Lordosis

What are some medical causes for sway back (lordosis)?

Lordosis, also known as sway back, is an exaggeration of the lumbar curvature of the spine.  Folks are often most familiar with this in reference to the posture of pregnant females.

First things first - let's start with what is anatomically normal.

An anatomically normal human vertebral column has 4 curvatures that can be observed from the lateral view.

The curvatures of the thoracic and sacral regions of the spine are present at birth while those of the cervical and lumbar regions develop postnatally, during the first year of life.

A person who has an exaggerated or excessive lumbar curvature would be said to have hyperlordosis (a.k.a. sway back, saddle back).  Some resources shorthand this to just "lordosis," although that technically refers to the normally-occuring curvature in this area.

There are several reasons a person may experience lordosis.  As I stated earlier, folks who are aware of this may know it best from the posture of pregnant females.  If a person has weight added to the anterior abdomen (as in the case of pregnancy or increased visceral fat in the abdomen), that person may develop lordosis as a result, to stabilize one's center of mass (as depicted in the image below).

Whitcome, et al. (2007) Nature

Other causes noted for lordosis (particularly in children) are achondroplasia and spondylolisthesis, neither of which involves addition of weight to the anterior abdomen and both of which affect the skeleton.  Achondroplasia is a bone growth disorder which causes the most common form of dwarfism, so while folks with this condition may experience lordosis, it likely wouldn't be considered the major symptom of this condition.  Spondylolisthesis is a condition in which a vertebra in the vertebral column slips out of position – this most often happens to L5 - and would then cause a change in the lumbar curvature resulting in lordosis.  There are other conditions a person may have (like forms of muscular dystrophy) that could result in lordosis, but that person would have a lot of other symptoms as well.  

References

"Achondroplasia" (2011) PubMed Health. Retrieved 8 February 2013 from http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002544/

"Lordosis" (2012) MedlinePlus. Retrieved 16 January 2013 from

http://www.nlm.nih.gov/medlineplus/ency/article/003278.htm

"Spondylolisthesis" (2012) MedlinePlus. Retrieved 16 January 2013 from

http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002240/

Whitcome, K.K., Shapiro, L.J., Lieberman, D.E. (2007) Fetal load and the evolution of lumbar lordosis in bipedal hominins. Nature. 450: 1075-1078. doi: 10.1038/nature06342.