Acute Haematogenous Osteomyelitis

Acute haematogenous osteomyelitis (AHO) is a blood-borne pyogenic infection of bone, classically of the metaphysis of long bones in children. It is an orthopaedic emergency: delayed treatment converts an easily curable infection into chronic osteomyelitis with sequestrum, deformity and lifelong morbidity. This is among the highest-yield infection topics for NEET PG.

Definition and classification

Osteomyelitis = inflammation of bone and marrow caused by an infecting organism. "Haematogenous" denotes spread through the bloodstream from a distant focus (skin pustule, infected umbilicus, URTI, otitis media, dental sepsis), as opposed to direct/contiguous spread (open fracture, surgery, diabetic foot) or vascular insufficiency.

By duration / Waldvogel classification:

Type	Onset	Hallmark
Acute	< 2 weeks	Pus, systemic sepsis, normal radiograph early
Subacute	2 weeks–3 months	Brodie's abscess, insidious, minimal systemic upset
Chronic	> 3 months	Sequestrum, involucrum, sinus, recurrence

Cierny–Mader (anatomical/host) classification is used mainly for chronic osteomyelitis and surgical planning:

• Stage 1 Medullary, Stage 2 Superficial, Stage 3 Localised, Stage 4 Diffuse.
• Host: A (normal), B (compromised – local/systemic), C (treatment worse than disease).

High-yield: AHO is predominantly a disease of children (boys > girls, ~3:1); in adults it is uncommon and tends to involve the vertebrae rather than long-bone metaphyses.

Etiology — organisms by age and host

Patient group	Most common organism(s)
All ages (overall commonest)	Staphylococcus aureus
Neonates	Staph aureus, Group B Streptococcus, E. coli
Children 1–4 yr (unvaccinated)	Haemophilus influenzae type b (now rare post-Hib vaccine)
Sickle cell disease	Salmonella (then Staph aureus)
IV drug users / spine	Pseudomonas aeruginosa, Staph aureus
Puncture wound through shoe sole	Pseudomonas aeruginosa
Prosthetic joints / implants	Staphylococcus epidermidis (coagulase-negative)
Immunocompromised / chronic	TB (Mycobacterium tuberculosis), fungi

High-yield: Two of the most repeated NEET PG facts — Salmonella in sickle cell osteomyelitis and Pseudomonas with calcaneal osteomyelitis after a nail prick through a shoe. Note: even in sickle cell disease, Staph aureus remains the single commonest organism overall, but Salmonella is disproportionately frequent.

Pathophysiology — why the metaphysis?

The seeding site is dictated by the unique vascular anatomy of the growing metaphysis:

1. Metaphyseal capillaries are non-anastomosing terminal loops of the nutrient artery.
2. Blood flow is slow and turbulent in venous sinusoids, where it makes hairpin turns → stasis → ideal nidus for bacterial lodgement.
3. These vessels lack a reticulo-endothelial lining / are relatively deficient in phagocytes, so bacteria proliferate unchecked.

Trueta's pattern by age (very high-yield):

Age	Vascular situation	Spread pattern
Infant (< 1 yr)	Vessels cross the physis into epiphysis	Septic arthritis common; epiphysis & joint involved
Child (1 yr–puberty)	Physis is an avascular barrier	Infection confined to metaphysis; subperiosteal abscess
Adult (physis fused)	Vessels again cross fused plate	Can reach subchondral bone & joint

Sequence of events (flow):

Bacterial lodgement in metaphysis → acute inflammation + oedema in rigid bone → rising intramedullary pressure → vascular thrombosis & ischaemia → pus formation (48–72 h) → pus tracks via Volkmann's canals to subperiosteal space → strips periosteum → cuts off cortical blood supply → bone death = sequestrum → periosteum lays down new bone shell = involucrum → pus drains through holes (cloacae) and skin sinuses → chronic osteomyelitis.

High-yield: Where the metaphysis is intracapsular (proximal femur, proximal humerus, radial neck, distal lateral tibia), subperiosteal pus ruptures directly into the joint → secondary septic arthritis. The hip is the classic example (proximal femoral metaphysis lies inside the capsule).

High-yield: Sequestrum = dead bone, radiologically dense (sclerotic and whiter) because it is avascular and cannot be remodelled/decalcified. Involucrum = living new bone laid down by periosteum around it.

Clinical features

Long bones most affected (in order): distal femur and proximal tibia (around the knee — fastest-growing physes) > proximal femur, proximal humerus.

• Severe constant pain near a joint, the child refuses to move the limb (pseudoparalysis) and refuses to bear weight.
• High fever, malaise, toxaemia, tachycardia; the child looks ill.
• Exquisite, well-localised metaphyseal tenderness is the single most important early sign.
• Warmth, swelling and overlying erythema appear later (after 2–3 days, once pus is subperiosteal).
• Sympathetic effusion of the adjacent joint may be present (sterile, unless true septic arthritis).

