Session 20: Pathogenesis of injury by Physical agents

Session 20: Pathogenesis of injury by Physical agents

• Alex Simon (MD)

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Learning tasks

Learning tasks

At the end of this session, students are expected to be able to:

Explain mechanical injuries.

Explain thermal injuries.

Explain electrical injuries.

Explain injuries due to ionizing radiations.

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Introduction

Introduction

Injury induced by physical agents is divided into the following categories

Mechanical trauma.

Thermal injury.

Electrical injury.

Injury produced by ionizing radiation.

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Mechanical trauma

Mechanical trauma

Mechanical forces may inflict a variety of forms of damage including

Soft tissue injuries

Bone injuries

Head injuries.

The type of injury depends on

The shape of the colliding object.

The amount of energy discharged at impact.

The tissues or organs that bear the impact.

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Soft tissue injuries

Soft tissue injuries

All soft tissues react similarly to mechanical forces.

The patterns of soft tissue injuries can be divided into

Abrasions.

Contusions.

Lacerations.

Incised wounds.

Puncture wounds.

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Abrasion

Abrasion

An abrasion is a wound produced by scraping or rubbing the skin surface, damaging the superficial layer.

Typical skin abrasions remove only the epidermal layer.

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Contusion

Contusion

A contusion (bruise) is a wound usually produced by a blunt trauma.

It is characterized by damage to vessel and extravasation of blood into tissues.

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Laceration

Laceration

A laceration is a tear or disruptive stretching of tissue caused by the application of force by a blunt object.

In contrast with an incision, most lacerations have intact bridging blood vessels and jagged, irregular edges.

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Incised wound

Incised wound

An incised wound is one inflicted by a sharp instrument.

The bridging blood vessels are severed.

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Puncture wound

Puncture wound

A puncture wound is typically caused by a long, narrow instrument.

It is termed penetrating when the instrument pierces the tissue.

It is termed perforating when it traverses a tissue to also create an exit wound.

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Gunshot wounds

Gunshot wounds

Contact wounds

Stellate-shaped

Contain soot and gunpowder (fouling)

Intermediate-range wounds

Powder tattooing (stippling) of the skin around the entrance site

Long-range wounds

No powder tattooing

Exit wounds

Typically larger and more irregular than entrance wounds

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Gunshot wounds

Gunshot wounds

Gunshot wounds are special forms of puncture wounds that demonstrate distinctive features important to the forensic pathologist.

For example, a wound from a bullet fired at close range leaves powder burns, whereas one fired from more than 4 or 5 feet away does not.

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Vehicular accident

Vehicular accident

Motor vehicle collisions

Frequently cause mechanical injury.

Frequently alcohol-related.

Injuries typically sustained result from

Hitting a part of the interior of the vehicle or being hit by objects that enter the passenger compartment during the crash, such as engine parts.

Being thrown from the vehicle.

Being trapped in a burning vehicle.

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Vehicular accident cont…

Vehicular accident cont…

The pattern of injury relates to whether one or all three of those mechanisms are operative.

For example, in a head-on collision, a common pattern of injury sustained by a driver who is not wearing a seat belt includes: trauma to the head (windshield impact), chest (steering column impact), and knees (dashboard impact).

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Vehicular accident cont…

Vehicular accident cont…

Common chest injuries stemming from such accidents include sternal and rib fractures, heart contusions, aortic lacerations, and (less commonly) lacerations of the spleen and liver.

Thus, in caring for an automobile injury victim, it is essential to recognize that internal wounds often accompany superficial abrasions, contusions, and lacerations.

Indeed, in many cases, external evidence of serious internal damage is completely absent.

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Thermal injury

Thermal injury

Both excess heat and excess cold are important causes of injury.

It includes

Thermal burns.

Hyperthermia.

Hypothermia.

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Thermal injury cont…

Thermal injury cont…

Many victims are children, in whom the cause of injury often is scalding by hot liquids.

The clinical severity of burns depends on the following important variables

Depth.

Percentage of body surface involved.

Whether internal injuries from inhalation of hot and toxic fumes are present.

Promptness and efficacy of therapy, especially fluid and electrolyte management and prevention or control of wound infections.

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Classification of burn cont…

Classification of burn cont…

Full-thickness burn.

Produces total destruction of the epidermis and dermis, including the dermal appendages that harbor cells needed for epithelial regeneration.

Full thickness burns includes

Third- degree burns.

Fourth-degree burns.

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Classification of burn

Classification of burn

Partial-thickness burns.

At least the deeper portions of the dermal appendages are spared.

Partial-thickness burns include

First-degree burns (epithelial involvement only)

Second-degree burns (involving both epidermis and superficial dermis).

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Morphology of burn injury

Morphology of burn injury

On gross inspection

Full-thickness burns are white or charred, dry, and anesthetic (as a result of destruction of nerve endings)

Partial-thickness burns, depending on the depth, are pink or mottled, blistered and painful.

Histologic examination

Coagulative necrosis adjacent to vital tissue, which quickly accumulates inflammatory cells and marked exudation.

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Complications of thermal injury

Complications of thermal injury

Infections.

Dehydration.

Hypovolemic shock.

Generalized edema, including pulmonary edema.

Organ system failure resulting from sepsis.

Contractures.

For inhalation injury

Upper airways inflammation and swelling.

May lead to partial or complete airway obstruction.

Pneumonitis.

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Hyperthermia

Hyperthermia

Prolonged exposure to elevated ambient temperatures can result in

Heat cramps.

Heat exhaustion.

