Genetics and LSTV
People are born with lumbosacral transitional vertebrae. Each lumbar and sacral vertebra you have was made during development stages prior to birth. They are not acquired later in life from trauma, degeneration, poor posture, heavy lifting or other lifestyle factors.
New research points to a group of genes responsible for normal development of the spine to play a large role in whether or not an individual vertebra takes on lumbar or sacral features. Groups of genes known as HOX10 and HOX11 have been studied as the body’s “developmental blueprint” responsible for laying down normal spinal anatomy and vertebral level identity during embryonic development.
In one study, researchers looked at the development of vertebrae and discovered that disruption of HOX genes could lead to transitional anatomy.
For the doctor this means that an LSTV is a true variation. For the patient this means your transitional vertebra was born with you since before you were born. Ok you may not have symptoms until your Junior or teenage years or even adulthood but the variant has always been there.
Genetics may play a role in you developing an LSTV but they do not predetermine if you will develop symptoms and be diagnosed with Bertolotti Syndrome (Bsy). Anatomy, biomechanics, activity, changes in adjacent segments and patient specific factors all play a role in symptom development.
HOX Genes Determine Vertebral Identity
So why does this happen? Well…did you know that vertebrae aren’t made, they’re programmed to BECOME vertebrae? During spinal development in the womb, genes from the HOX family play a role in deciding whether a developing segment will become lumbar, sacral or something else along the spine.
Experimental alteration of HOX expression (namely HOX10/HOX11 family members) has been demonstrated to alter vertebral patterning/segmentation. Basically, vertebrae start to take on the appearance of an adjacent region along the spine. It’s believed that this phenomenon allows for transitional anatomy to occur, whereby a vertebra assumes characteristics of both the lumbar spine and sacrum.
We know that the lowest lumbar vertebra can develop enlarged transverse processes and sacral-like characteristics forming a lumbosacral transitional vertebra. Instead of viewing this as a defect, think of this as an alteration of the master plan that your spine was programmed with while you were developing in your mother’s womb.
Of course scientists are still trying to determine the exact genes involved, but this lends credibility to the idea that LSTVs develop in uterous and are affected by the same processes that determine whether a vertebra is built to be thoracic, lumbar, sacral, etc.
Research is continuing to look at the genetics behind vertebral development and its relation to Bertolotti Syndrome. In the meantime we do know that lumbosacral transitional vertebrae are developmental variations that occur before birth.
Rare Congential Conditions and Developmental Variations Associated with Bertolotti Syndrome (BSy)
Hypermobile Ehlers Danlos Syndrome
Hypermobile Ehlers Danlos syndrome (hEDS) is a genetic disorder that affects connective tissue. Patients experience instability within their joints, ligaments, and muscles and chronic pain. The most common version of EDS making up about 90% of cases. The relationship between hEDS and LSTVs has yet to be determined.
Similar to how LSTVs affect spinal biomechanics, patients with hEDS also put abnormal strain on stabilizing structures within the pelvis and spine.
Spina Bifida Occulta
Spina bifida occulta is one of the most prevalent congenital variations of the spine. Occulta means hidden, and refers to cases where the lamina and spinous processes fail to fully fuse together.
Multiple case studies have reported the coexistence of Spina Bifida occulta and LSTVs. Because they both occur as a result of variations during embryonic development, both conditions are thought to represent deviations from typical vertebral segmentation.
Craniocervical Instability (CCI)
Craniocervical instability is abnormal movement between the skull and the top part of thecervical spine. Typically this involves excessive motion between the occiput (base of skull) and the first two cervical vertebrae (the atlas and axis). These structures are responsible for housing many neurological and vascular components that keep our bodies functioning normally.
CCI can be the result of trauma/injury, inflammatory diseases, congenital issues, or problems with connective tissue integrity. In recent years we have started to see more discussion regarding CCI as it relates to hypermobility disorders, like Hypermobile Ehlers-Danlos Syndrome.
