Neurosurgery notes/Infection/Spinal infection/Spinal epidural abscess (SEA)

Spinal epidural abscess (SEA)

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Spinal infection

Epidemiology

  • Incidence: 0.2–1.2 per 10,000 hospital admissions annually, possibly on the rise.
  • Average age: 57.5 ± 16.6 years.
  • Spinal level
    • Thoracic (≈ 50%)
    • Lumbar (35%)
    • Cervical (15%)
  • SEA may span from 1 to 13 levels
  • Location
    • 82% posterior to the cord,
    • 18% anterior
  • Often associated with
    • Vertebral osteomyelitis
      • All cases of anterior SEA,
      • 85% of circumferential SEA,
      • No cases of posterior SEA)
    • Intervertebral discitis.

Co-morbid conditions

  • Chronic diseases associated with compromised immunity were identified in 65%
    • Diabetes mellitus (32%),
    • IV drug abuse (18%),
    • Chronic renal failure (12%),
    • Alcoholism (10%),
    • Rarer
      • Cancer,
      • Recurrent UTI,
      • Pott’s disease,
      • Positivity for HIV.
  • Iatriogenic
    • Chronic steroid use
    • Recent spinal procedure
  • Trauma (e.g. GSW)
  • Skin infection (e.g. furuncle).

Clinical features

  • Pain
    • Excruciating pain localized over spine
    • Tenderness to percussion.
  • Radicular symptoms
    • Radicular symptoms follow with subsequent distal cord findings,
    • Sequence
      • Bowel/bladder disturbance → abdominal distension → weakness progressing → para- and quadriplegia.
    • Average time is
      • 3 days from back pain to root symptoms;
      • 4.5 days from root pain to weakness;
      • 24 hrs from weakness to paraplegia.
  • Fever, sweats or rigors are common,
    • But are not always present.
  • A furuncle (skin boil) somewhere on the body may be identified in 15%.
  • Encephalopathic.
    • This may range from mild to severe and may further delay diagnosis.
  • Meningismus with a positive Kernig sign
  • Patients with postoperative SEA may demonstrate surprisingly few signs or symptoms (including lack of leukocytosis, lack of fever) aside from local pain.

Pathophysiology of spinal cord dysfunction

  • Mechanical compression
    • Due to
      • Vertebral body collapse compression
      • Abscess compression
    • Not always found
  • Vascular mechanism
    • Arterial and venous pathology have been described (one autopsy series showed little arterial compromise, but did show venous compression and thrombosis, thrombophlebitis of epidural veins, and venous infarction and oedema of the spinal cord).
  • Infection of the spinal cord itself,
    • By extension through the meninges.

Source site of infection

  • Hematogenous spread
    • Most common source (26–50% of cases)
    • Either to the epidural space or to the vertebra with extension to epidural space.
    • Reported foci include
      • Skin infections (most common): furuncle may be found in 15% of cases
      • Parenteral injections, especially with IV drug abuse
      • Bacterial endocarditis
      • UTI
      • Respiratory infection (including otitis media, sinusitis, or pneumonia)
      • Pharyngeal or dental abscess
  • Direct extension from
    • Decubitus ulcer
    • Psoas abscess
    • Penetrating trauma, including: abdominal wounds, neck wounds, GSW
    • Pharyngeal infections
    • Mediastinitis
    • Pyelonephritis with perinephric abscess
    • Dermal sinus
  • Following spinal procedures
    • 3 of 8 of these patients had readily identified perioperative infections of periodonta, UTI, or AV-fistula
    • Open procedures: especially lumbar discectomy (incidence ≈ 0.67%)
    • Closed procedures
      • Epidural catheter insertion for spinal epidural anesthesia
      • Lumbar puncture
  • A history of recent back trauma is common (in up to 30%)
  • No source can be identified in up to 50% of patients in some series

