Lesson of the Week: False localising signs in the spinal cord

Other falsely localising symptoms or signs that may occur in spinal cord disease include low back and leg pain simulating lumbar disc disease but actually caused by cervical cord compression 1 and wasting of the intrinsic muscles of the hands innervated by the T1 segment yet caused by a lesion at the foramen magnum. 2 3 Benign extramedullary tumours at the foramen magnum are notorious for producing signs which do not accurately identify the anatomical site of the tumour, with sensory loss below C5 occurring in 15% of patients and atrophy localised to the hands in 13% of patients. 3 A sensory discrepancy has been well described in cervical disc herniation, 4 5 and in one series of five patients with painless cervical myelopathy the sensory level was consistently below T5. 6 This has been explained on the basis of pain and temperature fibres crossing obliquely to reach the contralateral spinothalamic tracts two or three segments higher. 7 Although this is true, it accounts only for sensory levels a few segments below the lesion, and does not explain the large discrepancies of up to nine segments below the lesion that have been described in cervical myelopathy, 6 of seven segments in other forms of cord compression, 3 or of 11 segments in our case. Ischaemia after arterial occlusion is a further unsatisfactory explanation, because anterior spinal artery compression would not produce a complete lesion of the cord, and there is no supporting evidence for this mechanism from pathological studies. 8 A more plausible hypothesis is that the myelopathy is caused by venous obstruction with ensuing stasis, resultant hypoxia, and subsequent loss of anterior horn cells. 2 Features of anterior horn cell damage rather than root compression at the level of the clinical signs have been shown by electromyography, 9 seen pathologically, 10 and induced experimentally at sites distant from the cord lesion. 2 The clinical neurophysiological signs in the case described above also suggest anterior horn cell damage far removed from the lesion.