Patients with hEDS have connective tissues (think ligaments, joint capsules) that may not provide the typical amount of structural support we expect. As a result these patients can have increased motion (mobility) in many joints throughout the body including the junction of the skull and cervical spine. When this happens some people may experience symptoms including neck pain/headaches, dizziness/light sensitivity/vision changes, balance issues, fatigue, neurologic symptoms, or autonomic dysfunction.
Not all patients with hEDS will develop CCI and not all patients with CCI have a connective tissue disorder. However, there is increasing discussion in the medical community about the link between ligamentous laxity and craniocervical instability.
Congenital Cervical Stenosis
Congenital cervical stenosis is a narrowing of the spinal canal that occurs within the cervical region of the spine. This narrowing occurs from birth and differs from the spinal stenosis that occurs with age.
Congenital cervical stenosis occurs within a different region of the spine than LSTVs. However, some patients may present with more than one congenital spinal variation that affect multiple regions of their spine.
Thoracic Outlet Syndrome (TOS)
Compression of nerves and/or blood vessels between the neck and upper chest defines Thoracic Outlet Syndrome (TOS). Congenital anomalies including cervical ribs and thoracic outlet abnormalities have also been associated with TOS.
While TOS and Bertolotti Syndrome occur in different areas of the body, both demonstrate how developmental changes to skeletal structure can impact biomechanics and result in nerve irritation/compression. The presence of both conditions together doesn't necessarily mean they are related, but both can serve as a reminder to consider congenital anatomical variations when evaluating patients with chronic pain.
Extra Lumbar Vertebrae (Typically L6)
An L6 vertebra refers to a developmental variation of spinal anatomy where an individual has six lumbar type vertebrae rather than five. It may alter the typical segmentation of the spine and affect the biomechanics of the lumbosacral junction.
Typically the lumbar spine transitions into the sacrum at some point in its inferior extent. The sacrum functions to provide relative stability to movements occurring at the level of the pelvis. However when an L6 vertebra is present this division of motion between the spine and pelvis may be altered.
Load, motion or mechanical forces may subsequently be distributed differently across the vertebrae, discs, facet joints, ligaments and surrounding soft tissues. Pain generation or accelerated degeneration may occur at adjacent levels in some cases due to this altered biomechanical loading.
An L6 can occur in One of 3 ways-
L6 Lumbarisation of S1
The superior segment of the sacrum doesn’t completely fuse with the rest of the sacrum and maintains more characteristics of a lumbar segment. This gives the appearance of having six lumbar vertebrae and a sacrum with one segment missing (Lumbarised). or-
True Extra Lumbar Vertebra
Some people naturally have an extra vertebral segment giving them 25 vertebrae rather than the more common 24 presacral vertebrae. This results in a ‘true’ sixth lumbar vertebra. (sacralisation) or-
Difference in Thoracic Ribs
In some people they only have 11 pairs of ribs and are missing the 12th set of ribs resulting in the lowest count of lumbar vertebra being called an L6 (sacralisation) These people have 11 sets of thoracic ribs and not the normal set of 12. they also have 6 lumbar vertebrae instead of the normal 5.
Effects of L6 Vertebra on Biomechanics
Depending on its anatomy, an L6 vertebra can influence loading patterns across the lowest segments of the spine.
When pseudoarticulation is present between the vertebra and sacrum abnormal motion may occur across a false joint. This joint was never intended to support normal pelvic or spinal loads and can lead to inflammation, arthritic changes or localised pain symptoms overtime.
Alternatively complete or partial fusion may exist at the segment changing how much an individual can move at that specific spot. Increased stress can occur at the disc, facet joints and supporting structures above due to compensatory hypermobility.
Take Home Message
An L6 vertebra can be more than just an anomaly in numbering. Its presence has the potential to influence biomechanics and contribute to symptoms in certain cases.
The spine and musculoskeletal system develop together. Abnormalities in vertebral development, segmentation, connective tissue integrity and skeletal structure can all coexist producing individual patterns of biomechanical stress.