Spinal hemangiomas form when blood vessels proliferate abnormally within the bone marrow spaces of a vertebra. The exact trigger for this overgrowth remains unknown, but the process involves a combination of vascular cell behavior, possible genetic changes, and hormonal influences. These growths are extremely common: MRI-based studies find them in roughly 41% of people scanned, and the vast majority never cause symptoms or require treatment.
How a Spinal Hemangioma Forms
A vertebral hemangioma begins as a cluster of blood vessels that multiply within the spongy interior of a vertebral bone. The resulting mass is made up of blood vessels of varying sizes, fat cells, smooth muscle, fibrous tissue, and remodeled bone. These vessels are typically thin-walled and sit within a soft, fluid-filled tissue matrix, hemmed in by the bony scaffolding (trabeculae) of the vertebra itself.
Most spinal hemangiomas fall into two structural categories. Capillary hemangiomas are built from many small, fine blood vessels. Cavernous hemangiomas contain fewer but much larger, dilated vessels. Less commonly, a hemangioma can be composed of tangled arterial and venous networks or clusters of abnormal veins. The capillary and cavernous types account for the large majority of cases found in the spine.
The Role of Genetics
No single gene mutation has been definitively identified as the cause of spinal hemangiomas, which is notable given how much progress researchers have made in pinpointing genetic causes for other vascular abnormalities. Twin studies comparing identical and fraternal twins found no strong evidence of an inherited predisposition in most people.
That said, genetics likely plays a role in at least some cases. A small number of families pass down a tendency to develop hemangiomas and vascular malformations in a dominant inheritance pattern, meaning only one copy of the altered gene is needed. Researchers mapped a region on chromosome 5q linked to hemangioma development in these families. Further investigation found that hemangioma tissue frequently shows a loss of genetic material in this same region, suggesting that a gene there normally acts as a brake on blood vessel growth. When that gene is lost or damaged, the brake is released and vascular proliferation can begin.
This “loss of function” model fits with the broader understanding of how benign tumors develop: a cell loses a protective signal and begins growing in an unchecked but non-cancerous way. Still, the specific gene on chromosome 5q has not yet been identified, and most people with spinal hemangiomas have no family history of them.
Hormonal and Pregnancy-Related Factors
Hormones, particularly estrogen and progesterone, appear to influence the growth of vertebral hemangiomas. These hormones can stimulate the formation of new blood vessels, a process called angiogenesis, which is the same mechanism that builds the hemangioma in the first place. This hormonal link helps explain why spinal hemangiomas are found slightly more often in women (43%) than in men (39%), though the difference is modest.
Pregnancy is the clearest example of hormones driving hemangioma behavior. The combination of rising estrogen and progesterone levels with a dramatic increase in blood volume, especially in the third trimester, can cause a previously silent hemangioma to enlarge rapidly. About 62.5% of pregnancy-related symptomatic cases occur during the third trimester, when blood volume peaks. After delivery, uterine contractions can redirect a sudden surge of blood back into the central circulation, potentially accelerating hemangioma expansion even further. This does not mean pregnancy causes hemangiomas to appear from nothing. Rather, it can transform a small, stable lesion into one large enough to press on the spinal cord or nerves.
Tumor or Birth Defect?
There has been a longstanding question about whether spinal hemangiomas are true tumors (new, abnormal growths) or congenital malformations (structural errors present from birth). The current international classification system draws a clear line between the two categories. Vascular tumors, including hemangiomas, arise from active overgrowth of the cells lining blood vessels. Vascular malformations, by contrast, are developmental errors where abnormal vessels form but the cells lining them behave normally and don’t multiply excessively.
Most vertebral hemangiomas in adults fall into the tumor category, meaning they result from a slow proliferation of vascular cells rather than a malformation present since birth. The cells are not cancerous and grow very slowly, which is why these lesions can sit quietly inside a vertebra for decades without causing any trouble.
Where They Tend to Appear
Spinal hemangiomas can occur at any level of the spine, and no single vertebra is dramatically more vulnerable than others. That said, T11 and T12, the two lowest vertebrae of the thoracic (mid-back) spine, show the highest prevalence, with about 3.6% of those vertebrae affected across study populations. Within the vertebral body itself, hemangiomas are roughly equally distributed between the front and back halves, and between the center and the periphery. They tend to sit at or just above the midpoint of the vertebral body’s height.
Why Most Stay Silent
The overwhelming majority of spinal hemangiomas never produce symptoms. They’re discovered incidentally when someone gets an MRI or CT scan for an unrelated reason. On imaging, they have a characteristic look: a pattern sometimes described as a “polka-dot” sign on cross-sectional CT images, created by the thickened bony trabeculae seen in cross-section among the vascular tissue.
A hemangioma typically becomes symptomatic only when it grows large enough to expand beyond the vertebral body and compress nearby structures like the spinal cord or nerve roots, or when it weakens the bone enough to cause a small fracture. The factors that push a quiet hemangioma toward aggressive behavior include rapid vascular expansion (as seen in pregnancy), a location that leaves little room between the lesion and the spinal canal, and a composition that is more vascular and less fatty. Lesions with a higher proportion of fat tend to be more stable, while those dominated by blood vessels and edematous tissue are more likely to enlarge.
Because the root cause of the initial vascular overgrowth is still not fully understood, there is no known way to prevent a spinal hemangioma from forming. The practical reality is that these are among the most common benign bone lesions in the human body, and for the vast majority of people who have one, it will never cause a problem or require any intervention.