Acute vs. Chronic Pain: Understanding the Differences

You're wondering why your pain is lasting longer than expected. Maybe you've been told that it has become "chronic," and that word worries you. Does that mean it's forever? The term "chronic" describes pain that has lasted or recurred for more than three months. It doesn't mean that your pain will always stay the same.

Understanding your pain helps you choose the right care. Duration is important, but so are the cause, your activities, and the impact of the pain on your life. Certain mechanisms and treatments may apply to both acute and chronic pain.

• Acute pain may cause you to protect an injured area. In some people, increased sensitivity of the nervous system contributes to persistent pain. Duration alone is not enough to identify this mechanism.
• The choice of treatment depends on the health issue, your needs, and how long the pain has lasted.
• Chronic pain can improve; its progression varies depending on the individual and the health condition.

This guide helps you understand your pain and how to manage it. To delve deeper into the mechanisms of persistent pain, consult our guide on chronic pain.

What is the difference between acute and chronic pain?

Acute pain is recent, while chronic pain has persisted or recurred for more than three months. The mechanisms may overlap. Chronic pain may accompany a persistent illness or changes in the nervous system, among other factors.

Duration helps classify pain, but it does not measure its severity or its impact on your life. The assessment also takes into account the possible cause, your overall health, and what you are able to do.

A sprained ankle, a back sprain, or a sudden awkward movement can cause acute pain. This pain may prompt you to protect the area and seek medical care. If the pain persists, a re-evaluation can help determine what is causing the problem.

How do medical definitions classify pain over time?

Medical definitions classify pain as acute, subacute, or chronic based on its duration. The ICD-11 medical classification defines chronic pain as pain that has lasted or recurred for more than three months.¹ ICD-11 is the eleventh version of the International Classification of Diseases. This definition comes from the International Association for the Study of Pain (IASP). Duration alone does not describe all the mechanisms underlying your pain.

Categories by Duration

The following guidelines are from the 2022 U.S. Centers for Disease Control and Prevention (CDC) guide. The definitions of the acute and subacute periods may vary among different guidelines.

These categories provide a common framework for professionals. They do not account for your recovery time and are not sufficient on their own to determine a course of treatment.

Primary Chronic Pain and Secondary Chronic Pain

Chronic pain encompasses several medical categories. Two main groups help explain this classification.

Secondary chronic pain is associated with an underlying health problem, such as a disease or nerve injury. An injury is damage to tissue. Certain types of pain that persist after surgery or trauma also fall into this category.¹

A diagnosis of primary chronic pain requires pain lasting more than three months that is not better explained by another medical condition. This pain is also accompanied by at least one of the following:

• Significant emotional distress, such as severe anxiety related to pain;
• A significant limitation in activities or participation in social life.²

This distinction helps in choosing treatment. It does not mean that primary chronic pain is imaginary or caused solely by emotions. Physical, psychological, and social factors can contribute to it.²

What protective role can acute pain play?

Acute pain may prompt you to protect a certain area, modify an activity, or seek medical care. However, its intensity does not directly indicate the severity of an injury.

The protective role of acute pain

When you touch a hot surface, a reflex may cause you to pull your hand away before you consciously feel the pain. This reflex travels through the spinal cord, which transmits messages between the brain and the body. The pain you feel afterward may prompt you to protect your hand. This process is described in a chapter on the detection and transmission of nerve signals.

Nociception refers to the nervous system’s detection and processing of stimuli that can damage tissue. Pain is a distinct experience, both sensory and emotional: it encompasses what you feel in your body and how that experience makes you feel. Pain is not a direct, automatic interpretation of nerve signals.³

What is the relationship between pain and tissue repair?

Pain and tissue repair progress at different rates. Pain intensity and medical imaging alone are not sufficient to assess the condition of the tissues.

What happens during healing

Tissue repair involves several overlapping stages. In a skin wound, the cessation of bleeding precedes or accompanies the onset of repair. Inflammation, the formation of new tissue, and the reorganization of tissue then play different roles over time.

Inflammation mobilizes cells that remove debris and contribute to tissue repair. It may be accompanied by pain, warmth, and swelling.

New cells and tissues form. Sensitivity may change during this period, but it alone does not indicate the strength of the tissues.

The remodeling phase: reorganizing the tissues. The tissues reorganize as the healing process progresses.⁴ These phases overlap, as explained in this review of the cells involved in repair. Their duration varies depending on the tissue, the injury, and the individual. Pain and healing do not always proceed at the same pace.

Pictures alone are not enough to explain the pain.