Heat stroke.

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Heat cramps

Heat cramps

Heat cramps result from loss of electrolytes through sweating.

Cramping of voluntary muscles, usually in association with vigorous exercise (hallmark sign).

Heat-dissipating mechanisms are able to maintain normal core body temperature.

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Heat exhaustion

Heat exhaustion

Heat exhaustion is probably the most common hyperthermic syndrome.

Its onset is sudden, with prostration and collapse, and it results from a failure of the cardiovascular system to compensate for hypovolemia, secondary to water depletion.

After a period of collapse, which is usually brief, equilibrium is spontaneously.

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Heat stroke

Heat stroke

Heat stroke is associated with high ambient temperature and high humidity.

Thermoregulatory mechanisms fail sweating ceases, and core body temperature rises.

In the clinical setting, a rectal temperature of 106°F or higher is considered a grave prognostic sign.

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Heat stroke cont…

Heat stroke cont…

The underlying mechanism is marked generalized peripheral vasodilation with peripheral pooling of blood and a decrease effective circulating blood volume.

Necrosis of the muscles and myocardium may occur.

Arrhythmias, disseminate intravascular coagulation, and other systemic effects are common.

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Malignant hyperthermia

Malignant hyperthermia

Malignant hyperthermia, although similar sounding, it is not caused by exposure to high temperature.

It is genetic condition resulting from mutations in gene that control calcium levels in skeletal muscle cells.

In affected individuals, exposure to certain anesthetics during surgery may trigger a rapid rise in calcium levels in skeletal muscle, which in turn leads to muscle rigidity and increased heat production.

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Hypothermia

Hypothermia

Due to prolonged exposure to low ambient temperature.

At about 90°F, loss of consciousness occurs, followed by bradycardia and atrial fibrillation at lower core temperatures.

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Hypothermia cont…

Hypothermia cont…

Chilling or freezing of cells and tissues causes injury by two (2) mechanisms:

Direct effects probably are mediated by physical disruptions within cells and high salt concentrations incident to the crystallization of the intra- and extracellular water.

Indirect effects are the result of circulatory changes, which vary depending on the rate and the duration of the temperature drop.

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Electrical injury

Electrical injury

Can arise from

Low voltage currents (i.e. in the home and workplace).

High-voltage currents carried in power lines.

Lightning.

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Electrical injury cont…

Electrical injury cont…

Electrical injuries are of two types

Burns due to heat produced by electrical energy.

Ventricular fibrillation or cardiac and respiratory center failure resulting from disruption of normal electrical impulses.

The type of injury and the severity and extent of burning depend on:

Amperage of the electric current.

Its path within the body.

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Injury by ionizing radiation

Injury by ionizing radiation

The main sources of ionizing radiation are

X-rays and gamma rays, which are electromagnetic waves of very high frequencies.

High-energy neutrons

Alpha particles (composed of two protons and two neutrons).

Beta particles, which are essentially electrons.

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Injury by ionizing radiation cont…

Injury by ionizing radiation cont…

Ionizing radiation is indispensable in medical practice, but this application constitutes a two-edged sword.

Radiation in this form is used in the treatment of cancer, in diagnostic imaging, and as therapeutic or diagnostic radioisotopes.

However, it also is mutagenic, carcinogenic, and teratogenic.

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Injury by ionizing radiation cont…

Injury by ionizing radiation cont…

Ionizing radiation may injure cells directly or indirectly by generating free radicals from water or molecular oxygen.

Ionizing radiation damages DNA; therefore, rapidly dividing cells such as germ cells, and those in the bone marrow and GI tract are very sensitive to radiation injury.

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Injury by ionizing radiation cont…

Injury by ionizing radiation cont…

DNA damage that is not adequately repaired may result in mutations that predispose affected cells to neoplastic transformation.

Ionizing radiation may cause vascular damage and sclerosis, resulting in ischemic necrosis of parenchymal cells and their replacement by fibrous tissue.

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Effects of ionizing radiation

Effects of ionizing radiation

Somatic effects which cause acute cell killing.

Genetic damage by mutations and therefore, passes genetic defects in the next progeny of cells.

Malignant transformation of cells.

Teratogenic.

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Key points

Key points

Soft tissue injury occurs due to mechanical forces.

Infections, dehydration, hypovolemic shock are common complications of burn.

Heat exhaustion is the most common hyperthermic syndrome.

Lightning is a classic cause of high voltage electrical injury.

Ionizing radiations are mutagenic, carcinogenic and teratogenic.

Injury due to ionizing radiation correlates with type of radiation, cumulative dose, and amount of surface area exposed.

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Review questions

Review questions

Explain five (5) forms of soft tissue injury.

Explain effects of injury due to ionizing radiation.

List three (3) characteristics of gunshot wound.

Differentiate between full thickness burn from partial thickness burn.

Explain how electricity cause the injury.

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References

References

Bezabeh M.; Tesfaye A.; Ergicho B. et al (2004): General pathology lecture notes for Health Sciences students. Ethiopia Public Health Training Initiative. Pg. 231-239.

Goljan E.;(2007): Rapid Review Pathology (2th Ed.) Elsevier Saunders, USA. Pg. 116-120.

Kumar V. ; Abbas A. K. ; Aster J. C.;(2013): Robbins and Contran Pathologic Basis of Disease (9th Ed.) Elsevier Saunders, USA. Pg. 287-293.

Mohan H.;(2010): Text book of Pathology (6th Ed.) Jaypee Brothers Medical Publishers, India. Pg. 242.

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