Organisms

  • Operative cultures are most useful in identifying the responsible organism,
    • These cultures may be negative (possibly more common in patients previously on antibiotics) and in these cases blood cultures may be positive.
  • No organism may be identified in 29–50% of cases.
  • Types
    • Staph. aureus
      • Most common organism (cultured in > 50%) possibly due to its propensity to form abscesses, its ubiquity, and its ability to infect normal and immunocompromised hosts (these facts help explain why many SEA arise from skin foci)
    • Aerobic & anaerobic streptococcus: second most common
    • E.coli
    • Pseudomonas aeruginosa
    • Diplococcus pneumoniae
    • Serratia marcescens
    • Enterobacter
    • Chronic infections
      • TB
        • Most common of these, and although it has become less widespread in the U.S. it is still responsible for 25% of cases of SEA,
        • It is usually associated with vertebral osteomyelitis, see Pott’s disease
      • Brucellosis
      • Fungal
        • Cryptococcosis
        • Aspergillosis
      • Parasitic: Echinococcus
    • Multiple organisms in ≈ 10%
    • Anaerobes cultured in ≈ 8%

Laboratory tests

  • CBC
    • Leukocytosis common in acute group (average WBC=16,700/mm3),
    • Normal in chronic (ave. WBC=9,800/mm3).
  • ESR elevated in most, usually > 30 CRP.
  • LP
    • Performed cautiously in suspected cases at a level distant to the clinically suspected site
    • Constant aspiration while approaching thecal sac to detect pus (danger of transmitting infection to subarachnoid space);
      • If pus is encountered, stop advancing, send the fluid for culture, and abort the procedure.
    • CSF protein & WBC usually elevated;
    • CSF glucose normal (indicative of parameningeal infection).
    • 5 of 19 cases grew organisms identical to abscess.
    • (C1–2 puncture may be needed to do myelogram)
  • Blood cultures: may be helpful in identifying organism in some cases.
  • Anergy battery: (e.g. mumps and Candida) to assess immune system.

Radiographic studies

Plain films

  • Usually normal
    • Unless there is osteomyelitis of adjacent vertebral bodies (more common in infections anterior to dura).
  • Look for lytic lesions, demineralization, and scalloping of endplates
    • May take 4–6 weeks after onset of infection

MRI

  • Imaging study of choice.
  • Differentiates other conditions (especially transverse myelitis or spinal cord infarction) better than myelo/CT
  • Typical findings
    • T1WI
      • Hypo- or isointense epidural mass, vertebral osteomyelitis shows up as reduced signal in bone.
    • T2WI
      • High intensity epidural mass
    • Enhances with gadolinium
      • 3 patterns of enhancement
        • Dense homogeneous
        • Inhomogeneous with scattered areas of sparse or no uptake
        • Thin peripheral enhancement
      • But may show minimal enhancement in the acute stage when comprised primarily of pus with little granulation tissue.
      • Unenhanced MRI may miss some SEA, gadopentetate dimeglumine enhancement may slightly increase sensitivity
    • Vertebral osteomyelitis shows up as increased signal in bone, associated discitis produces increased signal in disc and loss of intranuclear cleft.

Myelogram-CT

  • Usually shows findings of extradural compression
    • (e.g. “paintbrush appearance” when complete block is present).
  • In the event of complete block, C1–2 puncture may be needed to delineate upper extent (unless post-myelographic CT shows dye above the lesion).

CT scan

  • Intraspinal gas has been described on plain CT
  • Post-myelographic CT is more sensitive.

Treatment

General information

  • Controversial: Surgery + antibiotics VS antibiotics alone
    • In most cases, treatment consists of early surgical evacuation combined with antibiotics as the treatment of choice.
    • Argument for surgery + antibiotics: Although there are reports of management with antibiotics alone ± immobilization, rapid and irreversible deterioration has occurred even in patients treated with appropriate antibiotics who were initially neurologically intact.
      • 86% of those who deteriorated were initially treated with antibiotics alone.
    • Argument for antibiotics alone
      • At the time of surgery, instead of a true abscess, inflammatory tissue that is not easily or effectively debrided is encountered.

