The prognosis of obstetric brachial plexus palsy (OBPP) has been considered as being very good, with reports of complete or almost complete spontaneous recovery in the majority of patients. However, a systematic review1 showed that this claim was not based on scientifically sound evidence. A recent prospective study reported that one in five children with OBPP had permanent neurological deficit at 18 months of age2 and that this was the case even after surgical repair. The assessment of function in many studies of OBPP, whether of natural recovery or recovery after surgery, has tended to focus on motor activity, such as the degree of active range of motion and strength. In contrast, few studies have reported on sensory recovery, and the general conclusion is that sensibility is less affected than motor function. It is highly desirable to test sensation, as somatosensory feedback is critical for good hand function. Moreover, adults with OBPP have reported many problems, including pain, impaired sensation, and limitation of function, which interfered with activities of daily living.3
For these reasons, the study of adults with conservatively managed OBPP by Anguelova et al.4 is timely, and suggests that the commonly held view that sensory recovery is good in people with OBPP should be revised. A systematic investigation by this group of sensory function in adults with OBPP has shown that sensory discrimination, assessed using Semmes-Weinstein monofilaments and two-point discrimination, was worse in people with OBPP than in a comparison group. However scores for object recognition and locognosia were similar between groups. These findings are explained by suggesting that in OBPP, reinnervation almost always occurs to some degree.
Because OBPP occurs at a critical period, it is salutary to reflect on the substantial changes that take place in the central nervous system during infancy. Developing sensory and motor systems expect specific experiences through the environment to help shape their connectivity. Activity-dependent mechanisms during critical periods in postnatal life are key factors in development of sensory and motor systems. Information travelling from the receptors in the skin to the cortex maintains an orderly relationship of neighbouring sites in the periphery. This results in maps of the body surface in the component nuclei of the somatosensory system: i.e. the cuneate and gracile nuclei in the brainstem, and the ventroposterolateral nucleus of the thalamus, as well as in the primary somatosensory cortex, areas 3b, 1, and other areas. Neuronal activity is required for topographical specification and refinement of these maps. Thus a loss of (or disordered) peripheral sensory input might be expected to lead to disordered central representations. This is certainly the case in adult monkeys following a peripheral nerve lesion. However, such a lesion in infant monkeys does not appear to affect cortical representations of the hand in primary somatosensory cortex.5 Receptive fields of cortical neurons after nerve regeneration are larger than those typical of normal animals and may be spread across multiple digits. However, after nerve repair in fetal animals, the receptive fields of most of the neurons in area 3b are of normal size although they usually have a larger surrounding zone of weaker activation.5
The recovery of perceptual abilities after OBPP may well be related to the preservation of the central sensory representations though these may not be precise. Post-injury experience is a potent modulator of recovery,6 therefore sensorimotor experiences in the postnatal period are likely to have a profound effect on residual function in the affected arm.