Silas Weir Mitchell (1829-1914), justly regarded as one of the founding fathers of neurology, published accounts of phantom limbs in the 1860s and 1870s,1 around the time that  neurology was emerging as an independent clinical discipline (although earlier accounts of phantom limbs are recognised2). Phantom limbs are most often observed in the context of amputation, but reports of extra limbs occurring without amputation have also appeared. Two brief cases are presented here to illustrate the clinical heterogeneity of the supernumerary limb, the possible pathophysiology of which is briefly considered.

Case 1

A 59-year-old right-handed lady was referred to the neurology clinic with a history of three stereotyped episodes occurring over a 5-month period in which she had the sensation of having two arms and two hands on the left hand side, these symptoms lasting between 30 minutes and 2 hours.  She was not aware of any spontaneous movement of this extra limb, which was not visible to her, nor could she move it voluntarily.  She had a longstanding history of anxiety and her psychiatrist thought that the symptoms might be a reflection of this.  Neurological examination was normal.  MR brain imaging showed some high signal white matter changes.  As it was not possible to say with certainty whether these changes were ischaemic or inflammatory she underwent lumbar puncture which showed normal CSF contents with no unmatched oligoclonal bands.  She was then symptom free for around 18 months, when a further cluster of similar episodes occurred.  Repeat MR brain imaging showed no change from previously.  In the absence of a structural or inflammatory lesion, the working neurological diagnosis was somaesthetic migraine aura (both her children had migraine).  About six months later she started getting multi-coloured flashes in her vision and was seen by an ophthalmologist who found no ocular pathology and thought that the visual symptoms were likely to be migrainous.

Case 2

A 55-year-old lady (previously reported in abstract3) with acute motor and sensory axonal neuropathy which had required ventilatory support developed the sensation of two extra arms and legs during the prolonged recovery phase.  More than 6 months after the acute onset of neuropathy, nerve conduction studies showed uniform absence of sensory responses, and likewise absence of lower limb distal and proximal motor responses.  Median and ulnar nerve motor responses were markedly reduced (<1mV) with increased latencies and reduced velocities.  Awareness of the phantom limbs could be reduced by visualising her normal limbs.

The phenomenon described in these cases has been known by various names, including supernumerary phantom limb(s) (SPL), reduplication of body parts, and pseudo(poly)melia.  As these cases show, the phenomenology of SPL is heterogeneous: symptoms may be transient/episodic or persistent/prolonged, and may involve single or multiple limbs, or even other body parts (such as teeth4).  In most cases, SPL has only somaesthetic characteristics but in some cases the limb can be seen and voluntarily moved (i.e. multimodal SPL).   SPL has been described in association with stroke affecting the right hemisphere5,6 and in episodic form in epilepsy.7,8  A possible migraine-related case is that of Todd and Dewhurst,9 although their patient also had a diagnosis of epilepsy.  SPL has been described in acute inflammatory demyelinating polyneuropathy.10  Cancellation of SPL by vision of the real limb is reported.

The pathophysiology of phantom limbs has attracted much attention in recent years.  They have been variously regarded as delusional beliefs or hallucinatory perceptions,11 or as disorders of the body schema resulting from a failure to integrate neural impulses initiating motor action and from sensory feedback.12  The profound deafferentation indicated by the neurophysiological findings in Case 2, along with impaired motor conduction, would be in keeping with such an explanation.  Spatial distortions of body size occurring in migraine (macro- and microsomatgnosia, also known as “Alice in Wonderland” syndrome) have also been attributed to “pathologies of sensory input”,12 so a similar mechanism of failure to integrate motor command and sensory feedback might be occurring in Case 1.

Whilst structural reorganisation, with reinnervation of deafferented sensory cortex from other cortical regions, may occur in persistent cases of SPL associated with brain injury (e.g. after stroke), as has been suggested for phantom limbs in amputees,13 this may not be a necessary feature for the development of SPL, as suggested by transient forms.  Functional neuroimaging (fMRI) in a patient with a poststroke (subcortical capsulolenticular haemorrhage) SPL with somaesthetic, visual and intentional motor components showed modality-specific activations in motor, visual and somaesthetic areas, interpreted as cortical deafferentation from the subcortical lesion.14  fMRI was also undertaken in Case 2 during a motor paradigm task, which showed activation within the primary motor and supplementary motor areas only,3 as might be anticipated with an exclusively somaesthetic SPL.

References

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