It is interesting you, Hannah H. raising this point about  sensitivity to external stimuli. Movies such as, There’s Something About Mary, I am Sam depict characters with highly sensitive behaviors. While these are made believe characters they serve the purpose of exposing characteristics that are in real life. Hypersensitivity is not something that is a new idea in people with autism.  However, while working on our book project we completely overlooked this characteristic and it was because the DSM did not mention it-something you, Hannah pointed out.

I have decided to look this up in more detail. So here is a little information about this type of abnormality. The most common sensory problems autistic people experience are their hyper or hyposensitivities to sensory stimuli. Their senses seem to be too acute (in the case of hypersensitivity) or not working at all (in the case of hyposensitivity). An example that is widely reported is hyperhearing. Hyperhearing autistics generally are very light sleepers and are frightened by sudden unpredictable sounds. Other examples of abnormality is Hypertaste and Hypersmell as well as hypertactile.

Now as far as why it is not in the DSM I have not a clue. However I did find a paper that describe the reasons for the abnormality sensitivity. It seems emotional factors can magnify the global stimulation and cause, in some person, an enhancement in the perception of visual, auditory, smell and/or painful stimuli. The limbic system plays a role in the hypersensitivity as it is this system that is abnormal in autistic individual suffering from hypersensitivity. In reference to auditory sensitivity when a sound elicits abnormal responses, the result is the establishment of a subconscious reflex response with automatic and invariable activation of the limbic and autonomic nervous systems.

The abnormality is not the conscious sound but in fact the meanings and associations that are associated with the stimulus. The correlation observed between hypersensitivity to sound and the motions of placing the hands on the ears may result from the stimulation of the autonomic nervous system and the triggering of protective reflexes. There is a link between perceptual abnormalities and behaviors of stereotypy, fixations, and obsessions.

Behavioral manifestations to sounds are not associated to hypersensitivity of the auditory pathways, but rather to difficulties in the upper processing at the level of the cerebral cortex, involving systems that usually are impaired in autistic spectrum patients, such as the limbic system. Identical results occur with other changes in sensitivity and their associated behaviors, as fear and reality distortions, which are complex interactions originated from upper processings, instead of specific hypersensitive pathways.

The study is: Gomes, C., Rotta, N. T., Pedroso, F., Sleifer, P., Danesi, M.C. (2004). Auditory hypersensitivity in children and teenagers with autistic spectrum disorder. Archives of Neuro-Psychiatry, 62, 797-801.