The Nature of Pain

In the first of three articles on pain the nature of pain is seen to be complex and, in many respects, poorly understood.

Wouldn’t it be great if we couldn’t feel pain?

Well, no it wouldn’t. Our ability to perceive pain has evolved as a sophisticated response to our environment. Pain warns us that physiological damage is happening or has happened. It helps us to learn about situations and events likely to cause damage and even if we are injured pain serves to identify limit our activities until we have recovered. We do however have an insight into what our worl would be like without pain. In rare cases of congenital universal insensitivity to pain (CUIP), people are born with an inability to feel pain. Miss C, a Canadian and sufferer of CUIP, helped with experiments to try and understand the condition. Despite exhaustive investigations no cause could be found. Miss C died at the age of 29, after developing infections due to joint injuries which she could not feel. Even after death her post-mortem revealed no physiological abnormalities. CUIP remains a mystery to this day.

The case of Miss C is actually only one of several puzzles about pain. The sudden loss of a limb may result in a state known as episodic analgesia. Here the victim is perfectly aware of what has happened, but feels no pain at the site of the injury. The onset of analgesia (pain relief) is instant but only lasts for a limited time. This can result in the curious effect of the victim whincing at the minor pain of a tetanus injection whilst being unaffected, at least for a time, by the severity of limb loss.

Sometimes pain occurs after injury to the body has repaired itself. Neuralgia, for example, is a sudden intense pain that follows the track of a nerve. It is associated with nerve damaging diseases like shingles although, curiously, the pain starts after the disease has ended.

The vast majority, if not all people, suffer at some time with headaches. Headaches are a familiar if unwelcome feature of life and can often be traced to stressful events, over indulgence of alcohol, staring at this screen for too long and so on. Yet relatively little is known about the mechanism of headaches such as migraine which were originally thought to be due to dilation of blood vessels. However, it is now known that blood vessel changes are a result, not a cause of migraine. Headaches however stand as the prime example of how we experience pain without injury.

Whilst we tend to associate pain with the severity of injury this is not always the case. Certain cancers cause profound injury to the body but produce little pain until quite advanced. Yet, a paper cut on the finger can cause immediate discomfort. Likewise a damaged dental nerve can produce agonising levels of pain.

So-called phantom pains occur after the loss of a limb. People may try to walk on a phantom leg for example because it still feels so real. Phantom arms may feel as if they are hanging by the persons side. People with spinal injuries may also experience phantoms and may report the fatiguing sensation of cycling movements with their legs. Phantom limbs can appear to get stuck in awkward position and around 70 per cent of phantoms have a wide range of sensations such as heat, cold, itchiness and pain. An explanation for this is that the brain contains a kind of neural map of the body. When a limb is lost, the brain still regards it as being in place and tries to accommodate messages from nerves in way it can make sense of. This results in the sensation of limbs being intact.

Whilst a lot is known about pain it is clear that a great deal remains unknown. In the second of the articles on pain we explore psychological models that have been constructed to help explain the pain process. The third article looks at options in the treatment of pain.