Anatomical Aspects of Infections

Routes of Spread - Pathogen Parade

• Hematogenous: Via bloodstream (arteries, veins); distant spread. E.g., sepsis, osteomyelitis.
• Lymphatic: Via lymph channels to nodes; regional spread. E.g., TB, filariasis.
• Direct Extension: Invasion of adjacent tissues/organs. E.g., cellulitis, sinusitis to orbit.
• Along Fascial Planes: Tracks via low-resistance paths. E.g., Ludwig's angina, necrotizing fasciitis.
• Perineural: Spread along nerve sheaths. E.g., rabies, VZV, perineural tumor spread.
• Iatrogenic: From medical interventions. E.g., post-operative infections, contaminated devices.
• Transplacental (Vertical): Mother to fetus via placenta. E.g., TORCH infections.
• Canalicular: Along ducts/tubes. E.g., ascending UTI, cholangitis.

⭐ Batson's plexus of veins (valveless) allows hematogenous spread of pelvic (e.g., prostate cancer) and abdominal infections/metastases to the vertebral column and brain.

Anatomical Barriers - Body's Border Control

• Skin & Mucous Membranes: First line of defense.
  • Intact skin: Keratinized stratified squamous epithelium; physical barrier.
  • Mucous membranes: Line respiratory, GI, GU tracts; secrete mucus (traps pathogens), IgA.
  • Cilia: Respiratory tract; propel mucus & debris out.
• Fascial Planes & Compartments: Limit spread of infection.
  • Deep fascia: Dense connective tissue; encloses muscle groups.
  • Intermuscular septa: Divide limbs into compartments.
  • Compromise (e.g., surgery, trauma) ↑ infection risk.
• Body Fluids: Mechanical flushing & antimicrobial properties.
  • Tears, saliva, urine: Lysozyme, flushing action.

⭐ Fascial planes often dictate the pathway of pus collection and spread; e.g., prevertebral fascia can allow infection spread from pharynx to posterior mediastinum or even diaphragm (Danger space).

Key Infected Spaces - Trouble Hotspots

• Neck Spaces:
  • Retropharyngeal Space ("Danger Space"): Extends to posterior mediastinum. Risk of mediastinitis. Dysphagia, dyspnea.
  • Parapharyngeal Space: Lateral neck swelling, trismus. Carotid sheath contents at risk.
  • Ludwig's Angina: Bilateral infection of submandibular/sublingual/submental spaces. Woody induration, airway compromise.
• Hand Spaces:
  • Thenar Space: Thumb/index infections.
  • Midpalmar Space: Middle/ring/little finger infections.
  • Parona's Space (Forearm): Proximal spread from bursae.
  • 📌 Kanavel's Signs (Flexor Tenosynovitis): Finger flexed, Length tender, Extension painful, Symmetrical swelling.
• Subphrenic Spaces:
  • Between diaphragm & transverse colon. Common post-laparotomy. Shoulder tip pain.
• Ischiorectal Fossa (Perianal):
  • Lateral to anal canal. Common for abscesses. Painful perianal swelling.

⭐ Ludwig's angina is a rapidly spreading cellulitis of the floor of the mouth that can cause acute airway obstruction; a surgical emergency.

Organ-Specific Infections - Systemic Weak Spots

• Liver Abscesses:
  • Pyogenic: Portal vein (e.g., appendicitis); R lobe common (portal flow).
  • Amoebic: E. histolytica; "anchovy sauce" pus; often single, R lobe.
• Lung Infections:
  • Aspiration Pneumonia: Gravity-dependent sites (posterior segments of upper lobes, superior segments of lower lobes).
  • Tuberculosis (TB): Apices (↑O2 tension, ↓lymphatic drainage).
• Brain Infections:
  • Abscess: Hematogenous (grey-white matter junction); direct extension (e.g., sinusitis, otitis).
  • Cavernous Sinus Thrombosis: Valveless facial veins drain via ophthalmic veins ("danger triangle" of face).
• Osteomyelitis:
  • Children: Long bone metaphysis (rich vascularity, hairpin capillary loops, sluggish flow).
  • Adults: Vertebrae (Batson's venous plexus allows spread from pelvic/prostatic infections).
• Kidney (Pyelonephritis):
  • Ascending infection from lower UTI; Vesicoureteral Reflux (VUR) is a key risk factor.
  • Renal carbuncle: Staphylococcal cortical abscess, often hematogenous.

⭐ Batson's plexus, a network of valveless veins, facilitates the hematogenous spread of pelvic or prostatic infections/malignancies to the vertebral column, leading to osteomyelitis or metastases an important concept for understanding spinal infections and cancer spread.

High‑Yield Points - ⚡ Biggest Takeaways

• Danger triangle of face drains to cavernous sinus via ophthalmic veins; risk of septic thrombosis.
• Ludwig's angina: cellulitis of submandibular, sublingual, submental spaces; airway compromise risk.
• Retropharyngeal space infections ("danger space") can rapidly spread to posterior mediastinum.
• Psoas abscess, often from vertebral TB, tracks along psoas sheath to inguinal region.
• Hand infections: Felon (fingertip pulp), paronychia (nail fold), tenosynovitis (Kanavel's signs).
• Lymphatic drainage patterns dictate regional spread; e.g., axillary nodes from upper limb_