What is pain?

What is pain, and what exactly is the worst pain a human being can bear? These sound like simple questions until we think about the people behind the pain: the patient whose cancer has invaded bone, the mother recovering from a Caesarean section, the person waking from open-heart surgery, someone doubled over with a kidney stone, or a patient with pancreatitis who cannot find a comfortable position. Migraine can make light, sound and movement unbearable, while an apparently ordinary toothache can make an ordinary day feel impossible.

Pain is not simply a signal travelling from an injured body part to the brain. The International Association for the Study of Pain defines it as an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage (Raja et al., 2020). That distinction matters. Unlike temperature or blood pressure, pain does not come with a single physical quantity that an instrument can directly read. Clinical assessment therefore still depends heavily on what the person experiencing pain reports (Mannion et al., 2007).

This makes the idea of a 'worst pain' surprisingly difficult. The most painful condition is not necessarily the most dangerous one. Cancer, for example, can threaten life in ways that a disorder producing an extremely high pain score may not. Yet some disorders demonstrate just how intense human pain can become.

When 10 out of 10 is not the whole story.

Cluster headache provides a striking example. In an international survey of 1,604 people, cluster-headache attacks received an average pain score of 9.7 out of 10, higher than every other condition examined in that study (Burish et al., 2021). Severe trigeminal neuralgia can produce sudden facial pain often described as electric shocks (Ashina et al., 2024). These findings should not be treated as an Olympic table of suffering: the populations, conditions and circumstances differ. But they show that some neurological disorders can push subjective pain close to the upper limit of a conventional scale.

Cluster headache is also more than a severe headache. It is a neurological disorder involving complex brain and trigeminal pathways, and its attacks can recur in distinctive patterns (Wei & Goadsby, 2021). Calling it simply 'a headache' can therefore hide something important: the enormous burden carried by the person experiencing it.

My sixteen-years question.

In September 2026, over twelve days, I experienced fifteen cluster-headache attacks. 'Suicidal pain' is the closest phrase I have found for the intensity. I do not use it lightly. To me, the pain can feel like a dentist drilling into a perfectly healthy tooth.

My story began in August 2010, when excruciating pain suddenly appeared on the right side of my face, beginning around the upper jaw and moving towards my right eye. I assumed something was wrong with my teeth. A dentist examined me the following day. My teeth were fine.

That was the beginning of a sixteen-year journey through consultations, medicines, uncertainty and, eventually, a diagnosis of cluster headache. Between attacks, ordinary life continued: research, teaching, family and work. During this period I completed my MPhil-PhD research at Pondicherry University and later returned to Kashmir. The headaches came with me. Perhaps that is why I keep returning to one deceptively simple question: why can we explain so much about pain, yet still struggle to measure it?

We can see the brain, but can we see the pain?

Modern medicine can measure blood pressure, glucose, oxygen saturation, temperature and electrical activity in the heart. Researchers can also use techniques such as functional MRI and PET to study brain activity associated with pain (Tracey, 2011). We can identify receptors, inflammatory pathways and neural circuits involved in nociception and pain processing. Yet when a doctor asks, 'How bad is the pain?', the answer is still often a number from zero to ten (Mannion et al., 2007).

The difficulty is that the number is not a direct physiological reading. A person may call the same experience an eight one day and a ten on another. Anxiety, expectation, previous experience and other cognitive and emotional factors can alter how pain is perceived (Bushnell et al., 2013). Context matters too, the meaning we attach to a situation can change the experience of pain (Carlino et al., 2014). A pain score is therefore clinically useful, but it is not equivalent to a blood-pressure value such as 120/80.

This does not mean that science has failed to understand pain. It may mean that pain is not a simple physical quantity waiting to be measured. Pain is produced through interactions among sensory signals, the nervous system, the brain, emotions, expectations and context. The biology can be studied without reducing the whole experience to a single number.

A warm observation from Kashmir.

One night after returning to Kashmir, probably in April 2016, an attack began in the middle of the night. I had no painkiller immediately available. Out of frustration and desperation, I placed the right side of my face near the warmth of a kangri, the traditional earthen firepot carried beneath the traditional attire, the pheran in winter.

The warmth seemed to help. The attack eventually subsided. I did not conclude that I had discovered a treatment. One attack proves almost nothing, cluster attacks end spontaneously, pain fluctuates and coincidence is always possible. But it happened again, and then again. Over the years I began noticing what seemed to be a pattern. Warmth appeared to help during some attacks, particularly at night and in winter. If I had to describe my own experience numerically, I would say it has helped roughly six times out of ten.

Today I use an electric heat blower rather than a kangri. Does it work every time? No. Does it work for everybody? I have no idea. Do I know why it sometimes seems to work? No. And that last question is precisely why I think the observation belongs to science, not as evidence that warmth is a treatment, but as a question worth testing.

Pain can be modified by physiological, emotional and contextual processes (Bushnell et al., 2013; Carlino et al., 2014). Whether temperature has a meaningful effect on cluster headache, however, cannot be established from my experience. It could be coincidence, an effect of sensory signalling, changes in blood flow, altered attention, or something else entirely. The scientifically honest answer is, I do not know.

Why a patient himsekf is still part of the measurement?

This is where patient experience has an unusual scientific value. An observation can become a hypothesis, and a hypothesis can become an experiment. A patient's experience is not scientific evidence simply because it is sincere. But it is not scientifically worthless because it is anecdotal either. It can be the starting point for a question.

We are living through an extraordinary period of biomedical science. Researchers can map genomes, edit genes, visualise neural activity and search for biomarkers that could improve pain diagnosis and treatment. Yet current biomarkers and imaging methods are not replacements for self-report; they are being developed and validated alongside it (Davis et al., 2020).

Pain research has also revealed increasingly detailed neural mechanisms, including circuits involved in pain perception and pain relief (Tan & Kuner, 2021). But explaining the mechanism is not the same as measuring the experience. A scan may show changes associated with pain, while two people with similar scans may describe very different experiences. Conversely, a person can experience severe pain even when routine tests do not reveal an obvious structural cause.

Why the question needs to change?

Perhaps science has not failed to measure pain. Perhaps we have been asking pain to behave like temperature, blood pressure or blood glucose, quantities that can be represented by relatively stable physical measurements.

Pain is different. It happens to someone. It has intensity, duration, memory, anticipation and fear. It can arrive at three in the morning and turn a familiar bedroom into an unfamiliar place. It can disappear minutes later and leave no visible scar. The absence of a visible lesion does not, by itself, make the pain unreal.

The future of pain measurement will probably not come from a single machine that replaces the patient. It may come from combining self-reported experience with neurological, physiological and behavioural measures. Researchers are already working towards objective biomarkers, but these must be carefully validated before they can become reliable measures of individual pain (Davis et al., 2020).

Until then, the familiar question remains, 'Rate your pain from zero to ten.' It is an imperfect instrument, but it has an important advantage, only the person experiencing the pain can tell us what it feels like. Behind every number is a human experience, and sometimes a story that no scale can contain.

References

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2. Burish, M. J., Pearson, S. M., Shapiro, R. E., Zhang, W., & Schor, L. I. (2021). Cluster headache is one of the most intensely painful human conditions: Results from the International Cluster Headache Questionnaire. Headache, 61(1), 117–124. https://doi.org/10.1111/head.14021

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