There are quite a few possible underlying causes to sensory issues, and since they are interconnected it's hard to know where to begin. One common cause is elevated histamine levels in the blood (which can happen for a variety of different reasons, not necessarily about "allergies"). Among the many things that histamine
does is to regulate the "volume" of sensory input in the brain (ever had sensory auras from a migraine? that's histamine at work). A common source of histamine is an imbalance of gut flora- yeast and many bacteria produce histamine, as well as other toxins that directly alter sensory processing (such as acetyl-aldehyde given off by yeast- acetyl-aldehyde is what causes the feeling of a hangover). In this case healing the gut and restoring healthy gut flora can eliminate the sensory problems. Excessive ammonia in the system is another example.
Inflammation in the brain is also a major issue- the location of the inflammation is what determines what is affected (auditory processing, balance, etc). Histamine is a big part of the inflammatory response so there is a connection there. Neurological inflammation can also be caused by viruses in the brain (especially herpes viruses), or by auto-immune reactions to other pathogens. When the auto-immunity is caused by strep it is called PANDAS and can involved OCD and tics. If a person has a leaky blood brain barrier (BBB), then there are many chemicals (some produced by the body, some from outside) that can get into the brain and trigger inflammation. Mercury is a big one, as is aluminum. Both of those can also cause the BBB to become leaky and allow other toxins or pathogens in.
The chemicals that can be let in can be from foods that we eat (they may also be toxins produced by the imbalanced gut flora, which are often heightened after we eat food that feeds those bad bugs). Some foods contain histamine in them, and the histamine from those foods can aggravate the situation. There are several other categories of foods that can do this including phenols (think Feingold diet and red dyes), salicylates, glutamate (like MSG but there are many natural sources too), and oxalates. Oxalates are a major source for Roo and I. Allergies cause inflamation so it can be simple as an allergic reaction. People sometimes refer to sensory and other neurological issues as "brain allergies".
How you address these causes depends on the person's unique situation biologically as well a what can be made to work realistically. I also want to point out that healing from sensory processing issues does not mean that you lose your perceptiveness. Many people feel that they have a very powerful antenna that brings in more input than they can often handle and they get overwhelmed. Treating the underlying causes doesn't take away your antenna, it just puts you in control of the volume controls. Many people find that when they no longer get overwhelmed, they actually become "more" perceptive in many ways because they aren't needing to avoid or block out stimuli.
