Sensory Processing Disorder confuses the hell out of a lot of people. This info will help explain why your kid has to chew on all their clothes, cant stand their sister singing but will talk at 120 dB and not notice.
In short, SPD is a neurological disorder, close to - but not officially on - the autism spectrum. That's a sore point with parents. It has no "official diagnostic criteria." It's not in the DSM so may professionals will not diagnose it as a stand-alone disorder. In America particularly this has important ramifications for access to insurance coverage and accommodations made for the child at school. In Australia it means that some support services are not available and special Medicare funding can't be accessed. Major suckage.
It is considered that SPD can present on its own, without a co-morbid ASD diagnosis, ASD can exist without SPD symptoms and it can present with them. It is my personal opinion - and only an opinion - that those who diagnose it as a separate condition with no ASD are not looking hard enough.
SPD is an overload of information being received by all our senses: visual, auditory, touch, taste, smell and the ability to feel pain hunger and temperature (called the interoceptive sense). Lights are brighter and too stimulating. Sounds can seem louder or you can't filter out sounds in your environment to concentrate on one. Conversations can be impossible to follow. The feel of clothing can be close to painful. Food can be a battleground where things of different colours are not allowed to touch on a plate or foods that they will eat are extremely limited. F-Man wants to throw up at the very sight of yoghurt. Smells that other people don't notice can make you throw up or become extremely agitated.. You may not notice the temperature and dress in shorts and a t-shirt when it's freezing cold or a jumper when its 37 degrees Celsius. Again, F-Man. You may not feel pain.
You can be a seeker of these stimuli or you can be an avoider. Someone who is a seeker may crash into objects, love running, jumping, hard squishy cuddles. An avoider may hate being touched. Those who seek have senses which are under-stimulated (hyposensitive), whilst those who avoid have senses which are overstimulated (hypersensitive). You may be both a seeker in one sense and an avoider in another. Or even both in one sense, depending. Confused? I sure as hell was at the beginning of this journey.
Being exposed to these stimuli can cause massive meltdowns and it is often very difficult to work out exactly what has caused your child to start crying uncontrollably, be unable to calm himself or be calmed by others. It can cause hyperactive behaviour, aggression, strange noises, rocking, hand flapping, babbling nonsense: all the things we associate with autistic behaviour. These reactions are often what is called "stimming" (stimulation). It is a way of self soothing. We all do it - hair twiddling, tapping fingers on a table etc. In both SPD and ASD it manifests as what people see as strange, inappropriate or annoying behaviours.
The way to "treat" SPD is with Occupational Therapy and learning a new skill set, referred to as a "sensory diet." Behaviour management may also be used to deal with feelings of anxiety, aggression or fear that may accompany exposure to certain stimuli. Sometimes, for extreme cases, medications are used. It's not ever going to be my choice, but others find it works extremely well for their child.
The absolute best site I have found is Sensory Processing Disorder Parent Support. Run by one dedicated woman out of Canada, it has the most comprehensive information about everything you could possibly want to know: symptoms, who to see for a diagnosis, a huge list of OT therapies, a forum, links to stores for sensory and autism aids. You name it, it's here. She also runs a Facebook Page from which I have learnt so much valuable information from other parents. There are a large amount of posts each day and there is always something that will apply to you. JOIN THIS PAGE YOU MUST.
Pinterest has loads of SPD boards. LOADS. I have one. Here it is. They are full of links to therapy aids, OT therapies, personal stories, tips, sensory games etc etc. It is worth spending many hours going through them. I've had months of late nights finding very cool stuff.
The online stores I have found to supply a great range of products are listed on my Autism page.