Paraspinal abscesses are collections of purulent material within the paraspinal musculature, distinct from epidural abscesses. They require high clinical suspicion, prompt imaging diagnosis, and appropriate antimicrobial therapy with or without surgical drainage. The condition can be primary (hematogenous) or secondary (direct extension from vertebral osteomyelitis, discitis, or epidural abscess).

Key Recognition Features:

• Back pain with fever and elevated inflammatory markers
• Paravertebral muscle swelling and tenderness
• MRI showing rim-enhancing fluid collection in paraspinal muscles
• May coexist with vertebral osteomyelitis or epidural abscess

Critical Management Pathway:

1. Blood cultures and inflammatory markers (CRP, ESR, WCC)
2. Urgent contrast-enhanced MRI whole spine
3. CT-guided aspiration for culture and sensitivities
4. Empiric broad-spectrum antibiotics (adjust per culture)
5. Surgical drainage if failed medical therapy or neurological deficit

High-Yield Exam Points:

• Staphylococcus aureus most common organism (60-70% cases)
• MRI with gadolinium contrast is gold standard imaging
• Indications for surgery: failed medical therapy, neurological deficit, large abscess greater than 3 cm, coexisting epidural abscess requiring decompression
• Duration of antibiotics typically 6-12 weeks IV then oral

Paraspinal Muscle Anatomy

The paraspinal compartment contains multiple muscle layers susceptible to abscess formation:

Superficial Layer:

• Trapezius and latissimus dorsi
• Relatively resistant to deep infection

Intermediate Layer:

• Erector spinae group (iliocostalis, longissimus, spinalis)
• Most common site for abscess formation
• Supplied by posterior branches of spinal arteries

Deep Layer:

• Multifidus, rotatores, interspinales, intertransversarii
• Close proximity to neural foramina and epidural space
• Direct communication pathway for infection spread

Fascial Compartments:

• Thoracolumbar fascia creates potential spaces
• Infection can track longitudinally within fascial planes
• May extend from cervical to lumbar regions

Pathogenesis Mechanisms

Paraspinal abscesses are purulent collections within paraspinal musculature, most commonly from direct extension from vertebral osteomyelitis/discitis (50-60%) or haematogenous seeding (30-40%). Staphylococcus aureus is the causative organism in 60-70% of cases (MRSA increasing). MRI with gadolinium is the gold standard, showing rim-enhancing fluid collection. Initial management includes blood cultures, CT-guided aspiration for culture, and empiric broad-spectrum antibiotics (6-12 weeks IV then oral). Surgical drainage is indicated for failed medical therapy, neurological deficit, abscess greater than 3cm, or coexisting epidural abscess requiring decompression.

Hematogenous Seeding:

• Accounts for 30-40% of primary paraspinal abscesses
• Bacteremia leads to muscle microabscess formation
• Rich vascular supply facilitates bacterial deposition
• Common sources: skin infections, endocarditis, IV drug use

Direct Extension:

• Most common pathway (50-60% of cases)
• Vertebral osteomyelitis with posterior extension
• Discitis with paravertebral phlegmon progression
• Epidural abscess breakthrough via neural foramina

Iatrogenic Causes:

• Facet joint injections
• Medial branch blocks
• Trigger point injections
• Spine surgery with inadequate debridement

Microbiology

Staphylococcus aureus (60-70%):

• Most common pathogen overall
• MRSA increasing in prevalence (20-40% of S. aureus)
• Forms biofilm and tissue destruction
• Toxin production enhances virulence

Mycobacterium tuberculosis (10-20%):

• Higher prevalence in endemic regions
• Cold abscess formation (less acute inflammation)
• Granulomatous inflammation
• Requires extended treatment duration (9-12 months)

Gram-negative organisms (10-15%):

• E. coli, Pseudomonas, Klebsiella
• Associated with urinary source
• Elderly and immunocompromised patients
• Often polymicrobial

Streptococcus species (5-10%):

• Group A and B Streptococcus
• May follow pharyngitis or skin infection
• Generally more antibiotic sensitive

Anaerobes (rare):

• Bacteroides, Peptostreptococcus
• Associated with GI source or polymicrobial infection
• Foul-smelling purulent material

Classic Presentation Triad

Back Pain (98% of cases):

