Mental Health Symptoms and the Relationships Between Them

Most of us are familiar with mental health symptoms such as “depressed mood” or “insomnia”. Current classification systems such as the Diagnostic and Statistical Manual of Mental Disorders (DSM) and the International Classification of Disease (ICD) use lists of such symptom items as criteria for defining mental conditions.

Let us reflect on the questions of “What constitutes a single mental symptom?” and “Why are the relationships between mental symptoms important?”

Eric Chen

“Syndromes” consists of groups of “symptoms” co-occurring in an interesting way

In medicine, "symptoms” are subjective experiences attributed to changes in the body, usually due to ill health. Examples of symptoms are pain, nausea, loss of appetite etc. In contrast, "signs" refers to observations that can be detected by an observer (such as vomiting, or a mass). Symptoms are often distressing and motivate help-seeking. With the advent of empirical clinical observations, clinicians discovered that some unrelated symptoms (and signs) tended to co-occur. Patterned sets of "syndromes” (or co-occurring groups of symptoms) were observed and became important in the identification of specific illness entities and their underlying causes. For example, in the 18th century, it was recognised that sea voyages were associated with a syndrome of bleeding gums, dental problems, joint pain, bruised skin, and brittle hair. The distinctiveness of this combination led to the subsequent observation that this condition could be prevented by fresh fruits. Eventually, the role of vitamin C was discovered. In this case, the unique permutation of otherwise unrelated symptoms gave rise to identifying the disease entity and the underlying cause (we may call the links between these symptoms “salient links”). In this sense, the more unrelated the constituent symptoms are, the more informative the links between them are, and the more distinctive the disease entity becomes. In contrast, co-occurring symptoms which are expected consequences of one another will be less informative.

Salient Links and Expected Links Between Symptoms

Symptoms in mental health are often complex, as they are changes in subjective experiences that are attributed to a mental health condition (e.g depressed mood, lack of interest, hallucinations). For example, what constitutes a depressed mood, does it include thoughts or emotions? Some of the symptoms are related to one another as a matter of direct "link". For example, if I have insomnia, I am likely to have sleepiness and tiredness the next day (an understandable consequence of the lack of sleep). If I have social anxiety, I am expected to avoid social situations. If I lack a sense of pleasure, I will probably have a reduction in motivation. So sometimes the symptoms in mental health are linked to one another as a network of expected consequences (let us call these “expected links”).

In this case, delineating where one symptom stops and where the next symptom begins may be quite arbitrary. It is also important to consider whether there is one complex symptom with several sub-components, or several symptoms happening to co-exist together.

The Importance of Delineating Symptom Boundaries

Why are the numbers of "units" of symptoms important? Symptoms as units are important because

1. they contribute to the quantification of psychopathology;
2. they constitute diagnostic criteria; and
3. their co-occurrence patterns may give insight into underlying pathophysiology.

Orthogonal Symptom Dimensions

The emphasis on the reliability of psychiatric diagnoses demanded the operationalisation of psychopathological observations. Standardised Interviews were conducted as fixed questions with responses being coded into numbers. Detailed descriptive accounts of psychopathology are summarised as unstructured lists of itemised symptoms. Itemised symptoms are considered basic units of observation. These treatments resulted in many unstructured, decontextualized symptoms. Attempts to organise them were then made with quantitative statistical techniques such as factor analysis. These techniques return “statistical factors” consisting of co-occurring symptoms. These factors have become accepted ways to handle symptoms, giving rise to the notion of symptom dimensions (e.g. positive symptoms or negative symptoms in psychosis). In the identification of symptom dimensions, preference was made to identify independent factors. For example, In factor analysis, one can elect to focus only on the effects of “orthogonal” factors.

However, independence between two factors also means that the two dimensions (i.e. the groups of symptoms), do not co-occur more than by chance. As genetics studies increasingly revealed a massively complex structure of inheritance for many mental health conditions, the clinical concept of mental health disorders has moved from the expectation of highly specific disorders with distinctive, localised, unique pathologies towards broader, more flexible, more inclusive multidimensional entities which accommodate internal heterogeneity. The expectations of specific information in terms of salient symptom co-occurrence have been under-emphasised.

Symptom Boundaries Are Crucial in Symptom Network Analyses

In the past decade, an alternative research paradigm using network analysis to explore relationships between symptoms has attracted attention (Borsboom, 2017). This approach described correlative patterns between symptoms using the new technology of network analysis and produced weighted links between symptoms. This “symptom network” approach asserts that symptoms may mutually activate one another and support one another to produce a self-reinforcing network which may be sufficient to explain the persistence of symptoms after the precipitating causes have been resolved. This approach also led to efforts to identify "hub" symptoms which have strategic links to several other symptoms, as targets of intervention.

The question of what constitutes a unit symptom becomes crucial in network analysis, as the ambiguity between the notion of one symptom with two subcomponents or two symptoms associated by a "link" is crucial to the principle of analysis and its interpretation. Expected links between "symptoms" (such as between insomnia and sleepiness) would become redundant noise in these analyses. Thus the simple question of what constitutes a basic symptom unit should call for a fundamental review of the structure of symptoms.