A main difference between somatic symptom disorder and illness anxiety disorder is

Excessive and persistent health anxiety is a common, disabling problem that causes substantial suffering to the individual and their loved ones, and is costly due to high health service utilization [1]. Despite this, research to improve our understanding of health anxiety has been hampered by the lack of reliable, valid and clinically useful diagnostic criteria. In particular, DSM-IV-defined Hypochondriasis [2] was criticised because of its overly narrow criteria that were challenging to operationalise in clinical practice [3], [4]. Due to these limitations, DSM-5 replaced Hypochondriasis with two new disorders: Illness Anxiety Disorder (IAD) and Somatic Symptom Disorder (SSD) (see Supplementary Materials for criteria) [5].

DSM-5 IAD is characterized by excessive fears of having or acquiring serious disease(s), high health anxiety, and behavioural symptoms (e.g., avoidance and repeated body checking). Individuals with IAD can be classified as either care-seeking (for those who use excessive medical care), or care-avoidant (for those who rarely use medical care). In contrast, SSD is characterized by the presence of one or more distressing and disabling somatic symptoms that disrupt daily functioning. To meet SSD criteria, individuals also need to experience excessive thoughts, feelings or behaviours related to the symptoms and health concerns, as defined by at least one of the following symptoms: disproportionate or persistent thoughts about the seriousness of one's symptoms, high health anxiety, or excessive time and energy devoted to their symptoms or health concerns. High health anxiety can therefore be a symptom of SSD, but is not necessary for a diagnosis of SSD to be made.

Research is needed to evaluate the reliability, validity and clinical utility of the DSM-5 IAD and SSD classifications in health anxious samples, but to date, there has been limited work in this area. Of the few published studies [6], [7], [8], [9], [10], most have focused on evaluating the validity of the DSM-5 SSD diagnosis in patients with somatic symptoms or unexplained symptoms, rather than evaluating whether the diagnoses are useful when diagnosing patients who present with health anxiety. Those studies that have focused on health anxious patients suggest that IAD and SSD can be diagnosed reliably by trained interviewers, distinguished from non-clinical community samples who were not experiencing health anxiety [11], and that patients with IAD and SSD respond to cognitive behavioural therapy (CBT) [12], [13]. Given the limited research into IAD and SSD, there are several pertinent questions that need further study. First, it is unclear the extent to which DSM-5 IAD and SSD have improved the reliability of diagnosing health anxiety, compared to Hypochondriasis which had suboptimal reliability [14]. Reliable diagnostic criteria are essential for a valid classification system, and reliability studies of the new DSM-5 classifications could provide a fundamental step for advancing the literature on the prevalence, course and nature of health anxiety. Although one study found inter-rater reliability estimates of 0.85 for IAD and SSD [11], the IAD sample was small (n = 6); these findings now await replication.

Second, DSM-5 estimated that 25% of those previously diagnosed with Hypochondriasis would be diagnosed with IAD, and 75% would meet criteria for SSD [5], but there was scant empirical data to guide these estimates. There is a clear need to estimate the prevalence, overlap and co-occurrence of Hypochondriasis, IAD and SSD. Research that determines whether the IAD and SSD classifications are more sensitive to detecting clinically significant cases of health anxiety compared to Hypochondriasis could yield important insights into the relative utility of the DSM-IV and DSM-5 classifications. Third, there is a need to examine the prevalence of the IAD care-seeking and care-avoidant subtypes. Not only could this provide valuable information about the classification of health anxiety, it may also help to identify the patients who are at risk of using unnecessary and costly health services, which will be key to reducing the burden of health anxiety on health care systems.

