Chapter: Pathophysiology & Classifcation of PoTS (2021) pt I: Neuropathic POTS
Around 50% of POTSies seem to have peripheral sympathetic neuropathy aka Neuropathic POTS
This chapter, in the book Postural Tachycardia Syndrome (2021), written by Matthew G. Lloyd & Satish R. Raj, talks about subtypes of Postural Orthostatic Tachycardia Syndrome (POTS).
The Paper
So, POTS subtypes (aka endophenotypes) were created to try to understand the different pathologies that can cause POTS.. but, unfortunately, they aren’t actually very useful, because:
the definitions of each aren’t standardized
people often (usually?) seem to have multiple subtypes going on at once
multiple causes can still cause the same subtype
They illustrated the messy situation in their Figure 2 diagram:
Still, the subtypes do exist in the literature, and this chapter provides a rundown of each. I’m going to break it up into a few different posts just for easier navigation.
Neuropathic POTS
In 2000 a New England Journal of Medicine study by Giris Jacob et al. conclusively shown that a subset of POTS patients have peripheral sympathetic neuropathy, and they coined the term “neuropathic POTS.”
The medical literature, though, is a hot mess. Everyone uses different methods to investigate a patient’s autonomic activity, so it’s hard/impossible to actually compare much between the studies. Despite that, the authors estimate that about 50% of POTS patients seem to have “peripheral sympathetic neuropathy”/neuropathic POTS.
“The putative mechanism underlying PoTS in patients with autonomic neuropathy is excessive venous pooling due to inadequate venoconstriction in the legs, and perhaps the splanchnic vasculature, which causes reduced cardiac output. A larger sympathetic outfow is required in order to maintain blood pressure (BP), and causes excessive orthostatic tachycardia.”
Translation: The underlying wtf in neuropathic POTS is generally considered to be wimpy blood vessel constriction of the legs (and maybe the abdominal gastrointestinal organs,) which causes the blood to pool there instead of going back to the heart so the heart can pump it out again.
The authors then summarize a 2013 study of 24 POTS patients by Gibbons et al. They defined neuropathic POTS by “scoring” below the 5th percentile on both Intraepidermal Nerve Fiber Density (IENFD), plus another test, such as the QDIRT/QST.
“This study revealed several important insights into PoTS neuropathies:
the remarkable lack of concordance between the various nerve function tests observed in this study implies distinct pathophysiological mechanisms,
the clinical presentation of neuropathic PoTS is distinct from non-neuropathic PoTS in several ways (Table 2), and
neuropathic PoTS patients do not generally report symptoms of a painful small-fber neuropathy, and
neuropathic PoTS patients exhibited a smaller phase IV overshoot during the Valsalva maneuver than the non-neuropathic group.”
Table 2: Clinical markers that differ between neuropathic POTS and non-neuropathic POTS
“Lower resting and tilted heart rate
Lower measures of parasympathetic function
Lower phase 4 Valsalva overshoot
Lower anxiety and depression
Greater ability to carry out usual activities
Greater self-perceived overall health-related quality of life”
Next up will be their summary of Hypovolemic POTS. ->
Lloyd MG, Raj SR. Pathophysiology and classification of pots. In: Gall N, Kavi L, Lobo MD, eds. Postural Tachycardia Syndrome. Springer International Publishing; 2021:29-40. doi:10.1007/978-3-030-54165-1_5
Comments are open for discussion, as well as if you have a correction or clarification of anything in this post!