits were included in this analysis. This represented (averaged over 1996 and 1997) visits made by 197,983 uniquely identified children (88,702 in 1996 and 109,281 in 1997).

4. Inclusion Criteria: Cases
Patients were included in the study as cases with ADHD if at any time during either year they had an International Classification of Diseases, 9th edition (ICD-9), code of or (attention-deficit disorder not otherwise specified), or (inattentive type), (with hyperactivity/impulsiveness), or (with other specific manifestations). The code (attention-deficit hyperactivity disorder not otherwise specified) was also included to capture cases from the Diagnostic and Statistical Manual of Mental Diseases-IV (DSM-IV).

5. Inclusion Criteria: Controls
The control population consisted of all remaining subjects in the database who did not have an ICD-9 code of , , or or the DSM-IV code of in their diagnostic formulation at any time during the 2-year span of the study. There were 24.5 controls per case in this study.

6. Limitations
The validity of the diagnosis reported needs to be considered.
These data are from a single rural state, and the subjects are not currently traceable across years or type of claims.
The data set does not include uninsured children or children who did not access health care in the 2-year period included in this study.

7. Prevalence of ADHD by Age Group, Gender, and Payer Type
1996
1997
Mean
N (%)
Age Group
Infant to 5 yr
334 (1.2)
190 (0.6)
262 (0.9)
6 to 13 yr
2952 (10.8)
2387 (6.7)
2669 (8.7)
14 to 21 yr
1031 (3.1)
851 (2.1)
941 (2.6)
Gender
Male
3247 (7.5)
2570 (4.8)
2908 (6.1)
Female
1070 (2.4)
858 (1.6)
964 (2.0)
Payer Type
Public
1233 (5.8)
909 (6.0)
1071 (5.9)
Private
3084 (4.6)
2519 (2.7)
2801 (3.6)
ADHD = attention-deficit hyperactivity disorder

8. Results Prevalence and Base Costs of ADHD
The prevalence of ADHD in these claims data was 3.9% or 7,745 children (4,317 in 1996 and 3,428 in 1997).

9. Prevalence and Risk of 4 Most Common General and Neurologic Diagnoses in Children With ADHD Compared With Children Without ADHD (Controls)
Observed
Expected
Added
Diagnosis
N (%) N
Cases
AR%
Respiratory illness
ADHD
Control
312 (7.2)
18,672 (22.1)
955
-643
-206.2
267 (7.8)
19,513 (18.4)
632
-365
-136.7
Acute pharyngitis
271 (6.3)
16,422 (19.5)
840
-569
-210.0
ADHD
318 (9.3)
17,374 (16.4)
563
-245
-76.9
Respiratory illness: 465.xx; Acute pharyngitis: 462.xx
ADHD = attention-deficit hyperactivity disorder; AR% = attributable risk percentage

10. Prevalence and Risk of 4 Most Common General and Neurologic Diagnoses in Children With ADHD Compared With Children Without ADHD (Controls)
Observed
Expected
Added
Diagnosis
N (%) N
Cases
AR%
Vision
ADHD
Control
368 (8.5)
5146 (6.1)
263
105
28.5
ADHD
181 (5.3)
30,006 (2.8) 97 84
46.2
Suppurative otitis media
ADHD
259 (6.0)
15,717 (18.6)
804
-545
-210.4
235 (6.9)
16,449 (15.5)
533
-298
-126.7
Vision: 367.xx; Suppurative otitis media: 382.xx
ADHD = attention-deficit hyperactivity disorder; AR% = attributable risk percentage

11. Comorbidity With General and Neurologic Diagnosis
Prevalence
These data identify an important association between a diagnosis of ADHD and a decrease in the total risk of these comorbid conditions. This suggests the possibility of a protective effect from ADHD for several common illnesses.

12. Comparison of Mean Costs of Common General and Neurologic Illnesses Between Children With ADHD and Controls (Base Cost=$495, Cost Attributable to ADHD=$154)
Mean Cost per
Year ($)
ADHD
Control
Mean Cost
Attributable ($)
Mean Cost of
Comorbidity ($)
Respiratory illness
1996
1997
Average
1429
837
1133
831
488
659
222
108
164
481
159
320
Acute pharyngitis
1385
789
1087
792
445
618
183 65
123
476
153
315
ADHD = attention-deficit hyperactivity disorder

13. Comparison of Mean Costs of Common General and Neurologic Illnesses Between Children With ADHD and Controls (Base Cost=$495, Cost Attributable to ADHD=$154)
Mean Cost per
Year ($)
ADHD
Control
Mean Cost
Attributable ($)
Mean Cost of
Comorbidity ($)
Vison
1996
1997
Average
1060
940
1000
589
402
495
-20 22 1
354
348
351
Suppurative otitis media
1450
887
1168
838
504
671
229
124
176
494
193
343
ADHD = attention-deficit hyperactivity disorder

14. Costs
The cost of care for these comorbid general health conditions was increased.
The equation for the total annual cost of care ($1133) is composed of:
Base cost of care for a control child ($495)
Cost of care attributable to ADHD ($154)
Cost of care attributable to respiratory illness in controls without ADHD ($164)
The cost of comorbidity ($1133-$495-$154-$164=$320).

