6.3 about 28 minDeep Learning viewL6 p.14–27

Physical injury

Mechanical, thermal, electrical and radiation injuries each leave a recognizable pattern. Read the wound to read the force, and know why a large burn kills by fluid loss before it kills by infection.

Mechanical injury: reading the wound

L6 p.15–16

The type of wound tells you the force. The lecture's four skin wounds plus puncture and gunshot cover the forensic basics.

Lecture image

Four photographs: a bruise, a grazed forearm, a jagged hand wound, and a sutured straight cut.
Contusion (bruise), abrasion (graze), laceration (jagged tear with bridging strands), incision (clean-edged cut).L6 p.15

Wound typesL6 p.15–16Ref. · Robbins; forensic wound terminology

  • Abrasion

    Mechanism
    Scraping that removes superficial epidermis
    Edges / clue
    Graze; no deep structures

  • Contusion (bruise)

    Mechanism
    Blunt force ruptures vessels; blood leaks into tissue
    Edges / clue
    Skin intact; color changes with age

  • Laceration

    Mechanism
    Blunt force tears skin
    Edges / clue
    Irregular, jagged edges with bridging vessels and nerves across the wound

  • Incision

    Mechanism
    A sharp edge (knife)
    Edges / clue
    Clean, straight edges; no bridging

  • Puncture / stab

    Mechanism
    A narrow pointed object
    Edges / clue
    Small surface wound; depth exceeds width; internal damage underestimated

  • Gunshot

    Mechanism
    Projectile
    Edges / clue
    Entry smaller with an abrasion rim; exit larger and irregular; range read from soot and stippling

Lecture image

Three gunshot wounds: a star-shaped tear on the forehead, a small wound surrounded by fine dark speckles on the face, and a large irregular wound with everted edges.
Left: contact wound over bone, torn into a star shape by expanding gas. Middle: intermediate range, with powder stippling (tattooing) around a small entry. Right: exit wound, larger and irregular with everted edges and no abrasion rim.L6 p.16

Range is read from what surrounds the entry wound: soot and seared skin at contact range, a stippled zone of unburned powder (tattooing) at intermediate range, and a clean small wound with an abrasion rim at distant range. A contact wound over bone can be torn into a star shape by expanding gas.

Abusive head trauma

L6 p.17

Violent shaking of an infant tears the bridging veins that cross from the cortex to the dural sinuses, producing a subdural hemorrhage. The same acceleration–deceleration forces cause retinal hemorrhages, and squeezing the chest fractures the posterior ribs. The triad of subdural hemorrhage, retinal hemorrhages and rib fractures should prompt evaluation for abuse.

Lecture image

A fundus photo with retinal hemorrhages and a diagram comparing a subdural hematoma (between dura and arachnoid) with subarachnoid hemorrhage.
Retinal hemorrhages on fundoscopy; a subdural hematoma collects between the dura and the arachnoid, where torn bridging veins bleed.L6 p.17

Burns: depth, area and the fluid shift

L6 p.18–20

A burn is graded by depth and by the area of body surface involved. Depth decides healing and scarring; area decides how much fluid is lost and whether the patient goes into shock.

Lecture image

Four photographs of burns of increasing depth, from red skin to charred full-thickness injury.
First-degree (epidermis only), second-degree (into dermis, blistering, painful), third-degree (through the dermis, adnexa and nerves destroyed, painless), fourth-degree (into muscle and bone).L6 p.18

Burn depthL6 p.18

  • First

    Reaches
    Epidermis only
    Features
    Red, painful; heals without scar (sunburn)

  • Second

    Reaches
    Into dermis (superficial or deep)
    Features
    Blisters, very painful; nerves preserved

  • Third

    Reaches
    Through epidermis and dermis
    Features
    Adnexa and nerves destroyed, so painless; needs grafting

  • Fourth

    Reaches
    Into muscle and bone
    Features
    Charring; limb-threatening

The rule of nines estimates the burned area in an adult: head 9%, each arm 9%, each leg 18%, front of trunk 18%, back of trunk 18%, perineum 1%. The larger the area, the more plasma escapes through heat-injured capillaries.

Why a large burn causes shock within a day

  1. Large burn
  2. Heat-injured capillaries become leaky; mediators raise permeability
  3. Plasma and protein escape into burned tissue (edema) and evaporate from the surface
  4. Intravascular volume falls
  5. Hypovolemic shock (the leading early cause of death)
L6 p.18–19Midterm sim. p.8

Heat and cold

L6 p.20–21

Heat illness is a spectrum. Heat cramps are painful muscle spasms from salt loss during exertion. Heat exhaustion is volume depletion with a core temperature below 40 °C, sweating preserved and no major CNS change. Heat stroke is a core temperature above 40 °C with central nervous system dysfunction, and it comes in two forms.