Degenerative changes are alterations in tissue that are often related to aging. They can be detected on an MRI, a test that produces detailed images using a magnetic field. In a study of 3,369 people followed for six years, most of the changes observed in the lower back were not associated with future pain. At the initial examination, the pain scores associated with the various findings ranged from 0.06 to 0.83 out of 10. One exception was people without initial pain who had at least five types of changes: their pain increased more over six years, with an average score of 1.21 out of 10. These associations do not prove that a visible change causes a person’s pain. The images must be interpreted in conjunction with the patient’s symptoms and examination findings.⁵

A lumbar disc herniation occurs when part of the contents of a disc in the lower back protrudes beyond its normal boundaries. Some herniations resolve without surgery. In a review, nine studies provided data to calculate these rates: resolution was observed in 96% of herniations with a detached fragment and in 70% of herniations in which the contents had protruded without being completely detached. Complete resolution was less common, occurring in 43% and 15% of these same groups, respectively. The type of herniation alone does not determine the severity of your symptoms. These results do not predict the course of the condition for each individual.⁶

Why does pain persist even after healing has occurred?

Pain may persist after an injury has healed when changes in the nervous system and other factors continue to contribute to it. Increased sensitivity of the nervous system is one possible mechanism; it cannot be inferred from duration alone. An ongoing illness or injury can also perpetuate chronic pain.

The paradox of chronic pain

Tissue healing and the disappearance of pain do not always occur at the same time. Even if an injury has healed, persistent pain still requires an explanation tailored to your situation.

Hypersensitivity refers to an increased response in certain pathways of the nervous system. It can contribute to heightened sensitivity to touch, pressure, or movement. It does not, on its own, explain all cases of chronic pain.

A smoke detector that has become overly sensitive may go off in the presence of cooking steam. This image helps explain how a response can become more intense. It does not prove that your pain is a false alarm or that there are no underlying medical issues that need to be treated.

The two levels of sensitization

Nociceptors are nerve endings that respond to certain stimuli that can damage tissue. They can become more sensitive. A review on chronic lower back pain describes how inflammation can lower their activation threshold: even a weaker stimulus is then enough to activate them.⁷

Certain nerve pathways in the spinal cord and brain may respond more strongly to the signals they receive. This heightened response is called central sensitization.⁸ An image showing increased volume helps illustrate this change, though it cannot be used to diagnose it in a person.

Stimulation may then become more painful, or stimulation that is usually painless may start to hurt. The pain is still real, even when it does not indicate a new injury.

In some people, this heightened response may persist after the initial problem has resolved. Central sensitization mechanisms are reversible.⁸ This capacity for change offers hope for improvement, but it does not guarantee that every case of chronic pain will resolve or that recovery will occur within a specific timeframe.

How can you tell if pain is likely to become chronic?

Fear of movement, an increasing tendency to avoid activities, and constant focus on pain may be associated with a risk of long-term difficulties.⁹ These factors alone do not predict how your pain will progress. Support from loved ones, conflicts, and financial difficulties can also affect your experience and your access to care, as explained in this review of social factors and pain.

Factors to Discuss During the Evaluation

There are several factors that can make living with pain more difficult. Recognizing them helps tailor your care; they do not mean that you are responsible for your pain persisting.

• Fear of movement (“If I move, I’ll hurt myself even more”);
• Recurring thoughts about the worst-case scenario, accompanied by a feeling of being unable to cope—sometimes referred to as catastrophizing;
• Concerns about physical strength (“my spine is weak”);
• Stress, anxiety, or depression that require support.

• Prolonged avoidance of activities beyond the restrictions necessary for your condition;
• A prolonged period of rest without a reassessment of permitted activities;
• Alternating between periods of very intense activity and long periods of inactivity caused by symptoms.

• Lack of social support
• Conflicts at work or home
• Financial difficulties due to incapacity

When to Have Persistent Pain Re-Evaluated

Have your condition reevaluated if you notice any of the following changes. The urgent medical signs described below require immediate or prompt care—do not wait several weeks.

• Your pain shows no signs of improving after several weeks;
• You are increasingly avoiding activities because of pain
• Your mood significantly worsens
• You have persistent sleep problems
• You are needing more and more medication without seeing any improvement. Talk to your doctor or pharmacist before increasing your dose.

Can complex pain be acute?

Acute pain can become complex as early as the first few weeks. Its duration does not fully reflect all of its effects. For example, a person who has been in pain for two weeks may already experience:

• Significant emotional distress
• Marked avoidance behaviors
• Deep-seated negative beliefs

In these cases, an early intervention that takes into account pain, emotions, and difficulties with daily activities helps define care needs. It does not guarantee that the pain will not become chronic.