15. Mean Cost of Comorbidity (%)
Comparison of Mean Costs of Common Mental Illnesses Between Children With ADHD and Controls (Base Cost=$495, Cost Attributable to ADHD=$154)
Mean Cost per
Year (%)
ADHD
Control
Mean Cost
Attributable (%)
Mean Cost of Comorbidity (%)
Depression
1996
1519
1081
586
321
1997
1142
1042
547
-91
Average
1330
1061
566
115
Oppositional defiant disorder
1664
1366
871
181
1239
1148
653
-100
1451
1257
762 40
ADHD = attention-deficit hyperactivity disorder

16. Comparison of Mean Costs of Common Mental Illnesses Between Children With ADHD and Controls (Base Cost=$495, Cost Attributable to ADHD=$154)
Mean Cost per
Year (%)
ADHD
Control
Mean Cost
Attributable (%)
Mean Cost of
Comorbidity (%)
Bipolar
1996
1991
1382
887
492
1997
1475
1270
775 15
Average
1733
1326
831
253
Conduct disorders
1851
1279
784
454
1302
1140
645
-28
1576
1209
714
213
ADHD = attention-deficit hyperactivity disorder

17. Oppositional defiant disorder
Prevalence and Risk of 4 Most Common Mental Diagnoses in Children With ADHD Compared With Children Without ADHD
Diagnoses
Observed
N (%)
Expected N
Added
Cases
AR%
Depression
1996
ADHD
Control
836 (19.4)
2690 (3.2)
138
698
83.5
1997
655 (19.1)
2139 (2.0) 69
586
89.4
Oppositional defiant disorder
493 (11.4)
316 (0.4) 16
477
96.7
377 (11.0)
218 (0.2) 7
370
98.1
Depression: , 296.2x, 296.3x, 296.5x, , 298.0x, 300.4x, , 309.0x, 309.1x, , 311.xx, and 313.1x; Oppositional defiant disorder:
ADHD = attention-deficit hyperactivity disorder; AR%= attributable risk percentage

18. Prevalence and Risk of 4 Most Common Mental Diagnoses in Children With ADHD Compared With Children Without ADHD
Diagnoses
Observed
N (%)
Expected N
Added
Cases
AR%
Bipolar
1996
ADHD
Control
363 (8.4)
1148 (1.4) 59
304
83.8
1997
270 (7.9)
909 (0.9) 29
241
89.1
Conduct disorders
329 (7.6)
437 (0.5) 22
307
93.2
198 (5.8)
305 (0.3) 10
188
95.0
Bipolar: 296.xx; Conduct disorders: 312.xx
ADHD = attention-deficit hyperactivity disorder; AR% = attributable risk percentage

19. Prevalence: Comorbidity With Mental and Neurologic Disorders
The relative risks of a child with ADHD being diagnosed with an additional mental disease were all greater than 1, ranging from 3.2 (nondependent drug use) to 53.4 (oppositional defiant disorder).
Children with ADHD were 53 times more likely to have oppositional defiant disorder than controls.

20. Four Disorders
Oppositional defiant disorder, conduct disorders, learning disabilities, and tics
Have an attributable risk percentage above 90%
This suggests that ADHD represents an important risk marker for multiple mental health disorders.

21. Policy Implications: Costs
Attributable costs for mental disorders ranged from $204 (tic disorder) to $1262 (personality disorder).
The cost of care attributable to depression in a control child was $566 ($1061-$495).
The cost of comorbidity in children with ADHD and depression was $115 ($1330-$495-$154-$566=$115).

22. Clinical Implications
We have identified an ADHD comorbidity paradox
This was a decrease of 12 to 70% of the expected number of cases.
In contrast, there were 26 to 698 additional cases of vision problems, epilepsy, depression, oppositional defiant disorder, bipolar disorder, conduct disorder, adjustment disorder, anxiety disorder, nondependent drug use, tic disorders, personality disorders, epilepsy, and visual impairment.
For these illnesses, the number of observed cases was increased by 97 to 5286% over the expected number