Fourth, DSM-5 distinguishes individuals with high health anxiety based on the severity of their somatic symptoms. The IAD diagnosis is restricted to those without any somatic symptoms or those with mild symptoms, whereas SSD is diagnosed when ‘significant’ moderate to severe somatic symptoms are present [5]. The conceptual basis for this distinction was to include a diagnosis (IAD) that classified individuals who were disabled or distressed by illness fears, but were not experiencing somatic symptoms, and therefore would not have qualified for any other diagnosis. Although this rationale is compelling, empirical scrutiny of this distinction between IAD and SSD is needed, especially given some researchers have argued that limiting the IAD diagnosis to individuals with mild somatic symptoms is inconsistent with clinical observations (e.g., [15], [16]). Yet there has been little research on whether there are any clinically meaningful differences between people with high health anxiety who meet criteria for IAD versus SSD. In the only study to examine the similarities and differences between the IAD and SSD in health anxious patients, Bailer and colleagues [6] studied 96 treatment-seeking patients with health anxiety, and created a ‘probable’ post-hoc DSM-5 SSD or IAD diagnosis based on responses to the Structured Clinical Interview for DSM-IV [17] and self-report questionnaires. Individuals with probable SSD were less educated, more impaired, more likely to experience comorbid Panic Disorder and Generalised Anxiety Disorder (GAD), and attended more medical consultations compared to those with probable IAD. However, patients with IAD did not differ from patients with SSD on other demographic and clinical characteristics including health anxiety severity, illness behaviour, somatic symptom attributions, anxiety sensitivity, and comorbid depression. These findings led the authors to suggest that the differences between IAD and SSD are ‘quantitative rather than qualitative’, and concluded that SSD is a more severe disorder. These findings need to be replicated in other health anxious samples using prospective diagnoses based on the actual DSM-5 criteria rather than retrospective ‘probable’ diagnoses.

The primary aim of this study was to replicate and extend Bailer et al. [6] by examining the reliability, validity and clinical utility of the DSM-5 IAD and SSD diagnoses, and their overlap with DSM-IV Hypochondriasis in a cross-sectional sample of treatment-seeking patients with health anxiety [13], [18]. To achieve this aim, we compared health anxious patients who met DSM-5 criteria for IAD to those who met criteria for SSD on their demographics; clinical characteristics (e.g., onset, course, severity, impairment and comorbidities); cognitive and behavioural characteristics (e.g., body hypervigilance, avoidance, safety behaviours and maladaptive cognitions about bodily symptoms and health); and service utilization patterns. To extend research into IAD, we examined the rates of the DSM-5 care-seeking and care-avoidant IAD subtypes.

Participants

One hundred and eighteen patients recruited for two clinical trials of online cognitive behavioural therapy (CBT) for health anxiety were included [13], [18]. The study was approved by the St Vincent's Hospital Human Research Ethics Committee (HREC/14/SVH/294) and all participants provided electronic informed consent to participate. All patients were aged 18 years or over, Australian residents, self-identified as experiencing elevated health anxiety, and met criteria for either DSM-5 IAD or SSD1

Discussion

This cross-sectional study compared the demographic and clinical characteristics of DSM-5 Illness Anxiety Disorder (IAD) and Somatic Symptom Disorder (SSD), and explored the overlap between DSM-IV Hypochondriasis and the new DSM-5 diagnoses in a treatment-seeking sample of patients with health anxiety. Although we expected that the majority of individuals with high health anxiety would be diagnosed with SSD based on previous studies [6], [11], [12], only 45% met criteria for SSD, whereas 47% of

Conflict of interest

Funding for this study was provided by the National Health and Medical Research Council of Australia (NHMRC) in the form of an Early Career Fellowship awarded to Dr. Jill Newby (NHMRC grant number 1037787), and St Vincent's Clinic Foundation Grant awarded to Dr. Jill Newby, Dr. Alison Mahoney and Professor Gavin Andrews. Further support was provided by the Australian Government Research Training Program Scholarships awarded to Alison Mahoney and Mr. Shiu Wong.

Acknowledgements

Role of the funding source

Funding for this study was provided by the National Health and Medical Research Council of Australia (NHMRC) in the form of an Early Career Fellowship awarded to Dr. Jill Newby (NHMRC grant number 1037787), and St Vincent's Clinic Foundation Grant. Further support was provided by the Australian Government Research Training Program Scholarship awarded to Alison Mahoney.

Competing interest statement

All authors have no competing interests to report.

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