Nonsurgical management reserved for the following patients

  • Indicated
    • Those with prohibitive operative risk factors
    • Involvement of an extensive length of the spinal canal
    • Complete paralysis for > 3 days
    • Asymptomatic
      • Can trial abx IV first but need close monitoring
  • Antibiotics
    • S. Aureus or unknown
      • Empiric antibiotics
        • Ceftriaxone or cefepime (use when pseudomonas is a concern) PLUS
        • Metronidazole PLUS
        • Vancomycin
          • Until methicillin resistant S. aureus (MRSA) can be ruled out
          • Once MRSA is ruled out switch to synthetic penicillin (e.g. nafcillin or oxacillin)
        • ± Rifampin PO
    • Modify antibiotics based on culture results or knowledge of source (e.g. IV drug abusers have a higher incidence of Gram-negative organisms).
    • Duration of treatment
      • Minimum of 6 weeks.
        • Longer therapy may be warranted in complicated infections and for patients who have spinal implants or hardware.
        • Immobilization for at least 6 weeks during antibiotic therapy is recommended.

Surgery

  • Indication for surgery
    • A symptomatic epidural abscess
      • Considered a medical and surgical emergency.
    • Establishing diagnosis
    • Causative organism
    • Drainage of pus
    • Debridement of granulation tissue
    • Bony stabilization if necessary.
      • If there is osteomyelitis of vertebral body with bony destruction that can cause instability after laminectomy
  • Post SEA: SEA post to dura
    • Most SEA are posterior to the dura
    • Options
      • Extensive multiple-level laminotomies or laminectomy, taking care to preserve the facet joints.
      • Debridement of the spinal canal through fenestrations removing the ligamentum flavum and portions of adjacent lamina, and use of catheters can be considered.
    • For posteriorly located SEA and no evidence of vertebral osteomyelitis, instability will usually not follow simple laminectomy and appropriate postoperative antibiotics.
    • Thorough antibiotic irrigation is employed intraoperatively.
    • Primary closure is often employed.
    • Post-op drainage is not necessary in cases with only granulation tissue and no pus.
  • Anterior SEA
    • Usually has associated osteomyelitis (especially Pott’s disease)
    • Options
      • ACDF/Corpectomy
        • Titanium cage or no cage
    • Posterolateral extracavitary approach
      • To avoid transabdominal or transthoracic approach in these debilitated patients
      • Removal of devitalized bone usually followed by posterior instrumentation and fusion.
      • Strut grafting with autologous bone (rib or fibula) can be done acutely in Pott’s disease with little risk of graft infection.
      • Purulent osteomyelitis
        • Metal hardware is not contraindicated
          • Titanium is more resistant to harbouring bacteria than stainless steel for several reasons:
            • Titanium does not permit bacteria to form a glycocalyx on its surface
        • Grafting may run the risk of perpetuating the infection.
          • Can use beads of calcium sulfate bone void filler impregnated with antibiotic (e.g. Stimulan® Rapid Cure™ antibiotic beads).
  • Both ant and post SEA
    • If an abscess involves both the anterior and posterior epidural space, an anterior and posterior approach combined with spinal stabilization using posterior instrumentation is considered.
  • Recurrent infections
    • Reoperation and post-op suction-irrigation may be needed.

Outcome

  • Fatal in 4–31% (12%)
    • Higher for
      • Older patients
      • Patient paralyzed before surgery
  • Rare for neurological improvement even with surgical intervention within 6–12 hrs of onset of paralysis,
  • Reversal of paralysis of caudal spinal cord segments if present for more than a few hours is rare
    • (Exception: Pott’s disease has 50% return).
  • Mortality due to
    • Original focus of infection or
    • A complication of residual paraplegia (e.g. pulmonary embolism).

Differential diagnosis

  • Meningitis
  • Acute transverse myelitis (paralysis is usually more rapid, radiographic studies are normal)
  • Intervertebral disc herniation
  • Spinal cord tumors
  • Post-op SEA may appear similar to pseudomeningocele