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1. History — Critical First Step

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2. Rash Morphology — Narrows the Differential

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3. Differential Diagnosis — Organized by Likelihood

🔴 Can't-Miss / Life-Threatening (Rule Out First)

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🟡 Common Infectious Causes

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🟢 Non-Infectious

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4. Approach to Management

Step 1: Immediate Assessment (Triage)

• Vital signs — hypotension/tachycardia → resuscitate first
• Look for red flags: non-blanching rash, meningism, altered consciousness, mucosal involvement, signs of bleeding → admit + empiric treatment immediately

Step 2: Initial Investigations

Step 3: Directed Treatment (Based on Diagnosis)

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5. A Classic Example — Dengue Fever Rash

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Summary Algorithm

Fever + Rash (4 days)
        │
        ├── Non-blanching / petechial? → Rule out Meningococcemia / RMSF IMMEDIATELY
        │
        ├── Mucosal involvement? → Rule out SJS/TEN
        │
        ├── Travel history?
        │       ├── Tropical → Dengue, Chikungunya, Typhoid, Malaria + rash
        │       └── Animal/tick bite → RMSF, Lyme, Rickettsial
        │
        ├── Pharyngitis + LAD → EBV / Strep (Scarlet fever)
        │
        ├── Drug history → Drug reaction / DRESS
        │
        └── Quotidian rash + arthritis + high ferritin → Still's disease

Clinical Pearl: In any patient with fever + non-blanching rash, do NOT delay empiric Ceftriaxone and Doxycycline while awaiting workup — the cost of under-treating RMSF or meningococcemia is death.

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Thermoregulation

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1. Overview

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2. The Thermoregulatory Center

Hypothalamic Control

1. Peripheral signals — from thermoreceptors in the skin via peripheral nerves
2. Central signals — from the temperature of blood directly bathing hypothalamic neurons

Key Neural Pathways

Molecular Sensors in the POA

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3. Peripheral Thermoreceptors

Cold receptors outnumber warm receptors in the skin (~3–10:1), making humans more sensitive to cold.

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4. Heat Production Mechanisms

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5. Heat Loss Mechanisms

Evaporation becomes the dominant heat loss mechanism when ambient temperature exceeds body temperature — radiation and convection reverse direction.

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6. Effector Responses — The Feedback Loop

Response to Cold (Core Temp ↓ below set point)

1. Cutaneous vasoconstriction — reduces blood flow to skin, conserves core heat
2. Piloerection — traps insulating air (vestigial in humans)
3. Shivering — generates heat via muscle contractions
4. Non-shivering thermogenesis — BAT activation via sympathetic norepinephrine → UCP-1
5. Behavioral — seeking warmth, huddling, adding clothing

Response to Heat (Core Temp ↑ above set point)

1. Cutaneous vasodilation — increases blood flow to skin; heat transferred from core to surface
2. Sweating — eccrine sweat glands (cholinergic sympathetic); evaporation cools skin
3. Decreased metabolic rate (mild)
4. Behavioral — seeking shade, removing clothing, reduced activity

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7. Fever — A Regulated Rise in the Set Point

Mechanism of Fever

Exogenous pyrogens (LPS, viral proteins, toxins)
        ↓
Macrophages/monocytes activated
        ↓
Endogenous pyrogens released:
IL-1β, IL-6, TNF-α, IFN-γ
        ↓
Circumventricular organs (organum vasculosum of lamina terminalis — OVLT)
(lack blood-brain barrier)
        ↓
Phospholipase A2 → Arachidonic acid → COX-2 → PGE2
        ↓
PGE2 acts on EP3 receptors in POA
        ↓
Set point ↑ → body temperature perceived as "too cold"
        ↓
Vasoconstriction + Shivering → Temperature rises to new set point

Maintaining and Defervescing Fever (Harrison's, p. 637)

• Maintenance: Once blood temperature matches the new (higher) set point, the hypothalamus maintains it via the same heat-balance mechanisms used in the afebrile state.
• Defervescence: When pyrogen concentration falls (resolution of infection) or antipyretics are given (COX inhibition → ↓PGE2), the set point resets downward → vasodilation + sweating dissipates heat → "breaking the fever."

Fever vs. Hyperthermia — A Critical Distinction

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8. Temperature Measurement — Clinical Points

Fever is defined as a core temperature >38.3°C (101°F) in adults (Harrison's).

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9. Clinical Disorders of Thermoregulation

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Summary

TEMPERATURE STIMULUS
        │
        ├── Cold detected
        │       → Vasoconstriction → Shivering → NST (BAT/UCP-1)
        │
        └── Heat detected
                → Vasodilation → Sweating → Behavioral changes
                
POA (Hypothalamus) = Master thermostat
Set point controlled by PGE2 (fever) and normal neural signals
Heat balance = Heat production ↔ Heat loss (Radiation + Evaporation + Conduction + Convection)