As mortality rates associated with admission to the intensive care unit (ICU) have decreased in recent decades, there has been increasing interest in the long-term negative consequences of critical care. These effects, collectively termed “post-intensive care syndrome,” include emerging or worsening impairments in physical, cognitive, and psychological functioning following ICU admission.1  

A growing body of research points to a key role for chronic ICU-related pain in mediating dysfunction, according to the authors of a narrative review published in August 2019 in the British Journal of Anaesthesia.1 Up to 77% of patients are estimated to experience chronic pain in the year after discharge from the ICU. It is thought that chronic pain develops in 22% to 33% of patients after ICU admission.1

Pain intensity
Studies in which pain intensity was evaluated in patients who had been admitted to the ICU reported the following observations:

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  • Disabling pain associated with moderate (21.2%) to severe (36.4%) limitation in daily activities occurs in >50% of patients with chronic ICU-related pain assessed; using the Von Korff and colleagues pain grading system.2
  • Moderate to severe pain — assessed with the Brief Pain Inventory — may occur in 31% of patients 3 months after ICU discharge and in 35% of patients 1 year after discharge.3
  • Pain intensity — assessed with the Brief Pain Inventory — of 0 to 3 and 4 to 10 on a 0 to 10 scale may occur in 44% and 56% of patients with sepsis, respectively.4
  • Moderate and extreme pain/discomfort — assessed with the Euro Quality of Life-5-dimension — were found to occur in 33% and 7%, respectively of survivors of sepsis and septic shock.5

Findings regarding the trajectory of CIRP have been mixed overall, with one meta-analysis indicating improvements in pain within the first year when pain was assessed with the Short-Form-36, but not if pain was evaluated with the Euro Quality of Life-5-dimension tool.6 Such results may be explained by a CIRP trajectory that is population-specific. In a multicenter cohort study, 40% of patients admitted to a cardiac ICU reported chronic pain at 3 months; this rate was decreased to 9.5% at 2 years.7

Etiology and risk factors

“There is evidence for the presence of nociceptive, neuropathic, and nociplastic pain in ICU survivors,” noted the review authors.1 Studies have examined whether undertreated acute pain may underlie CIRP, but evidence supporting this notion is insufficient. Other studies have linked sepsis with the development of chronic pain. “The inflammatory state associated with sepsis…mediates neurological damage, as demonstrated in critical illness polyneuropathy,” note the review authors.1 “Many of the small molecules important in inflammation, including neuropeptides and cytokines, are pro-nociceptive [and] could lead to the development of chronic pain through altering membrane excitability, descending nociceptive control, and synaptic plasticity.”

In addition, emerging evidence indicates that small nerve fiber abnormalities may have a role in the development of chronic ICU-related pain. Small fiber dysfunction was observed in 42.5% of ICU patients 6 months after discharge and was associated with higher pain intensity and pain-related disability as well as with lower health-related quality of life.8