• Localized to affected spinal level
• Constant, progressive intensity
• Worse with movement and axial loading
• Night pain disrupting sleep
• May radiate to flank or lower limbs

Fever (70-85% of cases):

• Temperature greater than 38.0 degrees Celsius
• May be absent in immunocompromised
• Rigors suggest bacteremia
• Low-grade fever common in chronic abscess

Inflammatory Response (95% of cases):

• Elevated CRP (often greater than 100 mg/L)
• Elevated ESR (often greater than 50 mm/hr)
• Leukocytosis (WCC greater than 11,000/µL)
• Procalcitonin elevated in bacterial infection

Physical Examination Findings

Inspection:

• Paravertebral muscle swelling or fullness
• Erythema over affected area (variable)
• Loss of normal spinal contour
• Antalgic posture or gait
• Surgical scars (if post-operative)

Palpation:

• Exquisite tenderness over paraspinal muscles
• Palpable fluctuance (large superficial abscess)
• Warmth compared to contralateral side
• Muscle spasm and guarding
• Percussion tenderness over spinous processes

Range of Motion:

• Restricted spinal flexion, extension, and rotation
• Patient resists active and passive movement
• Splinting and protective muscle spasm

Neurological Assessment:

• Usually neurologically intact (key distinction from epidural abscess)
• May have radicular pain if foraminal compression
• Lower limb weakness suggests epidural extension
• Sphincter dysfunction rare unless epidural involvement

Red Flag Features

Indicating Epidural Extension:

• Progressive neurological deficit
• Bilateral radicular symptoms
• Bladder or bowel dysfunction
• Saddle anaesthesia
• Upper motor neuron signs

Indicating Sepsis:

• Hemodynamic instability
• Tachycardia greater than 100 bpm
• Hypotension (systolic less than 90 mmHg)
• Altered mental status
• Oliguria

Indicating Vertebral Osteomyelitis:

• Severe vertebral body tenderness
• Kyphotic deformity
• Mechanical instability pain pattern

Laboratory Studies

Imaging Studies

Plain Radiographs (limited utility):

• May show paravertebral soft tissue shadow
• Vertebral endplate erosion if osteomyelitis present
• Loss of disc height in discitis
• Useful to exclude fracture or deformity
• Not adequate to diagnose or exclude abscess

MRI with Gadolinium Contrast (Gold Standard):

• Sensitivity 95%, specificity 92% for paraspinal abscess
• T1-weighted: hypointense to muscle
• T2-weighted: hyperintense fluid signal
• Post-contrast T1 with fat saturation: rim enhancement
• STIR sequences: high signal inflammation and edema

MRI Key Diagnostic Features:

• Well-defined fluid collection within paraspinal muscles
• Peripheral rim enhancement (abscess capsule)
• Central non-enhancing purulent material
• Surrounding muscle edema and enhancement
• Assessment of epidural space for extension

CT with Contrast:

• Alternative if MRI contraindicated
• Hypodense fluid collection with rim enhancement
• Better definition of bony anatomy
• Useful for CT-guided aspiration planning
• Lower soft tissue resolution than MRI

Ultrasound:

• Operator-dependent modality
• Useful for superficial abscess detection
• Can guide needle aspiration
• Limited in obese patients or deep abscess
• Real-time imaging for drainage procedures

Tissue Diagnosis

CT-Guided Aspiration:

• Diagnostic yield 70-85% for organism identification
• Obtain before antibiotic initiation when possible
• Send for Gram stain, culture, AFB, fungal studies
• Minimum 5 mL purulent material preferred
• Simultaneous therapeutic drainage possible

Indications for Aspiration:

• Uncertain diagnosis
• Negative blood cultures
• Need for organism-specific therapy
• Large abscess amenable to drainage
• Treatment failure on empiric antibiotics

Open Surgical Biopsy:

• Reserved for failed percutaneous attempts
• When operative drainage planned
• Tissue sample superior to fluid aspirate
• Multiple samples from abscess wall and contents
• Fresh tissue for microbiological analysis

Other Differential Diagnoses:

Psoas Abscess:

• Located in iliopsoas muscle
• Hip flexion contracture and positive iliopsoas sign
• May extend to lumbar paraspinal region
• Often associated with vertebral osteomyelitis or GI source

Malignancy:

• Metastatic disease to paraspinal muscles
• Primary sarcoma (rare)
• Less acute presentation
• MRI shows mass effect without rim enhancement

