For those of you interested in the overlap between Ehlers-Danlos (EDS)/hypermobility spectrum disorders (HSD) and autism, sensory issues in EDS/HSD probably come as no great surprise. Nevertheless, this is an area of similarity that’s remained poorly explored and so I’m going to review some of the sensory differences we know occur in these hereditary connective tissue disorders that share a loose overlap with those we see in autism.
Like some folks on the autism spectrum, zebras are not always known for their coordinative prowess [1, 2]. This is because there is some kind of impairment in the sense of proprioception, which is the sense of knowing where your body is in space. Mind you, this doesn’t mean that some individuals may not be accomplished in particular areas like dance or sports. But they may be more prone to accidents in everyday life. The severity of these deficits also varies by individual.
Although we don’t know exactly why proprioception tends to be impaired in EDS/HSD, the reigning hypothesis posits that lax joints and muscles are worse at sending feedback to the brain about their relative positions, leaving the brain to do more guesswork. This may be due to damage as a result of overstretching of the muscles and tendons because of hypermobility.
But although this hypothesis makes some sense, we don’t actually know the mechanism that’s leading to proprioceptive impairment. It’s possible, for instance, that the stretch receptors that are part of the sensory system in the joints and muscles aren’t working well for other reasons (e.g., neuromuscular dysfunction unrelated to structural damage). Interestingly, stretch receptors in the skin that detect vibration don’t seem to be similarly impaired in EDS/HSD, suggesting that the root of proprioceptive impairment in the conditions lie in the muscles and joints and their sensory fibers .
Touch can be another sense that’s altered in EDS/HSD. One reason for this can be small fiber neuropathy (SFN), which occurs in the majority of zebras with varying degrees of severity . [It’s also found in some people with postural orthostatic tachycardia syndrome (POTS) (4).]
SFN occurs due to damage to small sensory nerve fibers in the skin (this may also be accompanied by damage to other fibers that are part of the autonomic nervous system) . SFN can lead to pain, tingling, prickling, and numbness. People with SFN also tend to be hypersensitive to pain in general (they have an exaggerated pain response), although they may also be underresponsive to pain within very small areas of the body (e.g., a pin prick) likely due to loss of nerve fiber complexity and therefore sensitivity. Animal models have shown that damage to peripheral sensory nerves results in an exaggerated response to normal amounts of pain-inducing factors (e.g., substance P) in the nerve-injured animals, showing that nerve damage may make these fibers hyperresponsive under normal conditions . Inflammation may further complicate this process.
As many of you know, the majority of individuals with EDS/HSD have some sort of dysautonomia or dysregulation of the autonomic nervous system. POTS is the most frequently diagnosed type of dysautonomia in EDS/HSD, although other conditions like orthostatic hypotension (OH), mixed POTS/OH, or orthostatic intolerance (OI) can also occur. Although dysautonomia can include a wide variety of symptoms, individuals with these conditions can often experience sensory hypersensitivity, particularly to light and sound . Light sensitivity in POTS can sometimes be due to abnormal pupil dilation or may also be linked with migraines that are not uncommon in the condition. But in general, all senses can be affected and it has been shown that cardiac activity has a significant effect on the processing of sensory information in the brain [reviewed in 8].
Cerebrospinal fluid (CSF) leaks and Chiari malformations are not uncommon in EDS/HSD and can also lead to light and sound sensitivity, as well as vertigo and balance issues [9, 10]. In addition, vertigo can occur sometimes in mast cell activation syndrome (MCAS), another common comorbid condition with EDS/HSD .
EDS/HSD seems to co-occur with neurodevelopmental conditions like autism and ADHD, as well as sharing family overlap (for instance, a mother may have EDS/HSD, while her son has autism). However, it’s usually been assumed that sensory issues in autism are due to a centrally-mediated cause (i.e., the brain) rather than peripheral issues. Meanwhile, EDS/HSD is assumed to be peripheral in origin but little attention is paid to the central nervous system (with exceptions of CSF leaks and Chiari malformations) despite it’s associations with autism and ADHD.
These spectrum conditions would likely benefit each other by comparison. It’s possible that some of EDS/HSD’s sensory issues are related to or influenced by dysregulation of the central nervous system. Meanwhile, it could behoove us to take a look at the function of the peripheral nervous system in autism and ADHD.