Primary Cause of Spondylodiscitis
The primary cause of spondylodiscitis in adults is hematogenous spread of infection, with bacteria seeding the highly vascular vertebral endplates via septic emboli from bacteremia or endocarditis 1.
Pathophysiology of Infection
The mechanism follows a predictable pattern in adults:
- Initial seeding occurs at the vertebral endplate through hematogenous spread, as the disc itself is avascular in adults
- The infection then extends from the endplate into the adjacent intervertebral disc space
- Subsequently spreads to involve the opposing vertebral body
- Can further extend to posterior elements, paravertebral soft tissues, and epidural space 1
Most Significant Risk Factors
Bacteremia and endocarditis represent the most significant risk factors for developing spondylodiscitis 1. This explains why blood cultures are positive in approximately 47% of pyogenic spondylodiscitis cases 2.
Alternative (Less Common) Routes of Infection
While hematogenous spread dominates, spondylodiscitis can less commonly result from:
- Direct extension from adjacent soft tissue infection
- Direct inoculation during spinal procedures or penetrating trauma 1
These iatrogenic or traumatic causes are distinctly secondary mechanisms compared to the predominant hematogenous route.
Predominant Pathogen
Staphylococcus aureus is the leading causative organism, accounting for:
- Up to 80% of pyogenic spondylodiscitis cases 3
- 64% of pyogenic cases in recent series 2
- Approximately half of all non-tuberculous cases 4
Worldwide, tuberculous spondylodiscitis remains the most common type globally, though pyogenic causes predominate in developed countries 5.
Distribution Pattern
The infection typically affects:
- Single level involvement: 65% of patients
- Multilevel contiguous infection: 20% of patients
- Non-contiguous infection: 10% of patients 1
The lumbar spine is most frequently affected (63%), followed by thoracic (25%) and cervical (12%) regions 2.
Clinical Implication
Understanding that bacteremia/endocarditis drives most cases explains why identifying and treating the primary source of bacteremia is critical to successful management, and why blood cultures should be obtained before initiating antibiotics whenever possible 6, 2.