Intradural abscess: A challenging diagnosis. Case series and review of the literature

Spinal intradural abscesses are extremely rare. To our knowledge, only a few cases have been described in the literature. We report 2 cases of spinal intradural abscesses in patients presenting to our institution with different symptomatology. Both cases involved the lumbar spine, with different etiologies: Case 1 was presumptively related to spondylitis phenomena, with surgery confirming the intradural localization of the abscess; case 2 was of probable iatrogenic nature (secondary to lumbar drain positioning). The aim of this report is to briefly discuss the clinical significance and pathogenesis of these cases.


Introduction
Spinal intradural abscesses are an extremely rare condition.To our knowledge, only a few cases have been described in the literature with long-term sequelae (eg, neurological impairment) and poor prognosis if untreated [1] .Most commonly, intradural abscesses have been described as a consequence of a hematogenous spread of an infection, however, they may also be secondary to iatrogenic causes (eg, epidural injections) or to spondylitis phenomena [2] .
✩ Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
We would like to report 2 cases of patients presenting to our institution with different symptomatology and a common diagnosis of intradural spinal collections.pain with bilateral cruralgia, not responsive to medical therapy.Neurological examination revealed chronic lateral hypoesthesia of the right thigh.Blood tests showed elevated values of C-reactive protein, normal white blood cell count with mild relative neutrophilia, and relative lymphopenia.

Medical history and examination
MRI of the spine demonstrated the presence of suspected intradural collections in the lumbosacral region at the L2-S2 level with low values of ADC, intense peripheral enhancement after gadolinium injection, and internal colliquative appearance.There was a pathological dural enhancement extended cranially up to T9-T10.In addition, there was a focal inflammatory/infective spondylitis involving the left facet joint at the level L2-L3 ( Fig. 1 ).
During surgery, a durotomy was therefore necessary, confirming the radiological suspicion of the intradural location of the collections.

Pathological findings and postoperative course
Postoperatively, the patient received empirical intravenous antibiotic treatment with vancomycin, ceftriaxone, and metronidazole, awaiting cultures.Follow-up MRI performed 2 days after the surgery showed markedly decreased abscess size.
The biological examination of the purulent sample did not show any micro-organism growth neither blood cultures were positive.
The last follow-up MRI (2 months later) showed the disappearance of lumbar intradural collections, with the presence of arachnoiditis as a sequela.

Case 2
A 70-year-old male patient with a history of a kidney tumor and brain meningiomatosis was treated with surgeries and stereotactic radiation therapy.During recovery due to a cerebro-spinal leak from the surgical site, a lumbar drain was positioned.Sudden onset of fever and increased inflammatory markers (elevated values of C-reactive protein and PCR) appeared and follow-up MR images of the spine demonstrated the presence of an intradural collection in the lumbosacral region (L3-L4 level), with marked restricted diffusion on DWI and mild postcontrastographic enhancement; in addition, there were signs of arachnoiditis affecting the roots of the cauda which were particularly glued together, in an antigravity position ( Fig. 2 ).The patient initiated i.v.meropenem and fosfomycin after lumbar puncture tested positive for Pseudomonas aeruginosa .
He has not undergone new surgical procedures in both the cranial and spinal areas due to his poor clinical conditions.

Discussion
Intradural spinal abscesses are extremely rare in comparison to epidural ones [3] .Most cases of intradural abscesses reported in the literature are hematogenous in origin and are more rarely associated with epidural injections and spondylodiscitis [ 4 ,5 ] ( Table 1 ).This can be partially attributed to the epidural space acting as a filter.This condition is more common in older males (over the fifth decade of life), with the lumbar as the most common site of involvement [2] .
In both our cases, MRI images led us to suspect that the localization of the infectious collections was inside the intradural space.Indeed, the dural profile appeared as a continuous ipo-intense line (0.5-1 mm) delimiting the epidural fat, on the external side, from the abscess collections, on the internal side.A pial involvement was present due to the residual adhesiveness of caudal roots with antigravity position appreciable at MRI images in both cases.
In case 1, the intradural localization of the abscesses, confirmed by surgery, was probably related to spondylitis phenomena and a secondary dural breach at the left L2-L3 in-terfacetal joint.As described in the literature it is possible that transient bacteremia from a self-limited infection site may have been the underlying cause of interfacetal inflammation [6] .As reported on spinal epidural abscesses, Staphylococcus aureus is the most predominant pathogen isolated [7] .In our patient, despite abundant purulent samples being collected, a negative bacterial culture was obtained.An increase of culture-negative patients is reported in clinical practice [8] and an empirical use of vancomycin represents an appropriate therapy in these cases [8] , as in our patient.
In case 2 due to the unstable clinical condition of the patient, who was bedridden, surgery was not recommended and antibiotic treatment alone was initiated.Follow-up MRI images showed a slight reduction in the size of the lumbar intradural collection.

Conclusions
Intradural abscess is a rare and potentially dangerous condition.Despite its rarity, a high level of suspicion must alert the radiologist when dealing with a patient with intense back pain, neurological symptoms, and increased inflammatory markers.Generally, if promptly diagnosed, surgical drainage, combined with antibiotic treatment, results in a favorable outcome.

AFig. 1 -
Fig. 1 -Magnetic resonance imaging of the lumbar spine.Sagittal T2 weighted MRI image (A) reveals 2 suspected intradural collections localized ventrally and dorsally to caudal roots, with hypointense margins and an internal inhomogeneous hyperintense component.Postcontrast sagittal (B) and axial (C) T1 weighted image showing intense peripheral postcontrast enhancement and internal colliquative appearance of the suspected intradural collections at L2-S2 level.Axial DWI(E)/ADC(F) demonstrating restriction within the mass.Axial (G) STIR sequence revealing pathological hypersignal of the left facet joint at level L2-L3 extended to the lamina and spinous process of L2 as well as to the musculature of the contiguous paravertebral lodge.Intraoperative photographs (D-H), (D) A L5-S1 laminectomy was performed.The epidural space and the dural surface appeared undamaged, proving the absence of epidural collections.A vertical durotomy was performed, which showed an intact fibrous capsule.(H) Yellowish purulent material oozing out after incision of the fibrous capsule, confirming the intradural location of the abscess.

Fig. 2 -
Fig. 2 -Magnetic resonance imaging of the lumbar spine.Postcontrast sagittal (A) and axial (F) T1 weighted image showing mild post-contrastographic enhancement of a suspected intradural collection in the lumbosacral region (L3-L4 level); Sagittal (C) and axial (G) DWI showed marked restricted diffusion with marked hyposignal on ADC (sagittal D and axial H); in addition, there were signs of arachnoiditis affecting the roots of the cauda which were particularly glued together, in an antigravity position, best appreciated on sagittal STIR (B) and T2 axial image (E).