Edited excerpt from the book Understanding Hypermobile Ehlers-Danlos Syndrome and Hypermobility Spectrum Disorder, by Claire Smith

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Excerpt taken from Chapter 1, Frequently Asked Questions, pages 33 - 36.

Section entitled:

'Why are people affected so differently?

At the beginning of the previous section (Genetics), we noted that describing the inheritance patterns of hEDS using what is known as a ‘classic’ or ‘simple genetics’ model is probably too simplistic. In fact, to fully understand the whole range of variations seen in the spectrum spanning joint hypermobility, through hypermobility spectrum disorders and hEDS, it is likely that geneticists will need to look towards a slightly more complex model, which allows for the expression of the single or multiple gene mutation(s) to be further influenced by other genetic, environmental, or constitutional factors. 

 

The Threshold Model

The potential influence of genetic, environmental or constitutional factors may be better explained using the ‘Threshold Model’ - whereby a single, highly penetrant, dominant, inherited gene mutation is considered to increase risk, but is not necessarily sufficient for the disorder to be expressed (for symptoms to show). The underlying connective tissue variant seen in the hEDS/HSD spectrum can vary in severity and is not always sufficient per se in causing the disorder outright but, when other genetic, environmental, or constitutional factors are added, the combination may be sufficient to reach the threshold for disease expression in a hypermobile person.  These factors impact on how specific segments of the genetic code are turned on or ‘expressed’ and whether, when, and how the process happens (1 & 2).  

 

Examples of environmental and constitutional factors include:

 

  • An individual’s ethnicity 

  • Their biological sex 

  • Age(ing)

  • The combination of hypermobility types inherited (see Box 5 below)

  • The physical character of an individual (build, strength, general health etc.)

  • Psychological characteristics

  • Sports activities they may pursue 

  • Dietary habits

  • Traumas and/or surgeries they undergo 

  • Periods of temporary immobility

  • How well an individual's brain interprets proprioceptive feedback

 

In this section, we will take a look at some of the above factors, which may contribute to the variation in manifestations and severity, in more detail:

Ethnic background 

People of different ethnic backgrounds have differences in their joint mobility, which may reflect differences in the structure of the collagen proteins. Hypermobility is more common in those from Asian  backgrounds, African Black populations (3) and those from Indian heritage, than in white Caucasians. For example, people from the Indian sub-continent often have much more supple hands than Europeans (4).

Bony, collagen-related, and neuropathic hypermobility

There are three main types of hypermobility, namely bony hypermobility, collagen-related hypermobility and neuropathic hypermobility (see Box 5), and each type has its own characteristics (5 & 6). Any one person will have varying combinations of these types, but one type may predominate (7). 

Muscle tone 

Muscle ‘tone’ refers to the muscles’ ability to resist stretching while in a passive, resting state. Without us being aware of it, muscle tone keeps our muscles in a partially contracted state, which helps maintain posture, protects against injury, and declines during REM sleep.  The more relaxed our muscles are, the more movement we have in our joints (4). Poor muscle tone results in decreased stretch reflex responses, and the limbs’ resistance to passive movement being decreased (8, 9). Those with HSD, and hEDS, need to try to retain or increase their muscle strength and tone as much as possible in order that their muscles can compensate for the effects of defective connective tissue in other supporting structures such as ligaments...’

End of excerpt -  In the book, the remainder of this section carries on to cover the remainder of the list, i.e:

 

  • Age

  • Sense of joint movement (proprioception)

  • Examples of other environmental and constitutional factors

  • Biological sex

Longer sections (2 x A4 pages) from the list include:

  • The effects of sex hormones

  • Hormones which are relevant to hypermobility

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References

1. Castori M. et al 2011- Symptom and joint mobility progression in the joint hypermobility syndrome (Ehlers-Danlos syndrome, hypermobility type) Clinical and Experimental Rheumatology. 2011;29:998–1005. [PubMed]

2. Rothstein M. et al 2009 - The Ghost in our Genes: Legal and Ethical Implications of Epigenetics. Health Matrix Clevel. Author manuscript; available in PMC 2011 Feb 7.Published in final form as: Health Matrix Clevel. 2009 Winter; 19(1): 1–62.

3. Birrel F.N. et al  1994 - High prevalence of joint laxity in West Africans. Br J Rheumatol. 1994 Jan;33(1):56-9.

4. Arthritis Research UK - Joint hypermobility. www.arthritisresearchuk.org/arthritis-information/conditions/joint-hypermobility.aspx  - Content Contributor Prof H. Bird

5. Knight I. 2011 - A Guide to living with Hypermobility Syndrome: bending without breaking. Singing Dragon ed. London

6. Celletti C. et al  2013a - The multifaceted and complex hypermobility syndrome (a.k.a. Ehlers-Danlos Syndrome Hypermobility Type): Evaluation and management through a rehabilitative approach Clin Ter 2013; 164 (4):e325-335. doi: 10.7417/CT.2013.1597

7. Morgan A. W. and Bird H. 2007 - Asthma and airways collapse in two heritable disorders of connective tissue Ann Rheum Dis. 2007 Oct; 66(10): 1369–1373. Published online 2007 Apr 5. doi:  10.1136/ard.2006.062224 PMCID: PMC1994284

8. Boundless.com 2015 -  “Muscle Tone.” - Boundless Anatomy and Physiology. Boundless, 22 Jan. 2015. from https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/muscle-tissue-9/control-of-muscle-tension-97/muscle-tone-545-5975/. Retrieved 22 Feb. 2015

9. NHS Direct Wales 2014 - Hypotonia - nhsdirect.wales.nhs.uk/encyclopaedia/h/article/hypotonia/

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