Presentation
Back pain. Prior surgery.
Patient Data
Posterior rod and screw fixation from L3-L5 noted.
At L5/S1, there is fluid within the intervertebral disc with enhancement which extends into the anterior prevertebral soft tissues. There is posterior extension into the epidural space with a triangular anterior epidural collection to the right of the midline.
There is a larger more extensive posterior collection, subdural in location rather than epidural, which extends from the level of L5/S1 superiorly for a distance of at least 20 cm. It extends superior to the imaged field-of-view at the inferior border of T11. On cross-section, this collection causes severe compression of the cauda equina from at least T12 - L1/2.
Similarly there is extensive fluid and enhancement within the posterior paravertebral space from approximately L3-S1. A small collection is identified within the right paravertebral soft tissues at the level of S1 with several smaller pockets of rim enhancing fluid more superiorly at the level of L5. This region is difficult to visualize due to artefact from the fixation rods.
A 2cm abscess in the right piriformis and left sacral ala are also noted.
Conclusion:
Extensive posterior spinal abscess throughout the imaged lumbar spine - the superior extent has not been imaged and MRI of the thoracic spine is recommended.
The abscess is subdural in location rather than epidural and causes severe crowding of the cauda equina between T12 to L1/2. The infection likely arises from discitis osteomyelitis at L5/S1 and osteomyelitis of the left sacral ala. Anterior prevertebral inflammatory phlegmon and posterior paravertebral inflammatory changes as described.
Case Discussion
This diabetic man had a recent (approximately 10 days earlier) amputation of a toe for osteomyelitis that grew two staphylococcus species. He was placed on intravenous antibiotics.
Following this MRI he went on to have surgery and subdural pus was drained.
GRAM STAIN Leukocytes ++
Cultures failed to grow any organisms, presumably due to prior antibiotic therapy.
Discussion
The interesting and important teaching point, in this case, is that the collection is subdural (a subdural empyema) rather than epidural (epidural abscess). It is critical to recognize this as if this is not appreciated the surgeon may not open the dura and merely shrug and assume that the collection is much much smaller than believed.
The subdural space, between the meningeal layer of the dura (theca) and arachnoid is also present in the spine although this is not as often involved in pathology and even less commonly recognize.d