Ich habe noch eine Quelle gefunden, leider in Englisch, und leider habe ich zur Zeit nicht Kraft, das zu übersetzen. Ich poste es trotzdem mal:
(Wie ich sehe, habe die Autoren eine recht weite Definition, die z.B. auch Utthoff einschließt, und Schlafstörungen etc.)
Gruß
Boggy

FATIGUE and multiple sclerosis

In: Ms in Focus, Nr. 1; Jan 2003 (Hrsg: msif – mulitple sclerosis international federation – www.msif.org – London)

Fatigue limits the lives of 85 per cent of people with multiple sclerosis. It is important that fatigue is taken seriously and its management is considered carefully.
What is fatigue?
MS fatigue is different for everyone; for some it causes overwhelming tiredness, whilst in others it worsens symptoms such as visual disturbance, difficulties with concentration, memory, mobility and muscle spasms. Fatigue may make it more difficult to deal with everyday activities.

Primary fatigue
Primary fatigue is experienced as a direct result of damage to the central nervous system. The body responds to this damage by slowing down reactions and thus causing fatigue. People with MS experience different types of fatigue including:
 Lassitude (Abgeschlagenheit/Mattigkeit): an overwhelming tiredness not directly related to participation in activity or exercise.
 ‘Short-circuiting’ fatigue (Kurzschluß-Fatigue): occurs in specific muscle groups; for example, the hand after writing for a short time.

 Heat sensitivity fatigue: a rise in body temperature can cause fatigue. This kind of Fatigue is one of the invisible symptoms of MS
fatigue can occur due to the seasonal changes in the weather but may also be triggered by other things; for example, taking a hot bath or eating hot meals.

Secondary fatigue
Fatigue can be experienced as a result of other factors that are not necessarily directly related to your MS. These can include:

 Sleep disturbance: this is often due to symptoms that can be alleviated or lessened; for example, spasms, pain, urinary urgency at night, depression or anxiety.
 Infection: may cause a number of symptoms that could bring on specific MS fatigue; for example, having a cold or flu may raise your body temperature.
 Exertion (Über)Anstrngung): the increased effort required by the body, if mobility or coordination is affected, can cause fatigue.

 Medication: there are many medications that may cause tiredness or drowsiness as a side effect. It is important to be aware of this. If you notice that there is a correlation between a change in fatigue levels and a change in medication, tell your general physician.
 Depression: may be due to nerve damage or because of the emotional impact of adjusting to MS.
 Local environment: the lighting and temperature within a work area is crucial, as poor lighting increases visual effort and heat frequently exacerbates fatigue.