Merck Manual

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Some Causes and Features of Personality and Behavior Changes

Some Causes and Features of Personality and Behavior Changes


Common Features*


Mental disorders

Symptoms that occur in episodes lasting a few weeks to a few months and that include mania, depression, or both

Episodes of mania:

  • Elation or irritability

  • Grandiosity (exaggerated sense of superiority, power, knowledge, or identity)

  • Talkativeness

  • Increased activity

  • A decreased need for sleep

  • Racing thoughts, jumping from one idea to another

  • Sometimes hallucinations or delusions of persecution

Episodes of depression:

  • Feeling guilty or worthless

  • Sadness, despair, and a pessimistic mood

  • Loss of interest in typical pleasures

  • Lack of energy

  • Sleeping too much or difficulty sleeping

  • Thoughts of death or suicide

A doctor's examination

Episodes of depression as described in bipolar disorder (see above) without a history of mania

A doctor's examination

Usually symptoms that develop slowly, at first with mildly disorganized thinking and difficulty coping with daily routines

Later symptoms:

  • Delusions and/or hallucinations

  • Often disinterest and lack of emotion

  • Increasingly disorganized thinking and behavior

  • Difficulty maintaining relationships and employment

A doctor's examination


Use of a drug, particularly

Agitation and sometimes panic or aggression

Sometimes hallucinations

With long-term use of alcohol:

  • Sometimes balance problems

  • Twitching eyes

  • An abnormal way of walking

With long-term use of amphetamines:

  • Sometimes paranoia

With short-term excessive use of amphetamines or cocaine:

  • An increased heart rate

  • Sometimes fever

A doctor's examination

Sometimes blood or urine tests to detect the drug

Withdrawal of a drug, particularly

Typically significant confusion and delirium

Shaking (tremors), headache, sweating, fever, and a rapid heart rate or palpitations

Sometimes seizures, hallucinations, and sleep disturbances

Onset after cutting down or stopping the drug

A doctor's examination

Sometimes blood or urine tests to detect drugs

Side effects of a drug

Vary, depending on the drug

Drugs with anticholinergic effects, such as

  • Constipation

  • Blurred vision

  • Light-headedness

  • Difficulty starting and stopping urination

  • Dry mouth

A doctor's examination

Sometimes stopping the drug to see whether the symptom goes away

Brain disorders

Symptoms that progress slowly

Loss of short-term memory, difficulty finding the right words, and poor judgment

Difficulty with daily activities (such as balancing a checkbook or finding their way around their neighborhood)

Usually in people over 60

A doctor's examination

Detailed testing of mental function involving a series of questions and tasks (neuropsychologic testing)

Brain infections such as


Usually confusion and fever

Pain and/or stiffness when the doctor bends the neck forward (more common in people with meningitis)

With herpes simplex encephalitis, hallucinations of bad odors and sometimes seizures

A spinal tap (lumbar puncture)

Often CT or MRI of the brain

Culture of samples of blood, urine, and material from the throat

Forgetfulness and headaches

Emotional instability in the weeks after a significant head injury

CT or MRI of the brain

Testing of IQ and executive functions such as the ability to plan and solve problems (neurocognitive testing)

With brain tumors, a headache that develops gradually and is often worse during the night or early morning and when lying flat

With hemorrhage, a headache that starts suddenly (called a thunderclap headache)

Often confusion and drowsiness

Sometimes seizures

CT or MRI of the brain

Weakness and/or numbness that comes and goes in different parts of the body

Sometimes partial loss of vision or double vision

Sometimes symptoms that are worsened by heat (such as a warm bath or hot weather)

MRI of the brain and spinal cord

Tremors of the hands and fingers while they are at rest

Stiffness and difficulty moving and maintaining balance

Slowed speech and limited facial expressions

A doctor's examination

Seizure disorders (typically complex partial seizures Focal-onset seizures )

Episodes of abnormal behavior

Usually confusion and staring

Sometimes involuntary chewing, smacking of the lips, and purposeless movements of the limbs

Typically no loss of consciousness and no general shaking of the body (convulsions)

Sometimes hallucinations of odor or taste

MRI of the brain


Symptoms that appear suddenly

Usually weakness or paralysis on one side of the body and unsteadiness when walking

CT or MRI of the brain

Bodywide (systemic) disorders

Hypoglycemia Hypoglycemia (a low level of blood sugar)

Weakness, sweating, and confusion

Almost always in people taking drugs for diabetes

Tests to measure the blood sugar (glucose) level

Swelling of the legs, loss of appetite, and nausea

Weakness that typically develops over several weeks

Blood and urine tests to evaluate how well the kidneys are functioning

Yellow color of the skin and/or whites of the eyes (jaundice)

Usually swelling of legs and/or abdomen

A reddish purple rash of tiny dots (petechiae)

Usually in people already known to have a liver disorder

Blood tests to evaluate how well the liver is functioning (liver tests)

Usually painful, swollen joints

Often a rash, particularly on the face or areas exposed to sunlight

Sometimes a headache

Blood tests to check for certain antibodies

Thyroid disorders, including

Typically in hyperthyroidism: Palpitations, excessive sweating, difficulty tolerating heat, an increased appetite, weight loss, shakiness (tremor), and sometimes bulging eyes

Typically in hypothyroidism: Fatigue, constipation, difficulty tolerating cold, decreased appetite, weight gain, slow speech, sluggishness, a puffy face, drooping eyelids, coarse and thick dry skin, and loss of eyebrow hair

Blood tests to evaluate how well the thyroid gland is functioning

Vitamin deficiency, such as deficiency of thiamine Thiamin Deficiency or vitamin B12 Vitamin B12 Deficiency

Disorientation, an impaired memory, and irritability

Abnormal sensations in the hands and feet

Other symptoms, depending on which vitamin is deficient

Blood tests to measure vitamin levels

* Features include symptoms and results of the doctor's examination. Features mentioned are typical but not always present.

CT = computed tomography; EEG = electroencephalography; IQ = intelligence quotient; MRI = magnetic resonance imaging; PET = positron emission tomography.