In this chapter, we consider the clinically related phenomena of fatigue, nervousness, irritability, anxiety, and depression. These complaints form the core of a group of “symptom-based” disorders with normal neurologic findings that are nevertheless a large part of medical practice. Although more abstruse than paralysis, sensory loss, seizures, or aphasia, they are no less important, if for no other reason than their frequency. In an audit of a large neurologic practice, anxiety and depressive reactions were the main diagnosis in 20 percent of patients, second only to the symptom of headache (Digon et al). Similarly, in two primary care clinics in Boston and Houston, fatigue was the prominent complaint over 20 percent of patients. Some of these symptoms represent only slight aberrations of function or a heightening or exaggeration of normal reactions to environmental stress or to diseases; others are integral features of the diseases themselves; and still others represent disturbances of neuropsychiatric function that are components of the diseases described in Chaps. 47 through 49 on psychiatric diseases as viewed through the perspective of neurology.
Of the symptoms considered in this chapter, fatigue is the most frequent, and the most vague. Fatigue refers to the universally familiar state of weariness or exhaustion resulting from physical or mental exertion. Lassitude has much the same meaning, although in some literature it connotes more of an inability or disinclination to be active, physically or mentally, a “weariness of spirit.” More than half of all patients entering a general hospital register a complaint of fatigability or admit to it when questioned. During World War I, fatigue was such a prominent symptom in combat personnel as to be given a separate place in medical nosology, namely combat fatigue, a term that came to be applied to practically all acute psychiatric disorders occurring on the battlefield. In subsequent wars, it has become a key element of the posttraumatic stress disorders. The common clinical antecedents and accompaniments of fatigue, its significance, and its physiologic and psychologic bases are best understood by considering the effects of fatigue on the normal individual.
Fatigue has three basic meanings as experienced in daily life: (1) biochemical and physiologic changes in muscles and a reduced capacity to generate force manifest as weakness, or asthenia; (2) a disorder in behavior, taking the form of a reduced output of work or a lack of endurance; and (3) a subjective feeling of tiredness and the mental discomfort that is associated with such an unnatural state for the individual.
The decreased productivity and capacity for work, which is a direct consequence of fatigue, has been investigated by industrial psychologists. Their findings demonstrate the importance of motivational factors on work output, whether the effort is of physical or mental type. Quite striking are individual constitutional differences in energy and capacity for work, which vary greatly, just as do differences in temperament. What should be emphasized for neurologic practice is that in the majority of persons complaining of fatigue, one does not find muscle fatigue or weakness. This may be difficult to prove, for many such individuals are disinclined to exert full effort in tests of peak power of muscle contraction or in endurance of muscular activity. Contrariwise, fatigue that is so often correctly attributed to psychologic causes may also be a manifestation of medical diseases such as anemia, occult cancer, and chronic infection of inflammatory processes or of neurologic disorders of various types.
The Clinical Significance of Fatigue