Memory loss can result from a number of neurological conditions. Alzheimer’s disease is not the only cause of memory loss. There are at least three clinically important memory systems:
1) Episodic
2) Semantic
3) Working
4) Procedural

Episodic memory is the most common clinical memory complaint and most thoroughly researched and assessed (e.g., recall of lists of words). Episodic memory involves the recall and recognition of personal episodes and experiences. What did you eat for breakfast? What was the most recent movie that you saw? Where did you go for vacation? Episodes are the scripts of your life that involve the sense of time and self.

Episodic memory involves three steps:
1) Encoding. Awareness and attention are required for an event or episode to be recorded.
2) Consolidation. The brain process that actually lays down the memory “trace” for later recall or recognition.
3) Retrieval. Recall or recognition of events and experiences.

Episodic memory often involves the hippocampus and adjacent brain structures. The most famous example of what happens when the hippocampi are removed was Henry Molaison (HM). He lived between 1926 and 2008 and had neurosurgery to control seizures in his mid 20s. This caused him to lose his ability to consolidate new experiences into long-term memory – called anterograde memory.

This is the memory system that is most problematic in early Alzheimer’s disease. However, there are a number of additional neurological disorders that may also induce episodic memory loss: traumatic brain injury, stroke, brain infections (e.g., encephalitis, meningitis), toxic or nutritional conditions, Transient Global Amnesia (profound sudden onset that is time limited), some forms of epilepsy, Lewy body disease, Parkinson’s disease, and hippocampal sclerosis (often mistaken as Alzheimer’s disease).

Semantic memory involves forgetting of acquired knowledge. This shows as loss of facts or concepts, loss of words, loss of word meaning, loss of knowledge of the use or relationships of things. This usually presents as problems in language like trouble naming objects, word finding, or word substitutions. This memory system worked very well in HM. Semantic memory deficits are usually associated with damage to the frontotemporal areas of the brain. Often semantic memory deficits result from strokes or head trauma causing sudden onset. In other cases the onset may be slow and progressive in conditions like primary progressive aphasia or Alzheimer’s disease.

Working memory is the active maintenance of information in the mind. It involves attention and concentration. Working memory is the multitasking capacity of the brain. Why am I in this room? It contributes to encoding of episodic memory. Working memory is better considered an executive function rather than a memory system and was fine in HM.

Procedural memory is cognitive and behavior skills. These memories are often not verbalized but rather performed automatically – skills that persist over time after learning them. These skills include things like how to use a computer, how to use a TV remote, driving, playing a musical instrument, art, and swimming. HM not only did well with procedural memory but was able to learn new skills that he did not know he had learned. Procedural memory deficits are sometimes evident in Parkinson’s disease, Huntington’s disease, and cerebellar disease. They may show up later in the course of Alzheimer’s disease.

As you can see, memory disorders are complex and not always caused by Alzheimer’s disease even in the elderly. Memory changes and treatments require carful, thorough neuropsychological, neurological, and medical evaluation.