Age of onset of dependence: Does it help our understanding of opioid dependence by generating meaningful categories or by acting as a useful dimension? A critical examination of the classic debate in psychiatry.

Background: Category vs. dimension is a classic debate in psychiatry. Applying age of onset of dependence (AOOD) to categorize opioid dependence into early- (EO) and late-onset (LO) types provides a unique opportunity to critically examine this debate.

Aim: To study if EO and LO subjects differ significantly on 'validating variables' from five explanatory domains: Clinical (severity), genetic (family history), psychological (sensation-seeking and impulsivity), neuropsychological (attention-concentration and executive functions), and neurophysiological (P300-evoked response potential).

Materials And Methods: In a cross-sectional design, 60 ICD-10 DCR-diagnosed opioid-dependent male subjects (30 with AOOD≤20 years and 30 with AOOD≥22 years) comprised the two index groups (EO and LO, respectively), with their respective age-matched control groups (EOC and LOC). They were administered an extensive battery of instruments and tests based on the above domains.

Results: The two groups differed significantly on only three out of nearly 30 variables tested. However, there emerged a clear and consistent pattern of continuum of scores across the groups and across all the variables: The EO subjects were the most impaired or affected, the LO subjects were intermediate, and the control groups fared the best. Further, nine test variables correlated significantly and meaningfully with AOOD when the dichotomy was abolished and the sample was combined into one.

Conclusions: These results suggest that, in this particular case, the variable AOOD is more meaningful when it is used as a dimension rather than for generating categories perforce.