As you know, all sentence can be written in many different ways. Here are a few samples.
Fifteen elderly patients suffered senile dementia due to Alzheimer's disease. This variation makes the ending clearer.
Senile dementia due to Alzheimer's disease was suffered by fifteen elderly patients. This variation puts the disease first, which puts an emphasis on the disease. The weakness though is that it is a passive sentence.
There were fifteen patients, who suffered from senile dementia due to Alzheimer's disease. This variation uses a relative clause. So, if you wanted to make one element of the sentence subordinate, this is the way to go.
Senile dementia due to Alzheimer's disease affected fifteen patients. This variation personifies dementia and still retains the active voice.