DSM-5 Handbook of differential diagnosis
Chapter 1
Differential diagnosis step by step

Step 1: rule out malingering and factitious disorder

If the patient is not being honest regarding the nature or severity of his or her symptoms, all bets are off regarding the clinician’s ability of arrive at an accurate psychiatric diagnosis.
Most psychiatric work depends on a good-faith collaborative effort between the clinician and the patient to uncover the nature and cause of the presenting symptoms.

Two conditions in DSM-5 are characterized by feigning
These two are differentiated based on the motivation for the deception.

• Malingering
  When the motivation is the achievement of a clearly recognizable goal
• Factitious disorder
  When the deceptive behaviour is present even in the absence of obvious external rewards

The clinician’s index of suspicion should be raised when

• There are clear external incentives to the patient’s being diagnosed with a psychiatric condition
• The patient presents with a cluster of psychiatric symptoms that conforms more to a lay perception of mental illness rather than to a recognized clinical entity
• The nature of the symptoms shift radically form one clinical encounter to another
• The patient has a presentation that mimics that of a role model
• The patient is characteristically manipulative or suggestible

It is useful for clinicians to become mindful of tendencies they might have toward being either excessively sceptical or excessively gullible.

Step 2: rule out substance etiology (including drugs of abuse, medications)

Whether the presenting symptoms arise from a substance that is exerting a direct effect on the central nervous system.
Virtually any presentation encountered in a mental health setting can be caused by substance use.

The determination of whether psychopathology is due to substance use often can be difficult because although substance use is fairly ubiquitous and a wide variety of different symptoms can be caused by substances, the fact that substance use and psychopathology occur together does not necessarily imply a cause-and-effect relationship between them.

The first task is to determine whether the person has been using a substance

This entails history taking and physical examinations for signs of substance intoxication or substance withdrawal.
It is usually wise to consult with family members and obtain laboratory analysis of body fluids to ascertain recent usage of particular substances.

Patients who use or are exposed to any of a variety of substances can and often do present with psychiatric symptoms.

Once substance use has been established, the next task is to determine whether there is an etiological relationship between it and the psychiatric symptomatology

This requires distinguishing among three possible relationships between the substance use and the psychopathology

• The psychiatric symptoms result from the direct effects of the substance on the CNS
• The substance use is a consequence (or associated feature) of having a primary psychiatric disorder
• The psychiatric symptoms and the substance use are independent

In diagnosing a substance-induced disorder, there are three considerations in determining whether there is a causal relationship between the substance use and the psychiatric symptomatology

• You must determine whether there is a close temporal relationship between the substance or medication use and the psychiatric symptoms.
  The determination of whether there was a period of time when the psychiatric symptoms were present outside the context of substance/medication is the best method.
  What happens to the psychiatric symptoms when the person is no longer taking the substance or medication
  • Persistence of the psychiatric symptomatology for a significant period of time beyond periods of intoxication or withdrawal or medication use suggest that the psychopathology is primary and not due to substance/medication use.
    Sometimes, it is not possible to determine whether there was a period of time when the psychiatric symptoms occurred outside of periods of substance/medication use.
  • It may be necessary to assess the patient during a current period of abstinence form the substance or to stop the medication suspected of causing the psychiatric symptoms.
• You must consider the likelihood that the particular pattern of substance/medication use can result in the observed psychiatric symptoms.
  Consider whether the nature, amount, and duration of substance/medication cause are consistent with eh development of the observed psychiatric symptoms
• You should consider whether there are better alternative explanations for the clinical picture
  A history of many similar episodes not related to substance/medication use, a strong family history of the particular primary disorder, or the presence of physical examination or laboratory findings suggesting that a medical condition might be involved.

In some cases, the substance use can be the consequence or an associated feature (rather than cause) of psychiatric symptomatology

The substance-taking behaviour can be considered a form of self-medication for the psychiatric condition.
Individuals with particular psychiatric disorders often preferentially chose certain classes of substances.

The hallmark of a primary psychiatric disorder with secondary substance use is that the primary psychiatric disorder occurs first and/or exist at times during the person’s lifetime when he or she is not using any substance.

In other cases, both the psychiatric disorder and the substance use can be initially unrelated and relatively independent of each other

There are high prevalence rates of both psychiatric disorders and substance use disorders.
Even if initially independent, the two disorders may interact to exacerbate each other and complicate the overall treatment.

This independent relationship is essentially a diagnosis made by exclusion.
A lack of a causal relationship in either direction is more likely if there are periods when the psychiatric symptoms occur in the absence of substance use and if the substance use occurs at times unrelated to the psychiatric symptomatology.

