Natural History Of Diseases and Levels of Prevention

By : Sourav GoswamiModerator : Dr P R Deshmukh

Framework

• 1.Definition • 2.Understanding Natural History of Disease

• 3.Its Importance • 4.Stages• 5.Application : Levels of Prevention/Screening/prognosis/Evaluation

Definition

•Natural History of a disease signifies the way in which a disease evolves over time from the earliest stage of its prepathogenesis phase to its termination as recovery,disability or death,in the absence of treatment or prevention.

Natural history of disease

Natural history of Hepatitis B infection

SUSCEPTIBLE HOST

SUSCEPTIBLE HOST

ON EXPOSURE

ON EXPOSURE

ENTRY OF HBVENTRY OF HBV

DEVEL-OP

HEP-B

DEVEL-OP

HEP-B OUTCOMEOUTCOME

CIRRHOSIS

CIRRHOSIS

HCCHCC

CARRIERCARRIER

DEATHDEATH

Natural History of Typhoid

Entry of S.typhi

Entry of S.typhi

Incuba-tion

period 14 days

Incuba-tion

period 14 days

Palpabl-e

spleen ,Rash

Palpabl-e

spleen ,Rash

Headache, Fever,

Pea-soup stool

Headache, Fever,

Pea-soup stool

COMPLICATIONS:HemorrhagePerforationPeritonities

COMPLICATIONS:HemorrhagePerforationPeritonities

DEATH/DISABIL-ITY(carri

e-r)

DEATH/DISABIL-ITY(carri

e-r)

RecoveryRecovery

Susceptible

host

TIME

Incubation period

Death

Recovery

Exposure Onset

Latent Infectious Non-infectious

Infection

No infection

Clinical disease

Infectious period

the time during which the host can infect another susceptible host

•Non-infectious periodthe period when the host’s ability to transmit disease to other hosts ceases

•Incubation periodthe time interval between invasion by an infectious agent and appearance of the first sign or symptom of the disease in question

Latent period It is used in non-infectious diseases as the

equivalent of incubation period in infectious disease

-”Period from disease initiation to disease detection”

Stages of Natural History of DiseaseThe natural history of disease can

be divided into two stages :

1. Pre-pathogenesis phase2. Pathogenesis phase

1. Pre-Pathogenesis Phase / Stage of susceptibility

In this stage, the disease has not developed but the ground has been laid by the presence of factors that favor its occurrence, for eg :1.Alcohol consumption for Cirrhosis of liver2.High Cholesterol, obesity, Type A personality:

Heart Disease3.Smoking, Hypertension, High Cholesterol :

Stroke4.Radiation, Smoking, Immune suppression:

Cancer

Pathogenesis phase

• 1. Asymptomatic (Early Pathogenesis) phase

• 2. Early, Discernible Disease • 3. Full-Blown (Classical) Disease • 4. Termination - a) Complete Recovery • b) Chronic Disease • c) Life With Residual

Disability • d) Death

Why is it important to study natural history of disease?• 1. For prevention of disease •2.Adjusting lead time & length bias

for proper implementation of screening program

• 2.Forecasting prognosis• 3. Evaluation of intervention

Prevention

Levels of prevention

• In general, there are mainly three major levels of prevention, depending on the phase of the natural history of the disease :

• 1. Primary prevention (also primordial prevention )

• 2. Secondary prevention• 3. Tertiary prevention

• Primary prevention seeks to prevent the onset of specific diseases via risk reduction:

• (a) by altering behaviors /exposures that can lead to disease,(eg : cessation of smoking ) or

• (b) by enhancing resistance to the effects of exposure to a disease agent (eg : Vaccination )

• It can be done by : (1) Health Promotion

(2) Specific protection

Health Promotion• “The process of enabling people to

increase control over, and to improve health” (WHO)

It is not directed against any particular disease, but is intended to strengthen the host through a variety of approaches :

• 1.Health education• 2.Environmental modifications• 3.Nutritional intervention• 4.Lifestyle and behavioural changes

1) HEALTH EDUCATION : Most cost effective intervention. Now people have moved to behavior change communication.2) ENVIRONMENTAL MODIFICATION : # provision of safe water

#Installation of sanitary latrines #Control of insects & rodents etc

3)NUTRITIONAL INTERVENTIONS : # Food distribution & nutritional improvements of vulnarable groups ( viz. Mid-day meal in schools ,Khichri on Anganwadi etc ) etc4) LIFE STYLE & BEHAVIOURAL CHANGE : # motivation for healthy lifestyle

Contd……

Specific protection• immunization to protect against specific diseases

• fortification of foods with specific nutrients (as salt with iodine),

• use of condoms to protect against STDs,

• use of chemoprophylactic drugs to protect against particular diseases (as malaria, meningococcal meningitis, etc)

#Primordial Prevention

• This is prevention of the emergence of risk factors in countries or population groups in which they haven't yet appeared.

• By “individual and mass education”• It addresses BROAD HEALTH DETERMINANTS rather than preventing personal exposure to risk factors, which is the goal of primary prevention.

Contd ……

•Thus, outlawing alcohol in certain countries/areas would represent primordial prevention, whereas

•a campaign against drinking and would be an example of primary prevention.

