I just read a review on BRAIN SAGGING DEMENTIA. It doesn’t seem fair that another body part might sag as we age, but people who have a cerebrospinal fluid leak may develop symptoms that look like bvFTD, but this is a potentially reversible cause. Not having enough of that fluid to support the usual position of the brain inside the skull leads to a sagging that can be seen on an MRI and looks different from behavioral variant FTD’s shrinkage of cortex.

Most patients with suspected bvFTD will get an MRI of brain as part of the routine workup, so looking for the signs above (Figure 2 from the review article) may help.
It is important to note that the prevalence is not known and that only 70 patients were identified in all the available literature. It is not yet clear how rarely or commonly this occurs. Until more neurologists and neuroradiologists know to look for this, we may not be able to establish the real epidemiology. I remember that the 60 Minutes episode on Normal Pressure Hydrocephalus caused a rush to reverse Alzheimer’s disease diagnoses. Normal Pressure Hydrocephalus does cause cognitive impairment and might be missed in favor of a snap diagnosis of Alzheimer’s disease, but it happens very rarely.