The presentation and neurobiology of depression in young adults is an important but understudied area of psychiatric research. Many young adults with early-onset recurrent major
depressive disorder (MDD) have a strong genetic loading for mood disorder and, in the longer
term, may be at high risk of developing bipolar affective disorder (BPAD). The objectives of
this thesis were two-fold: firstly, to assess the prevalence and clinical validity of bipolar
spectrum disorder (BSD) in a consecutive sample of young adults with recurrent depression;
and secondly, to carry out a neurocognitive study comparing clinically recovered (euthymic)
young adults with recurrent MDD, euthymic BSD patients and well-matched controls.
Eighty-seven young adults presenting with recurrent depression were recruited from
consecutive referrals to a psychiatric clinic at a University Health Service. Of these, 14 had
BPAD, 27 had BSD and 46 had recurrent MDD. The classic criteria used to assess the
validity of psychiatric diagnoses (namely, clinical phenomenology, clinical course, family
history and treatment response) were applied to the BSD group of patients. This provided
only modest support for the validity of the BSD criteria according to clinical parameters.
However, on neurocognitive testing, there were significant differences between BSD patients,
MDD patients and controls in terms of performance on tests of attention, executive function
and declarative memory. This finding suggested that the diagnsotic criteria for BSD were
able to identify a sub-group of young adults with recurrent depression and strong bipolar
features (such as a family history of bipolar disorder or a personal history of antidepressantassociated hypomania) who performed less well than young adults with more straightforward
unipolar depression on tests of prefrontal and hippocampal functioning. The implications of
these findings for the concept of bipolar spectrum disorder, and for our understanding of the
neuropsychology of mood disorders, are discussed. Limitations of this work and directions
for future research are also considered.