After deciding that a presentation is due to the indirect effect of a substance or medication, you must then determine which DSM-5 substance-induced disorder best describes the presentation.

Step 3: rule out a disorder due to a general medical condition

Determine whether the psychiatric symptoms are due to the direct effects of a general medical condition.

This differential diagnosis can be difficult for four reasons

• Symptoms of some psychiatric disorders and of many general medical conditions can be identical
• Sometimes the first presenting symptoms of a general medical condition are psychiatric
• The relationship between the general medical condition and the psychiatric symptoms may be complicated
• Patients are often seen in settings primarily geared for the identification and treatment of mental disorders in which there may be a lower expectation for, and familiarity with, the diagnosis of medical conditions

Virtually any psychiatric presentation can be caused by the direct physiological effects of a general medication condition, and these are diagnosed in DSM-5 as one of the mental disorders due to another medical condition

You should direct the history, physical examination, and laboratory tests toward the diagnosis of those general medical conditions that re most commonly encountered and most likely to account for the presenting psychiatric symptoms.

Once a general medical condition is established, the next task is to determine its etiological relationship, if any, to the psychiatric symptoms

There are five possible relationships

• The general medical condition causes the psychiatric symptoms through a direct physiological effect on the brain
• The general medical condition causes the psychiatric symptoms through a psychological mechanism
• Medication taken for the general medical condition causes the psychiatric symptoms, in which case the diagnosis is a Medication-induced mental disorder
• The psychiatric symptoms cause or adversely affect the general medical condition
• The psychiatric symptoms and the general medical condition are coincidental

There are two clues suggesting that psychopathology is caused by the direct physiological effect of general medical condition

Neither of these is infallible.

• The nature of the temporal relationship.
  Requires consideration of whether the psychiatric symptoms begin following the onset of the general medical condition, vary in severity with the severity of the general medical condition, and disappear when the general medical condition resolves.
• A general medical condition should be considered in the differential diagnosis if the psychiatric presentation is atypical in symptom pattern, age of onset, or course.

If you have determined that a general medical condition is responsible for the psychiatric symptoms, you must determine which of the DSM-5 mental disorders due to another medical condition best describes the presentation.

Step 4: determine the specific primary disorders

Determine which among the primary DSM-5 mental disorders best accounts for the presenting symptomatology.

Step 5: differentiate adjustment disorders from the residual other specified or unspecified disorders

Many clinical presentations do not conform to the particular symptom patterns, or they fall below the established severity or duration thresholds to qualify for one of the specific DSM-5 diagnoses.
In such situations, if the symptomatic presentations is severe enough to cause clinically significant impairment or distress and represents a biological or psychological dysfunction in the individual, a diagnosis of a mental disorder is still warranted and the differential outcomes down to either an Adjustment disorder or one of the residual Other specified or Unspecified categories.

• If the clinical judgment is made that the symptoms have developed as a maladaptive response to a psychosocial stressor, the diagnosis would be an Adjustment disorder.
• If it is judged that a stressor is not responsible for the development of the clinically significant symptoms, then the relevant Other specified or Unspecified category may be diagnosed, with the choice of the appropriate residual category depending on which DSM-5 diagnostic grouping best covers the symptomatic presentation
  • The differentiation of residual categories
    If the clinician wants to indicate the specific reason, the name of the disorder is followed by the reason why the presentation does not conform to any of the specific disorder definitions
    If the clinician chooses not to indicate the specific reason why the presentation does not conform to any of the specific disorder definitions, the Unspecified disorder designation is used.

Step 6: establish the boundary with no mental disorder

Establish the boundary between a disorder and no mental disorder.
Generally, the rule of thumb, if the comorbid psychiatric presentation warrants clinical attention and treatment, it is considered to be clinically significant.

Differential diagnosis and comorbidity

Differential diagnosis is generally based on the notion that the clinician is choosing a single diagnosis from among a group of competing, mutually exclusive diagnosis to best explain a given symptom presentation.

Very often, DSM-5 diagnoses are not mutually exclusive, and the assignment of more than one DSM-5 diagnosis to a given patient is both allowed and necessary to adequately describe the presenting symptoms.
The use of multiple diagnoses is in itself neither good or bad as long as the implications are understood.
There are sex different ways in which two so-called comorbid conditions may be related to one other

• Condition A may cause or predispose to condition B
• Condition B may cause or predispose to condition A
• An underlying condition C may cause or predispose to both conditions A and B
• Conditions A and B may, in fact, be part of a more complex unified syndrome that has been artificially split in the diagnostic system
• The relationship between conditions A and B may be artifactually enhanced by definitional overlap
• The comorbid is the result of a chance co-occurrence that may be particularly likely for those conditions that have high base rates