Secondary prevention

• It include all actions undertaken at the stage of early pathogenesis so as to halt the progress of disease at it’s earliest stage,

• It is done by “early diagnosis and prompt treatment”

• eg : Screening for Cancer/ treatment of Tuberculosis-early diagnosis & prompt treatment/Diagnosis & treatment of malaria

Tertiary Prevention • It signifies interventions done in the late

pathogenesis phase.• “All measures available to reduce or limit

impairments and disabilities,minimise sufferings caused be existing departures from good health and to promote the patient’s adjustment to irremediable conditions” ( Last,, A Dictionary of Epidemiology )

• It can be attained by : a) Disability limitation & b) Rehabilitation

Disability limitation (impairment/disability/handicap)• Impairment is defined as "any loss or

abnormality of psychological, physiological, or anatomical structure or function."

• Impairment is a deviation from normal organ function; it may be visible or invisible (screening tests generally seek to identify impairments).

• Disability is defined as "any restriction or lack (resulting from an impairment) of ability to perform an activity in the manner or within the range considered normal for a human being."

• An impairment does not necessarily lead to a disability, for the impairment may be corrected.

• For example, I am wearing eye glasses, but do not perceive that any disability arises from my impaired vision. A disability refers to the function of the individual (rather than of an organ, as with impairment).

• Handicap is defined as "a disadvantage for a given individual, resulting from an impairment or a disability, that limits or prevents the fulfillment of a role that is normal (depending on age, sex, and social and cultural factors) for that individual."

• Handicap considers the person's participation in their social context.

• For example, if there is a wheel-chair access ramp at work, a disabled person may not be handicapped in coming to work there

Disability limitation• Concept of disability:

DISEASEAccident

(1)

DISEASEAccident

(1)

IMPAIRMENTLoss of foot

(2)

IMPAIRMENTLoss of foot

(2)

DISABIL-ITYCannot walk

(3)

DISABIL-ITYCannot walk

(3)

HANDICAPUnemploye

d(4)

HANDICAPUnemploye

d(4)

Contd…• Disability limitation includes all measures

to prevent the occurrence of further complications, impairments, disabilities and handicaps or even death. For example :

• When we apply plaster cast to a patient who has suffered Colle’s fracture, we are actually trying to prevent complications and further disability like mal-union or non-union (4)

Rehabilitation• “Rehabilitation” (Re =restore into, habitat

= the original home or environment of the person)

• “The combined and coordinated use of medical,social,educational and vocational measures for training and retraining the individual to the highest possible level of functional ability”

• It includes Physiotherapy,speech therapy,audiology,psychology, vocational work etc

Rehabilitation contd…• The following areas of concern have been

identified :• 1)Medical rehabilitation – restoration of

function• 2)Vocational rehabilitation- restoration of

the capacity to earn a livelihood• 3)Social rehabilitation –restoration of family

& social relationships• 4)Psychological rehabilitation –restoration

of personal dignity and confidence

Examples of rehabilitation

•Establishing schools for the blind,•provision of aids for the crippled, •reconstructive surgery in leprosy,•change of profession for a more suitable one etc

Knowledge of Natural History of disease helps in adjusting lead-time & length bias…..which helps in implementing proper screening measures

HOW SCREENING WORKSPathologyBegins

Symptom appears

DEATH/

DISABILITY

DEATH/

DISABILITY

SCREENING TEST &EARLY DIAGNOSIS(pre-symptom)

X

Concept of lead time

Possible detection by Screening

PreDetecta-

ble

PreDetecta-

ble

Detectable Pre-ClinicalDetectable Pre-Clinical ClinicalClinical Disability

DeathDisability

Death

AGE 20 30 40 50 60

Clinical detection

LEAD TIME

Survival time after diagnosis

> lead time

Length-time BiasAggressive Disease

Onset Clinical

Presentation

Death

Clinical

PresentationDeathOnset

1 yr sympto

Screening interval

1 year

6 mo.asymt period

2 year asym period

4 yr sym

Less Aggressive Disease

How do we conclude – the screening program is effective?

• We need to know the natural history of the disease

• for evaluating the Lead time & Length time bias

Prognosis: How much time do I have doc???

•Prognosis is the prediction of the course of a disease

and •is expressed as the probability that a particular event will occur in the future

Prognosis contd …..• Predictions are based on defined groups of

patients and the outcome may be quite different for the individual patients

• However, knowledge of the likely prognosis is helpful in determining the most useful treatment.

• Prognostic factors are characteristics associated with outcome in patients with the disease in question.

• For example, for a patient with AMI, the prognosis is directly related to heart muscle function.

Rates commonly used to describe PrognosisRate Definition5 year survival Percent of patients surviving 5 years

from some point in the course of disease

Case fatality Percent of patients with a disease who die of it

Disease-specific mortality

Number of people per 10,000 (or 100,000) population dying of specific disease

Response Percent of patients showing some evidence of improvement following an intervention

Remission Percent of patients entering a phase in which disease is no longer detectable

Recurrence Percent of patients who have return of disease after a disease free interval

Application of natural history of disease : Evaluation of interventional measuresEvaluation helps in 1)Providing feedback on the effectiveness of a program 2)helps to determine whether the program is appropriate for the target population

•

• 3) is there any problems with its implementation and support, and

• 4)whether there are any ongoing concerns that need to be resolved as the programme is implemented.

• 5)It helps in Comparing intervention modalities

Reference

• 1) AFMC (Association of Faculties of Medicine of Canada) Primer on Population Health-A virtual textbook on Public Health concepts for clinicians

• 2)Epidemiology by Leon Gordis( Fifth Edition)

• 3)Park’s testbook of Preventive and social Medicine( 23rd edition )

• 4)Text book of Public Health and community medicine by Armed Force Medical College

Thank you !