What Causes Lethargy?

Patient Presentation
A 3 year-old male came to clinic with a 2-day history of cough and runny nose. He had a fever to 38.4°C and complained of a sore throat that worsened with the cough, but denied any other pain. “He’s totally lethargic and is just lying around,” according to his mother. She stated that he was sleeping poorly because of the cough. He was drinking and urinating. He was in daycare. The past medical history was non-contributory.

The pertinent physical exam showed a tired appearing male with temperature of 37.8°C, heart rate of 98, and respiratory rate of 22. HEENT showed copious rhinorrhea, mild erythema of the posterior pharynx without exudate and shotty anterior cervical nodes. His lungs had coarse breath sounds and transmitted upper airway sounds, but he was in no respiratory distress. Neurological examination was normal and the patient was quieter but carried on an appropriate conversation.

The diagnosis of a viral upper airway infection was made. The mother was counseled about symptomatic treatment and reasons to call or return to clinic such as signs of respiratory distress. The patient was tested for novel coronavirus (COVID-19) and eventually was negative also.

Lethargy is a common word used to describe a person who is drowsy, sluggish, listless and apathetic. Concentration may be difficult and they may have problems doing simple tasks. Many people actual mean fatigue or lassitude or being more tired when they use the word. In medical terms, lethargy is usually used to describe patients who have some type of excessive tiredness and usually have mental status changes with decreased alertness or arousal. Patients and health care providers both usually qualify the term and give more description to better communicate what is implied when using the term. This is true of many words such as “irritable” or “diarrhea” which again are used frequently but may be less precise as to their meaning, or that have different meaning for laypersons or health care professionals. In medicine, true excessive tiredness can be caused by many problems. History is very important. Is this an acute change with a profound decrease in alertness (e.g. sepsis, meningitis, drug toxicity, etc.), or is this more chronic and the patient and family have noticed a slower onset that is just now causing changes to their activities of daily living but not their mental alertness (e.g. anemia, hypothyroidism, etc.).

Learning Point
The differential diagnosis of fatigue (a review can be found here) and lethargy are large with much overlap. Every organ system can be a potential cause, as the body obviously wishes to conserve energy and rest more when it is ill. Also any extreme state can often cause derangement or distress that manifests in some type of mental status change. One must consider what is meant by the term and especially the onset.

The differential diagnosis for lethargy includes:

  • Acute disease presentations
  • Altered homeostatic states
    • Fever – very common cause
    • Dehydration
    • Malnutrition
    • Insufficient sleep
    • Temperature regulation – too hot or cold
  • Chronic disease of major organ systems and/or organ failure
    • Central nervous system
      • Myasthenia gravis
      • Post-ictal state
      • Unrecognized seizures
    • Cardiac
      • Arrhythmias
      • Congenital heart disease
      • Endocarditis
      • Myocarditis
    • Endocrine
      • Addison’s disease
      • Cushing’s disease
      • Hypothyroidism
      • Hyperthyroidism
    • Gastrointestinal
      • Hepatitis
      • Pancreatitis
    • Hematologic
      • Anemia
    • Pulmonary
      • Respiratory failure
      • Severe asthma
    • Renal
      • Pyelonephritis/Urinary tract infection
      • Renal disease
  • Drug – side effects, abuse
  • Electrolyte abnormalities
    • Hypoglycemia
    • Hyperglycemia
    • Hyperammonemia
    • Metabolic abnormalities
  • Infections – acute and chronic
  • Mental health
    • Anxiety
    • Depression
    • Substance use/abuse
  • Trauma
    • Abuse and neglect
    • Traumatic brain injury

Questions for Further Discussion
1. What other terms are commonly used imprecisely by laypersons and healthcare providers?
2. What causes fever? A review can be found here

Related Cases

To Learn More
To view pediatric review articles on this topic from the past year check PubMed.

Evidence-based medicine information on this topic can be found at SearchingPediatrics.com and the Cochrane Database of Systematic Reviews.

Information prescriptions for patients can be found at MedlinePlus for these topics: Fatigue and Traumatic Brain Injury.

To view current news articles on this topic check Google News.

To view images related to this topic check Google Images.

To view videos related to this topic check YouTube Videos.

Illingworth RS. Common Symptoms of Disease in Children. Blackwell Scientific Publications. Oxford, England. 9th Edit. 1988.

Sheldon SH, Levy HB. Pediatric Differential Diagnosis. Raven Press. New York. 2nd Edit. 1985.

Donna M. D’Alessandro, MD
Professor of Pediatrics, University of Iowa