What could cause frequent lightheadedness and leg spasms while standing up which leads to a sudden fall in an elderly person?
Looks like orthostatic hypotension
This is Dr. Sameer, Cardiologist.
The symptoms you are telling about lightheadedness upon a change of posture from lying to standing seems to fit into orthostatic hypotension.
In this BP falls when there is a change in position due to the accumulation of blood in lower limbs, causing lightheadedness. This is generally because of a fluid deficit in body & I'll advise you to keep yourself well hydrated.
Also, maintain a track of your blood pressure readings both in lying or sitting and in standing position. If the bp ranges are lower than I would suggest changing Losartan+hydrochlorothiazide (50+12.5) to plain Losartan 50mg.
These symptoms can also be because of some heart blocks. To rule that out, I'll suggest putting a Holter monitor (ECG monitoring for 24hrs).
I hope it was helpful to you.
Thanks and take care.