Other symptoms or conditions commonly mistaken for fatigue include hypersomnolence, weakness, motor fatigue, depression, fatigue due to medical illness, and medication effect.
Although some sleep disorders may contribute to fatigue (see “Secondary causes” in the “Pathophysiology” section), they are most commonly associated with excessive daytime sleepiness or hypersomnolence. Several carefully phrased questions regarding the patient’s likelihood of falling asleep in quiet or sedentary situations can often help distinguish hypersomnolence from fatigue (see the “Diagnostic workup” section).
Weakness may also be perceived as fatigue. The distribution of the weakness is an important feature that can help to distinguish between the 2 symptoms. If focal weakness is reported, a thorough examination can typically distinguish this entity from fatigue. More generalized symptoms or a strong diurnal variation in symptoms suggests fatigue as opposed to weakness. Another symptom associated with weakness is motor fatigue, a loss in the maximal capacity to exert force during exercise (Schwid et al 1999). Like weakness, motor fatigue is typically focal, involving specific muscle groups utilized in the motor task. This entity is somewhat distinct from classic definitions of fatigue, which are more generalized in their descriptions.
Although depression may cause fatigue in and of itself, it is also associated with other symptoms commonly mistaken for fatigue. This distinction can be difficult. Hypersomnolence and anhedonia, both common consequences of depression, may be perceived as fatigue by patients or clinicians. A careful depression screen and thorough interview may help to distinguish between some of these symptoms.
Other medical entities, including several infectious and metabolic causes, may independently cause fatigue and need to be assessed separately with a thorough review of symptoms and examination. Likewise, medications including beta-interferons, antispasmodics, anticonvulsants, and pain medications may contribute to fatigue symptoms (see “Secondary causes” in the “Pathophysiology” section). A thorough review of the medication list is essential.