phantom limb

phantom limb After loss of a limb more than 90% of patients experience a vivid illusory persistence of the limb in the form of a ‘phantom’. The phantom appears immediately in the majority and after a delay of a few days or weeks in the rest. It can then persist for months, years, or even decades.

The phantom often has a ‘habitual’ position (for example a phantom arm may be partially flexed at the elbow with the forearm pronated) but spontaneous changes in its posture are common. The extent to which voluntary and involuntary movements occur in the phantom varies from patient to patient.

Phantoms can appear not only for a limb, but for almost any body part: breast, penis, face, jaw, or even for internal organs. Patients can experience phantom menstrual cramps after hysterectomy, the spasmodic pain of appendicitis after appendectomy, or ulcer pains after gastrectomy; even phantom erections have been reported after the penis is removed.

The term ‘phantom limb’ was coined in 1872 by Silas Weir-Mitchell, who published the first paper on this subject anonymously for fear of ridicule by his peers. Since then a fascinating clinical lore has built up and there have been hundreds of case studies. But a systematic scientific study began only towards the end of the twentieth century. Animal studies have combined with systematic psychophysical testing and brain imaging in human amputees, to move the study of phantom limbs from vague clinical phenomenology into an era of experimental research.

The phantom is enhanced by the presence of referred sensations: stimuli applied to other parts of the body that are experienced as arising from the phantom. For example, after arm amputation, touching the face will often evoke precisely localized sensations in the phantom fingers, hand, and arm. The points that evoke such sensations are topographically organized (consistently ‘mapped’) and the referral is modality-specific, meaning for instance that heat on the face will elicit heat in the phantom digits and that vibration is felt as vibration. This face-to-phantom-hand referral probably occurs because the face is right next to the hand in the complete map of the whole body's skin surface on the somatosensory region of the cerebral cortex (described as a result of electrical brain stimulation studies by neurosurgeon Wilder Penfield in the 1930s).

The sensory input from the face skin ordinarily activates only the face area of the cortex, but if the adjacent hand cortex is denervated (cut off from any sensory input), then the input from the face starts activating the original hand area as well. This is a striking demonstration of plasticity in the adult human brain: that new neural links can be made. The observation also implies that even though the hand area is now being activated by sensory input from the face skin, higher brain centres still continue to interpret the signals as arising from the hand. Changes in somatosensory cortex topography — occurring over distances of 2–3 cm — have also been shown in the same patients using functional brain imaging techniques — especially magnetoencephalography (MEG); this allows researchers to correlate perceptual phenomena described by the patient (such as referred sensations) with the anatomical sites of activity.

These demonstrations of ‘plasticity’ in the adult brain can also be seen in monkeys in which one arm has been deafferented (all sensory nerve pathways interrupted). Indeed, the human studies were inspired by the animal experiments.

Sensations may also be referred the other way around between the hand and the face, after the trigeminal nerve that supplies the face is cut — an occasional last resort for severe neuralgia. The patient then has a map of the face on the hand. Again, after leg amputation, stimuli applied to the genitals are referred to the phantom foot. This is consistent with the representation of the foot next to the genitals in Penfield's original maps of the somatosensory cortex.

Vivid ‘movements’ in the phantom are reported by some patients. These sensations are very ‘real’ to the patient — so much so that volitional movements of the phantom hand can interfere with a dissimilar movement performed by the normal hand, in a manner identical to the interference between hands that occurs in normal people. The patient cannot for example rub his belly with his real hand while ‘tapping his head’ simultaneously with the phantom. These movement sensations in the phantom probably arise from ‘feed forward’ or corollary discharge: when the motor areas of the patient's cerebral cortex send a command to the missing arm, a copy of the command is sent to the cerebellum and parietal lobes so that intention can be compared with action. These commands may initially be experienced as movements, but the prolonged absence of visual confirmation, and of sensory input from muscles and joints of the missing arm, may lead eventually to a ‘paralysed’ phantom that the patient can no longer move.

Sometimes the phantom will develop a painful clenching spasm and the patient cannot voluntarily ‘unclench’ his imagined fist even with intense effort. If a mirror is propped up vertically on the table, in the plane that separates the right from the left half of the body, and if the patient views the reflection of his normal hand in the mirror, the reflection of the hand is seen superimposed on the felt position of the phantom — giving the visual illusion that the phantom has been resurrected. If he now moves the normal hand the phantom is suddenly ‘animated’ and is vividly felt to move. Sometimes this can lead to the unclenching of a previously clenched, painful phantom, suggesting a promising new therapeutic approach for phantom pain. The usefulness of the procedure requires detailed evaluation, but the illusion suggests that a great deal of interaction can occur between visual sensations and those from the limb.

Phantom limbs are also seen in a small percentage of patients with congenitally missing arms or legs, suggesting that at least the basic scaffolding for one's body image may be innately specified. Indeed, the phenomenon provides a valuable opportunity to investigate how nature and nurture interact in the construction of body image by the brain. A patient with leprosy whose hand gets whittled away gradually with progressive sensory loss does not have a phantom hand. But if the stump is then amputated, what emerges is not a phantom stump but a whole phantom hand. It is as though the original image of the hand had survived but was inhibited by the stump, only to be resurrected when the stump is amputated!

In summary, at least four factors seem to contribute to the vividness of the phantom: stump neuromas (nodules of scar tissue and curled up nerve endings); remapping of somatosensory areas in the brain leading to referred sensations; a genetically-specified ‘body image’ that partially survives limb loss; and monitoring of corollary discharge associated with motor commands sent to the phantom. The combination of systematic psycho-physical testing with brain imaging techniques in human amputees, together with animal studies on somatosensory remapping, has rapidly advanced the study of phantom limbs. Such research will allow investigation not only of how the brain remodels itself continuously in response to bodily injury, but also how the activity in the brain somatosensory ‘map’ leads to conscious experience of body image and somatic sensations.

J. Vaid, and V. S. Ramachandran

Bibliography

Melzack, R. (1992). Phantom limbs. Scientific American, 266, 120–6.
Ramachandran, V. S. (1998). The perception of phantom limbs: the D. O. Hebb lecture. Brain, 121, 1603–30.

See also amputation; body image; cerebral cortex; magnetic brain stimulation; pain; proprioception; somatic sensation.