Neonatal AHO is deceptive — minimal systemic signs, the only clue may be pseudoparalysis of a limb, irritability on handling, or refusal to feed. High index of suspicion is essential; multifocal involvement is common.

Diagnosis and investigations

Blood / inflammatory markers

• CRP rises within 6–12 h, peaks at ~48 h, normalises in ~1 week with treatment → best for early diagnosis and monitoring response.
• ESR rises in 24–48 h, peaks day 3–5, normalises in 3–4 weeks → slower, lags behind CRP.
• Leucocytosis with neutrophilia (may be normal in neonates/early disease).
• Blood culture is positive in ~50–60% and should be taken before antibiotics.

High-yield: CRP is the most useful marker to monitor treatment response (falls fastest); a persistently high or re-rising CRP suggests inadequate drainage or abscess. Procalcitonin is more specific for bacterial sepsis but less sensitive than CRP.

Imaging — know the timeline cold:

Modality	Earliest change	Notes
Plain X-ray	Soft-tissue swelling at 3–5 days; bony changes (osteopenia, periosteal reaction) only at 10–14 days (need ~30–50% bone loss)	Normal X-ray does NOT exclude AHO
MRI	Earliest detector of bone involvement (within 24–48 h) — marrow oedema (↓T1, ↑T2/STIR)	Investigation of choice; shows abscess, soft tissue
Radionuclide (Tc-99m MDP / bone scan)	Increased uptake 24–48 h	Useful when multifocal or site unclear; less specific
Ultrasound	Subperiosteal collection/abscess	Good in neonates/infants; guides aspiration

High-yield: X-ray changes LAG by 10–14 days — this is one of the most repeated single-best-answer points. MRI is the earliest and most sensitive imaging investigation.

Confirmatory / definitive: Aspiration of the metaphysis (subperiosteal then intramedullary) with Gram stain and culture is the gold standard for organism identification. Aspiration of frank pus is also the key indication that surgical drainage is required.

Diagnostic approach (stepwise):

Clinical suspicion (febrile child, refuses to use limb, point metaphyseal tenderness) → blood culture + CRP/ESR/CBC → X-ray (often normal early, but excludes fracture/tumour) → MRI (confirm + map abscess) → diagnostic aspiration for Gram stain & culture → start empirical IV antibiotics.

Management

Principles (Nade's rules of treatment):

1. Appropriate antibiotic is effective before pus forms.
2. Antibiotics do not sterilise avascular tissue / abscesses — these need surgery.
3. Surgery that drains pus prevents bone death.
4. Continue antibiotics after surgery.
5. Restore vascularity.

Empirical IV antibiotics are started immediately after cultures, then tailored:

• Standard empirical cover (child): IV cloxacillin/flucloxacillin (anti-staphylococcal) + a 3rd-gen cephalosporin (e.g. cefotaxime) to cover Strep/Haemophilus.
• MRSA suspected/high prevalence: Vancomycin (or clindamycin/linezolid).
• Sickle cell disease: add cover for Salmonella (e.g. a fluoroquinolone or 3rd-gen cephalosporin).
• Neonate: anti-staphylococcal + aminoglycoside/cephalosporin (cover GBS, Gram-negatives).

Duration: classically IV until clinical improvement and CRP normalising (typically 1–2 weeks), then oral to complete 4–6 weeks total. Modern protocols favour earlier IV-to-oral switch once afebrile and CRP falling, provided a reliable oral agent and compliance.

High-yield: The empirical antibiotic in a healthy child must always cover Staph aureus (the commonest organism), because it dictates the choice in the absence of culture.

Supportive: analgesia, IV fluids, splintage of the limb (rest, comfort, prevents pathological fracture and contracture).

Indications for surgical drainage / decompression:

• Frank pus on aspiration (or abscess on imaging).
• No clinical improvement within 24–48 h of adequate IV antibiotics.
• Presence of a subperiosteal or soft-tissue abscess.
• Drainage = incision, cortical drill holes / windowing to decompress the medulla, evacuate pus, lavage; limb immobilised afterwards.

High-yield: AHO caught before pus forms is curable with antibiotics alone; once an abscess is present, surgical drainage is mandatory — antibiotics cannot penetrate dead/avascular bone or pus.

Subacute and chronic forms — exam favourites

Brodie's abscess (subacute osteomyelitis):

• A localised, walled-off chronic metaphyseal bone abscess (often upper tibial metaphysis), usually Staph aureus.
• Insidious pain, little/no systemic upset, low-grade markers.
• X-ray: well-defined lytic cavity surrounded by a rim of reactive sclerosis ("halo"/"penumbra sign" on MRI).
• Treatment: antibiotics ± curettage/evacuation.

Garré's sclerosing osteomyelitis: non-suppurative, markedly sclerotic, thickened cortex; chronic low-grade; dense bone with no obvious cavity.