Inflammatory Myositis:

• Autoimmune etiology
• Bilateral symmetric involvement
• Muscle enzyme elevation (CK, aldolase)
• Muscle biopsy shows inflammatory cells without infection

Conservative (Medical) Management

Empiric Antibiotic Regimens (Australian eTG):

Standard Regimen (no MRSA risk):

• Flucloxacillin 2 g IV 6-hourly PLUS
• Ceftriaxone 2 g IV daily OR gentamicin 4-7 mg/kg IV daily
• Covers MSSA, Streptococcus, Gram-negatives

MRSA Risk Factors Present:

• Vancomycin 25-30 mg/kg IV loading, then 15-20 mg/kg IV 12-hourly (adjust to trough) PLUS
• Ceftriaxone 2 g IV daily
• MRSA risk factors: previous MRSA, IV drug use, recent hospitalization, nursing home resident

Penicillin Allergy:

• Vancomycin (as above) PLUS
• Gentamicin 4-7 mg/kg IV daily OR ciprofloxacin 400 mg IV 12-hourly

Duration of Therapy:

• Total 6-12 weeks antibiotic therapy
• IV therapy: 4-6 weeks minimum
• Transition to oral when: afebrile 3-5 days, CRP decreasing, clinically improving
• Oral options: flucloxacillin 1 g 6-hourly, cephalexin 1 g 6-hourly, or per sensitivities

Indications for Surgical Intervention

Absolute Indications:

• Progressive neurological deficit
• Spinal cord compression (coexisting epidural abscess)
• Septic shock despite medical therapy
• Spinal instability requiring stabilization

Relative Indications:

• Abscess greater than 3 cm diameter
• Failed medical therapy (persistent fever, rising CRP after 3-5 days)
• Inability to obtain microbiological diagnosis
• Coexisting vertebral osteomyelitis requiring debridement
• Immunocompromised host with large abscess

Conservative Management Criteria:

• Abscess less than 3 cm
• No neurological deficit
• Hemodynamically stable
• Microbiological diagnosis obtained (blood culture or aspiration)
• Improving inflammatory markers on antibiotics

Surgical Techniques

Monitoring and Follow-Up

Inpatient Phase:

• Daily temperature and inflammatory markers
• CRP and ESR every 3-4 days
• Weekly full blood count
• Antibiotic levels (vancomycin, gentamicin)
• Neurological examination daily

Response to Treatment Criteria:

• Afebrile by day 5-7
• CRP declining (expect 50% reduction by week 2)
• Clinical improvement (pain reduction, mobility)
• No new neurological deficit

Outpatient Follow-Up:

• Weekly review first 4 weeks
• CRP and ESR monitoring
• Ensure antibiotic compliance
• Assess for recurrence or complications
• Repeat MRI at 6-8 weeks to confirm resolution

Early Complications (less than 4 weeks):

Sepsis and Septic Shock:

• Incidence: 10-15% of cases
• Risk factors: delayed diagnosis, MRSA, immunocompromise
• Management: ICU admission, vasopressor support, source control
• Mortality: 5-10% if septic shock develops

Epidural Abscess Extension:

• Occurs in 20-30% of paraspinal abscesses
• Presents with progressive neurological deficit
• Requires emergency surgical decompression
• Associated with worse neurological outcomes

Vertebral Osteomyelitis:

• Coexistent in 40-50% of cases
• May lead to vertebral collapse and instability
• Requires longer antibiotic duration (12 weeks)
• May need staged anterior reconstruction

Late Complications (greater than 4 weeks):

Recurrence:

• Incidence: 5-10% of treated cases
• Risk factors: inadequate drainage, short antibiotic course, immunosuppression
• Presents with recurrent pain and fever
• Requires repeat imaging and extended antibiotics or surgery

Chronic Pain:

• Incidence: 20-30%
• Mechanisms: muscle fibrosis, epidural scarring, facet joint arthropathy
• Management: multimodal analgesia, physiotherapy, pain clinic referral

Spinal Deformity:

• Kyphotic deformity if vertebral collapse
• Scoliosis from asymmetric muscle scarring
• May require corrective fusion surgery

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This comprehensive guide provides Gold Standard coverage of paraspinal abscess for FRACS examination preparation, emphasizing evidence-based diagnosis and management strategies with Australian clinical context.