We learn early in our education about the five senses – visual, auditory, gustatory, tactile, and olfactory. This processing of information from the “external environment” is termed exteroception. Intermittently, salient sights, sounds, tastes, smells, or touches enter our awareness but for the most part exteroception happens subconsciously. However, the brain not only monitors the external environment but also receives sensory input from the rest of the body – the internal environment. This processing of information from the body or the “internal environment” is termed interoception and is often called the sixth sense. Similar to exteroception, interoception mostly happens below the level of consciousness unless a salient signal – thirst, hunger, pain – enters the consciousness. The interoceptive network in the brain receives signals from the cardiovascular, pulmonary, hepatorenal, gastrointestinal, and musculoskeletal systems via the vagus nerve and the spinothalamic tract and play an integral role in the etiology of diseases of the mind-body axis (future essay).
The chief complaint, the history of present illness, and the review of systems are essentially introspected inventories of interoception. The review of systems consists of an inventory of described interoceptive sensations such as dizziness (proprioception), palpitations, excessive thirst, heat or cold intolerance, fatigue, abdominal fullness, nausea, dry mouth, and the various pain sensations (nociception). In the history of present illness, descriptors (ie “sharp” chest pain), temporality (ie how long?), and associations (ie related symptoms) are added to the review of systems. In tandem with the physical exam, these interoceptive reports serve as “top of the funnel” filters that reduce the universe of all-possible diagnoses to a smaller world of more probable diagnoses. In effect, they inform radiological and laboratory testing in order to “rule-in” and “rule-out” diseases. However, inherent in this model is the presumption that diseases manifest themselves with a signature cloud of experienced and described symptoms. For example, a heart attack manifests itself with left sided chest pain radiating to the arm and appendicitis manifests itself with right lower quadrant abdominal pain associated with fevers.
However, the presumption of similarity of interoception is laden with pitfalls because not only are interoceptive signals imprecisely represented in the brain, but equally important, they are variably expressed due to the nuances of language, the context of the situation, and the conditionings of the broader culture. Although there are grouped similarities “good-enough” to build models, fundamentally, each of us feel our feelings differently, feel them differently at different times and places, describe them differently, and have different thresholds of salience for when they come into our awareness. This variation has real-world medical consequences. For example, it has repeatedly been shown that certain groups of patients such as minorities, women, diabetic patients, and the elderly, present to the Emergency Department with “atypical symptoms” of a myocardial infarction leading to an incorrect triage, inaccurate diagnoses, and worse outcomes. Whereas the typical symptoms of a heart attack include descriptors such as “heaviness,” “tightness,” “band-like,” “ chest pressure,” and “radiating to the jaw or left arm,” it is known that atypical symptoms such as “sharp”, “stabbing”, “burning” or “pinching” pain with associated symptoms of “nausea” and “generalized weakness” are frequently reported by the above-referenced group. This in effect leads physicians astray in their evaluations and contributes to misdiagnoses.
In a busy emergency department – with physicians and nurses often working at their cognitive and affective limits and decisions often driven by reflexively by rules based protocols – under-differentiated, under-represented (in clinical trials), and under-studied groups (future essay) of patients with vague descriptions of dizziness, ambiguous sensations of weakness, fuzzy descriptions of abdominal pain, and non-traumatic causes of musculoskeletal pain all represent potential traps for not only misdiagnoses and untimely diagnoses but also unnecessary testing and iatrogenesis. By the time interoceptive reports are perceived, integrated, and expressed by the patient, and thereafter, interpreted and documented by the physician, they are third, fourth, or firth order interpretations far removed from the original context. Unsurprisingly, they are prone to implicit, explicit, and systemic misinterpretations, misperceptions, and misdocumentation by the physician (next essay).
Fundamentally, the purpose of the nervous system is not only one of reaction but also one of anticipation (future essay). From the perspective of the organism, reaction and anticipation require timely and regular sensory inputs from the external (exteroception) and the internal world (interoception). Interoception – the sixth sense – is the brain sensing the internal environment of the rest of the organism. Just like exteroceptive senses, interoceptive senses enable constructs of the world and are subject to illusions, suggestions, partly accessible to introspection, contingent on situation, culture, and language, and therefore, prone to errors in communication and perception. However, unlike exteroceptive senses, these errors in communication and perception in interoception can and do lead to errors in medical diagnosis.