Chronic osteomyelitis hallmarks (mnemonic SICCD):

• Sequestrum (dead avascular dense bone)
• Involucrum (new living bone shell)
• Cloaca (opening in involucrum through which pus drains)
• Chronic discharging sinus
• Deformity / pathological fracture

Treatment of chronic OM = thorough surgical debridement/sequestrectomy (remove the sequestrum, the nidus that antibiotics cannot reach) + prolonged culture-directed antibiotics ± dead-space management (antibiotic-impregnated beads, bone grafting, Ilizarov bone transport).

High-yield: The sequestrum is the source of persistent/recurrent infection and must be removed surgically — chronic OM cannot be cured by antibiotics alone. Long-standing draining sinuses can rarely undergo malignant change → Marjolin's ulcer (squamous cell carcinoma).

Complications

• Septic arthritis (esp. intracapsular metaphyses, infants).
• Chronic osteomyelitis with recurrence (commonest sequel of delayed/inadequate treatment).
• Pathological fracture through weakened bone.
• Growth disturbance: physeal damage → limb-length discrepancy, angular deformity.
• Metastatic infection / septicaemia, septic shock, DVT (especially CA-MRSA, PVL-positive strains → severe disease, multifocal, deep vein thrombosis, septic emboli).
• Amyloidosis (long-standing chronic suppuration).
• Marjolin's ulcer in a chronic sinus tract.

Key differentials

Condition	Distinguishing pointer
Septic arthritis	Pain on any joint movement, joint held flexed, effusion; vs OM where joint is relatively spared early. Both can coexist.
Acute rheumatic fever	Flitting polyarthritis, responds dramatically to salicylates, carditis, ASO titre
Ewing's sarcoma	Can mimic OM (fever, ↑ESR, lamellated "onion-peel" periosteal reaction, diaphyseal); biopsy decisive
Acute leukaemia	Bone pain, anaemia, thrombocytopenia, blasts on smear
Cellulitis	Superficial, diffuse skin erythema/oedema, no deep point bony tenderness, normal marrow on MRI
Trauma / fracture	History, X-ray; absence of systemic sepsis
Sickle cell vaso-occlusive crisis	Mimics Salmonella OM; both common in SCD — crisis is usually multifocal/symmetrical, OM more localised with persistent fever

High-yield: Differentiating vaso-occlusive bone infarct vs osteomyelitis in sickle cell disease is clinically difficult and a classic exam vignette; persistent fever, focal worsening, and positive cultures favour infection (Salmonella).

Recently asked / exam angle

• "Earliest investigation to detect osteomyelitis" → MRI (not X-ray).
• "When do X-ray changes appear in acute osteomyelitis?" → 10–14 days (soft-tissue swelling earlier, ~3–5 days).
• "Commonest organism in acute osteomyelitis" → Staphylococcus aureus (all ages).
• "Organism in sickle cell osteomyelitis" → Salmonella (Staph aureus still commonest overall).
• "Osteomyelitis after nail-prick through shoe" → Pseudomonas aeruginosa.
• "Commonest site of AHO in children" → metaphysis around the knee (distal femur / proximal tibia).
• "Best marker to monitor response/treatment" → CRP.
• "Why metaphysis affected" → slow turbulent flow in venous sinusoids of non-anastomosing terminal capillary loops.
• "Sequestrum vs involucrum" → dead dense avascular bone vs living new periosteal bone.
• "Brodie's abscess site/appearance" → upper tibial metaphysis, lytic cavity with sclerotic rim.
• "Indication for surgery in AHO" → pus on aspiration or no response to IV antibiotics in 24–48 h.
• Image-based: lytic cavity with sclerotic margin = Brodie's abscess; onion-peel periosteum = Ewing's (differential).

Rapid revision

1. AHO = blood-borne infection of the metaphysis in children; Staphylococcus aureus is the commonest organism at all ages.
2. Metaphyseal predilection is due to slow, turbulent blood flow in non-anastomosing venous sinusoids.
3. Trueta: infants → vessels cross physis → septic arthritis; children → physis is a barrier → metaphyseal abscess; adults → vertebral involvement.
4. CRP rises first (6–12 h) and falls fastest → best for early diagnosis and monitoring; ESR is slower.
5. Plain X-ray changes lag 10–14 days; soft-tissue swelling visible by 3–5 days.
6. MRI is the earliest and most sensitive imaging modality (marrow oedema within 24–48 h).
7. Aspiration with Gram stain & culture is the definitive diagnostic step and pus indicates surgery.
8. Empirical treatment = IV anti-staphylococcal cover immediately after cultures; add Salmonella cover in sickle cell disease, Pseudomonas cover for shoe-puncture wounds.
9. Surgery (drainage/decompression) is indicated for frank pus or failure to improve within 24–48 h of IV antibiotics.
10. Total antibiotic course is typically 4–6 weeks (IV then oral once CRP falling).
11. Sequestrum (dead, dense, avascular bone) is the nidus of chronic OM and must be excised; involucrum is the new living bone shell.
12. Chronic OM features = sequestrum, involucrum, cloaca, sinus; long-standing sinus → Marjolin's ulcer (SCC); Brodie's abscess is the subacute form (lytic cavity + sclerotic rim, upper tibia).