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Science Saturday: Sensory Processing Disorder

Today’s topic is Sensory Processing Disorder. I didn’t have a specific question to answer, so I decided to make this post an overview of our current understanding of SPD. As with all of these posts (so far), it’s important to know that I’m affected by the topic at hand. I’ll do my best to remain objective, but I do have some biases when it comes to SPD.

Sensory Processing Disorder as a Diagnosis

SPD was first described as a construct by Ayres in 1964. At that time, it was known as Sensory Integration Dysfunction. Now, SPD has several subtypes, including Sensory Modulation Disorder.

Sensory Modulation Disorder

Three main categories compose SMD: sensory overresponsivity, underresponsivity, and sensory seeking. Sensory Over-Responsivity (SOR)  can be defined as:

…exaggerated responses to one or more types of sensory stimuli not perceived as threatening, harmful, or noxious by children and adults who are typically developing.

Analysis by different authors of data from a 2004 study that investigated the prevalence of SPD among kindergarten children in a Colorado school district found the prevalence of sensory over-responsivity to be between 2.8 and 6.4%.

Experiencing Sensory Under-Responsivity (SUR) might look like:

…quiet and passive, appearing to disregard stimulation by not responding…[they may have] inadequate body awareness, poor endurance, and movements that are not appropriately graded.

And lastly, sensory seeking, or Sensory Craving (SC), is “…craving sensory stimulation and appearing to be obsessed with obtaining additional sensory input.”

SBMD and SDD

The other two subtypes of SPD are Sensory-Based Motor Disorder, which includes dyspraxia and postural disorder, and Sensory Discrimination Disorder, which can affect one or more sensory systems. A diagnosis of Sensory Processing Disorder can include symptoms in each of the subtypes; in fact, it’s often a mishmash of seemingly contradictory symptoms that fit into different types.

SPD and its subtypes are included in the 2005 version of the Diagnostic Classification of Mental Health and Developmental Disorders of Infancy and Early Childhood (DC:0-3R). However, it’s not included as its own disorder in the Diagnostic and Statistical Manual (DSM), which only perpetuates the idea that SPD symptoms are just behavioral and temperamental issues, rather than a real, neurological diagnosis.

Neurological and Physiological Research

Fortunately, there is evidence supporting SPD as a valid diagnosis. A 2013 study compared the brains of boys with and without SPD (sample size of 40) using diffusion tensor imaging. Significant differences in white matter microstructure were found in the posterior corpus callosum, corona radiata, and thalamic regions. This study used the cardiac vagal tone index to measure the physiologic reactivity of 15 children. The results were significant and showed that the kids with SPD had less effective vagal tone than kids without SPD. This indicates that people with Sensory Processing Disorder may have impaired parasympathetic functioning. I have no reason to doubt this result, I just wish the sample size were larger.

Not Significant?

I wanted to include this study, and as I read through it, I reached the part where they break it to you that the results were not statistically significant. Darn! (Remember when I said I was biased?) In any case, it’s still interesting, and this is part of what science is about; you can still learn something from insignificant results.

An EEG study with a sample size of 53 tested kids’ brain waves by introducing a “conditioning” stimulus followed by a “test” stimulus. The idea is that your brain becomes accustomed to stimuli that it deems safe, so the test stimulus elicits a smaller reaction than the conditioning stimulus. This is called sensory gating. As a group, the kids with sensory processing disorder showed less sensory gating than the comparison group (p= 0.04), but it wasn’t statistically significant when an adjusted alpha level was used. If the results are robust enough, it indicates that people with SPD don’t get conditioned by stimuli like people without SPD. Every instance of a startling stimulus produces the same reaction as the first one did.

Sensory Processing Disorder and Autism

The vast majority of people with Autism Spectrum Disorder (ASD) also have sensory challenges. Yet, not everyone with SPD also has ASD. Research is beginning to tease apart where the overlaps lie. Using the same imaging technique as in the study above, researchers found differences between the white matter in boys with autism and the white matter in boys with SPD alone. Compared to a control, both groups’ white matter differences overlapped in tracts thought to contribute to sensory processing. However, the boys with ASD had additional white matter differences in tracts linked to social processing. Other research suggests that assessing cognitive styles may be a good way to differentiate between Sensory Processing Disorder and Autism Spectrum Disorder.

Sensory Processing Disorder and Mental Health

If you’ve been keeping up with my blog (if not, welcome), you know that I have sensory processing disorder and depression. I stumbled upon a couple of studies that show correlations between sensory dysfunction and mental disorders. This one, for example, shows that both under-responsiveness and over-responsiveness correlate with depression and impulsivity. Another one suggests that under and over-responsiveness may be “trait” markers for people with major affective disorders. That last one seems like a stretch to me, but my own perception of those studies is that a.) perhaps there’s some neurological connection between the white matter changes of SPD and mental illnesses, and/or b.) it’s tough to live with SPD, leading many with the disorder to develop secondary disorders.

This is a complex topic, and I’d love to hear your thoughts. Let me know what you think in the comments.

3 thoughts on “Science Saturday: Sensory Processing Disorder

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