Fatigue or tiredness is extremely common in Multiple Sclerosis (MS) and is an important cause of disability. The mechanism for this symptom is incompletely understood. Suffice it to say that the complaint is multifactorial and related to pro-inflammatory chemicals, neuro-hormonal activity and where relevant, muscle weakness.
Many patients with chronic disease have fatigue, particularly those with cancer or immune disorders. However, in MS the fatigue is often disproportionately present, i.e. the patient may have little on neurological examination but much in the way of fatigability.
Often patients with depression have lethargy. They're sluggish. Indeed, depression and fatigue in the MS patient can occur concurrently. But the depression may be lifted through medication though the fatigue may remain.
The matter of fatigue needs to be sorted out in more detail via sub-classification, not only in MS but in any disease. As per my reference above, fatigability can be purely muscular. The classic example of such fatigue is the patient with Myasthenia Gravis who has muscles that tire very easily after repetitive effort or via heavy lifting. This striking problem is not exhibited in MS. Rather, the MS patient may have a more gradual and less pronounced decline in strength in a given muscle after heavy exercise, work or after a long day of activity. This is akin to the kind of problem a stroke patient might have in a body side that has been affected by a cerebrovascular accident (CVA).
Moreover, the MS patient typically has a kind of central fatigue, whereby there is a feeling of energy loss in many muscle groups. Recall that the muscles per se or their receptors are not damaged in MS. It is the Central Nervous System (CNS-brain and spinal cord) that is dysfunctional. The muscles and peripheral nerves supplying them are indirectly affected by damage to these Central Nervous System structures. Not only can muscles suffer strength deficits via the CNS plaques of MS but spastic and involuntary/uncoordinated movements of same muscles can impair their functional power.
Heat can worsen MS by exacerbating weakness and fatigue. An MS patient in the throes of an acute MS attack may also suffer sudden increase in tiredness associated with new findings on neurological exam or a deterioration of old problems. This speaks to a chemical surge in inflammation that can impair overall energy. Likewise, any infection, new disease (diabetes, emphysema) or fever due to many causes, such as a drug reaction, can promote MS fatigue.
In the last 30 years, with a better understanding of a condition known as Excessive Daytime Sleepiness, another distinction in MS fatigue has been made. Sometimes, tiredness is different from physical exhaustion. It's tiredness associated with a predisposition to fall asleep. There is a laundry list of causes for such hypersomnia; many such factors are linked to MS. They include: