A few weeks ago, I had a patient in his 90s whose chief complaint was "lassitude".

Much to my surprise, there is an ICD 10 code for lassitude: R53.1

When I look up R53.1 it says:

Asthenia NOS
excludes: age-related weakness (R54)
muscle weakness (M62.8-)
sarcopenia (M62.84)
senile asthenia (R54)

Having ruled out muscle weakness, sarcopenia and not being quite sure how to distinguish senile asthenia from asthenia NOS, we agreed on a treatment.

I do not yet know if it is successful.