Two kinds of heat stroke

Classic (non-exertional)

  • Elderly, chronically ill, underventilated apartments in a heat wave
  • Hot, dry skin (anhidrosis)
  • Altered mental status, coma, seizures

Exertional

  • Young, healthy athletes and military recruits
  • May still sweat
  • Rhabdomyolysis, acute kidney injury, DIC, lactic acidosis, hypocalcemia
L6 p.20

Lecture image

Toes with dark necrotic tips (frostbite) above two water-damaged pale hands (trench foot).
Frostbite: ice crystals form in tissue and microvascular stasis leads to necrosis of the digits. Trench foot: a nonfreezing wet-cold injury.L6 p.21
  • Frostbite: tissue below 0 °C. Ice crystals form in the extracellular space, drawing water out of cells; microvascular collapse, sludging and stasis then deprive tissue of oxygen and cause necrosis. A prefreeze phase of anesthesia precedes the freeze–thaw phase.
  • Trench foot / immersion foot: a nonfreezing injury from prolonged wet cold, without ice crystal formation.
L6 p.21

Electrical, drowning and altitude

L6 p.22–24

Electrical injury follows Ohm's law (I = V/R): current, not voltage alone, injures. Wet skin lowers resistance and raises current. Alternating current at a given voltage is about three times more dangerous than direct current because it causes tetanic muscle contraction that locks the victim to the source; DC tends to throw the victim off. The most common cause of death is cardiorespiratory arrest (ventricular fibrillation or respiratory muscle tetany); deep tissue is burned along the current path.

Lecture image

A hand charred black along the path of an electric current.
An electrical burn chars tissue along the current path; most of the damage is internal.L6 p.22
  • Drowning: suffocation from immersion. In wet drowning, laryngospasm relaxes and water is aspirated; surfactant is washed out, causing atelectasis and pulmonary edema. In dry drowning, intense laryngospasm asphyxiates without aspiration. Alcohol, seizures and trauma are major risk factors.
  • High-altitude (acute mountain) sickness: above ~8000 ft (2440 m), low barometric pressure lowers inspired PO₂. Headache is the most common symptom, with fatigue, dizziness, anorexia, nausea and insomnia. Severe forms are high-altitude pulmonary edema (HAPE, from hypoxic pulmonary vasoconstriction) and cerebral edema (HACE).
L6 p.22–24

Radiation and UV

L6 p.25–27

Ionizing radiation has enough energy to eject electrons and ionize atoms; it injures DNA mainly by generating hydroxyl radicals from water. The most radiosensitive tissues are those that divide fastest: lymphoid tissue, bone marrow, gastrointestinal mucosa and germ cells.

Teaching schematic

  1. < 0.5 Gy

    Subclinical

    No systemic effect; stem-cell mutations may predispose to cancer

  2. 0.5–2 Gy

    Radiation sickness

    Lethargy, nausea; transient drop in lymphocytes and neutrophils

  3. 2–6 Gy

    Hematopoietic

    Marrow hypoplasia; anemia 2 weeks later

  4. 3–10 Gy

    Gastrointestinal

    Death of gut epithelium, hair loss, diarrhea within hours

  5. > 10 Gy

    Cerebral

    Hemorrhagic brain necrosis; death within hours; convulsions

Thresholds are approximate and sources differ; higher doses add the lower-dose effects.

Acute radiation syndrome by whole-body dose: subclinical, hematopoietic, gastrointestinal and cerebral syndromes (approximate thresholds).L6 p.26

Radiation effects by tissueL6 p.25

  • Skin

    Acute effect
    Erythema, edema, blistering
    Chronic effect
    Radiodermatitis, cancer

  • Bone marrow

    Acute effect
    Lymphopenia first, then thrombocytopenia, marrow hypoplasia
    Chronic effect
    Acute leukemia

  • GI mucosa

    Acute effect
    Diarrhea, mucosal necrosis
    Chronic effect
    Fibrosis, strictures

  • Gonads

    Acute effect
    Germ cell death
    Chronic effect
    Atrophy, fibrosis, infertility

  • Lung

    Acute effect
    Acute pneumonitis
    Chronic effect
    Interstitial fibrosis

Nonionizing (ultraviolet) radiation does not ionize but damages DNA by forming pyrimidine (thymine) dimers that distort the helix. UVB is the main carcinogen. The dimers are normally removed by nucleotide excision repair; unrepaired damage, together with inactivation of p53 and activation of RAS, produces actinic keratosis, basal cell and squamous cell carcinoma, and melanoma.

Lecture image

Four clinical photos labelled basal cell, squamous cell and melanoma skin cancers.
UV-related skin cancers: basal cell carcinoma, squamous cell carcinoma and melanoma.L6 p.27

Knowledge check · 6.3

Answer, read why, and try again whenever you like. Answers are kept in both study modes.

0 of 4 correct
  1. Q1Burn shock (fluid loss)Midterm sim. p.8L6 p.18–19

    A 50-year-old firefighter has third-degree burns over 70% of his body. He dies 24 hours later.

    What is the most likely cause of death?

  2. Q2Abusive head trauma triadCh. VI cases p.9L6 p.17

    A 2-month-old is brought in lethargic with a bulging fontanelle. Fundoscopy shows bilateral retinal hemorrhages, and MRI shows a crescent of blood between the dura and arachnoid.

    Which additional finding on a chest x-ray would support the diagnosis?

  3. Q3Laceration vs incisionL6 p.15

    A forensic pathologist examines a skin wound that is irregular and jagged with thin strands of tissue bridging across the defect.

    Which wound is this?

  4. Q4UV, pyrimidine dimers and NERL6 p.27Ref. · Robbins (UV carcinogenesis, XP)

    A 68-year-old fair-skinned farmer who worked outdoors for decades develops a scaly, ulcerated plaque on his lower lip. A relative has an inherited condition that caused dozens of skin cancers in childhood.

    Which molecular event links the sun exposure to his tumor, and which defect explains his relative's early cancers?

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