Conversely, a person may have been experiencing pain for six months and still be able to carry on with their activities, with few limitations and a good quality of life. Their pain is considered chronic based on its duration, even if its effects on daily life remain limited.

What treatments can help when pain persists?

Treatment for persistent pain includes appropriate activities, education about pain, and support for sleep and well-being. These measures address your current needs; they do not guarantee that you will avoid chronic pain.

Act early, act effectively

Adjust your movements and activities to accommodate your health condition. Some injuries require temporary protection. Ask which activities you can continue and how to resume them gradually.

A trial compared three approaches among 186 employees with recent low back pain and no specific identified cause. Continuing normal activities as tolerated resulted in better recovery at three and twelve weeks than two days of bed rest or the specific back mobilization exercise program studied. The results specifically addressed pain, daily functioning, and days missed from work.¹⁰ This comparison does not automatically apply to all injuries or all exercises.

Understanding the mechanisms of pain can help you discuss your fears and possible activities. A review of five trials, involving 460 people with chronic muscle or joint pain, compared pain education combined with exercise to exercise alone. The combination led to greater short-term improvements in pain, functional ability, fear of movement, and catastrophic thinking. The certainty of the results was moderate for fear of movement, low for pain and functional ability, and very low for catastrophic thinking.¹¹ These results covered a period of up to twelve weeks and do not demonstrate the prevention of chronic pain following recent pain. To learn more about this topic, see our article on understanding chronic pain.

Emotions, sleep, and pain can interact. In a small study involving 36 healthy adults, a group experiencing muscle soreness and a sleepless night became more sensitive to pressure than the control group without muscle soreness. However, the groups did not report any clear difference in the perceived intensity of their muscle soreness.¹² This experiment does not allow us to attribute all pain to sleep deprivation. A review of chronic pain associated with nerve damage also describes the benefits of integrating psychological support with other forms of care, as needed.¹³ Talk to your healthcare provider about sleep disorders, anxiety, or depressive mood.

Seek medical advice if the pain does not improve or limits your activities. A physical therapist can assess your condition, tailor your treatment, and refer you to another professional if necessary. The following signs require special medical attention.

Signs That Require Medical Attention

• A new weakness that is progressing rapidly, especially in the legs;
• If you have back pain, new difficulty urinating, loss of bladder or bowel control, or numbness between your legs.

These symptoms may indicate severe compression of the nerves in the lower back. A review on cauda equina syndrome emphasizes the importance of an urgent evaluation. Do not wait for a routine physical therapy appointment.

• A progressive loss of strength, even if it occurs more slowly;
• An unexplained fever accompanied by pain, especially if there is significant discomfort or a red, warm, and swollen area;
• Unexplained weight loss;
• Intense nighttime pain that wakes you up;
• New pain, or pain that changes if you've had cancer before.

These signs alone are not sufficient to make a diagnosis. A professional interprets them in the context of your overall situation.

• Pain varies with your activities
• You have pain-free moments
• Pain gradually decreases, even if slowly
• You can function despite the discomfort

A gradual decrease in pain and a return to normal activities may indicate a favorable prognosis. It is also helpful to describe pain-free periods or how the pain varies depending on activity. None of these factors rule out a medical emergency. Muscle or joint pain may improve over time with appropriate care, but the course of the condition varies.

What should we take away from this?

Chronic pain lasts or recurs for more than three months, and it can improve. Its duration alone does not indicate its cause, severity, or impact on your life.

• The three-month threshold defines chronic pain. The diagnosis of primary chronic pain includes additional criteria.
• Increased sensitivity of the nervous system may contribute to persistent pain. Certain nerve pathways then respond more strongly to stimuli. This mechanism cannot be inferred from duration alone.⁸
• The nervous system retains the ability to change. This ability, known as neuroplasticity, involves changes in the way nerve cells respond and communicate. The reversibility of certain mechanisms does not guarantee that every chronic pain condition will disappear.⁸
• Treatment takes into account fear, avoidance, and stress. These factors help tailor care, though they do not guarantee the prevention of all chronic pain.
• Complex pain can be acute. Its effects on emotions and daily activities can be significant as early as the first few weeks.

If you are experiencing persistent pain affecting your back, neck, shoulders, or knees, consult a professional who can assess your overall situation and guide you towards a suitable approach.

Where can I find more resources on pain?

Physioactif's resources explain the mechanisms of pain, pain management strategies, and the psychological aspects of chronic pain.

• Understanding Chronic Pain: A Complete Guide
• The Neuroscience of Pain
• Pain Management Strategies
• The Psychology of Pain

References

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