'Senile Dementia' is a general term which groups all the different types of dementia experienced by the elderly into one category. Of course, the most common form of dementia is Alzheimer's Disease, accounting for roughly two thirds of cases in the over-65s. Alzheimer's Disease is a degeneration of nerve cells and their connections. Protein deposits tend to accumulate in the brains of Alzheimer's sufferers, but it hasn't yet been determined if this is a cause or effect of the problem. Progression of the disease can be gradual. I'm not going to write any more about this form of dementia, because there is so much information about it elsewhere.
However, there are several other types of dementia. Another form is Vascular Dementia. Vascular Dementia describes a number of conditions linked by a vascular cause (e.g. Hypertension, Stroke, Atrial Fibrillation). Often caused by a series of mini-strokes in the brain (Multi-infarct dementia), the disease tends to progress in a series of steps depending on the severity of each progressive stroke. Small steps leave the symptoms looking much like Alzheimer's, but it's equally possible for a major stroke to cause significant sudden loss of abilities. Each stoke cuts off or blocks blood flow to part of the brain, depriving it of oxygen and nutrients, leading to the death of that section. These dead areas, or lesions, are what a CT scan picks up.
Then there is Lewy Body Disease (Dementia with Lewy Bodies), in which small deposits of protein accumulate in the nerve cells of the cerebral cortex. The sufferer experiences persistent hallucinations, perhaps seeing a person in the same chair over and over. Here, the confusion can fluctuate such that a Carer might believe that the sufferer is 'pretending'. Parkinson-type symptoms like tremors or rigidity are associated with this type of dementia. As the disease progresses over several years, the sufferer experiences memory loss and his or her language skills will deteriorate.
Fronto-Temporal Dementia (grouped with Pick's Disease and Primary Progressive Aphasia) is a rare and slowly-progressing condition where the sufferer's personality changes, perhaps resulting in a loss of inhibition or sudden inappropriate rudeness or maybe a loss of emotional warmth and empathy. Over-eating or a craving for sweet foods is an indicator here (maybe I should get myself diagnosed?), as is a change in sexual behaviour. In this group, memory loss is not usual in the early stages, but a decline in ability to communicate sometimes gives the impression that memory problems exist. As the disease progresses, reading and writing skills deteriorate along with numerical skills. The sufferer will likely develop problems handling money. Some sufferers develop obsessive behaviours, with repeated hand-washing or similar activities. This group of diseases is characterised by more localised damage in the frontal or temporal lobes. Primary Progressive Aphasia attacks cells in areas of the frontal lobe particularly associated with speech. Pick's Disease is caused by abnormal or swollen cells or abnormal protein deposits within cells in the fronto-temporal areas.
In Mum's case, she has what's known as a 'mixed pathology', with the problem chiefly being Vascular Dementia but with some other damage present that could be due to one or several of the other forms listed above. I've certainly noticed a marked loss of empathy, her increased selfishness, and her preferred diet of sweet foods. This and her deterioration of abilities to deal with money give me reason to suspect that there's a fronto-temporal aspect to her condition, too. Then again, I have observed Mum having Lewy-Body hallucinations on many occasions over the past 5 or 6 years. She sees someone standing in a street and is convinced that she saw them standing in exactly that spot yesterday, and will not budge from the assertion, even if I tell her we were 300 miles away yesterday.