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We have read with interest the article entitledUnilateral focal lesions in the rostrolateral medulla influence chemosensitivity and breathing measured during wakefulness, sleep, and exercise by Morrellet al,1 which shows that unilateral ischaemic lesions of the rostrolateral medulla may lead to an abnormal ventilatory CO2 response and sleep apnoea. We have recently conducted a similar study on five patients with syringobulbia. Syringobulbia has a predilection for autonomic nuclei of the cardiorespiratory network localised in the caudal medulla and this may cause severe respiratory and cardiovascular abnormalities.2 On occasion, a syrinx may extend to the rostral and ventral medulla.3
Out of five patients with syringobulbia studied with MRI, ventilatory CO2 response and polysomnography, one showed bilateral syringomyelic cavities in the caudal dorsal medulla with unilateral extension to the rostrolateral medulla (figure). This 40 year old patient showed the following respiratory abnormalities: end tidal CO2, 47.3 mm Hg; p 0.1, 0.21; ventilatory CO2 response, 1.78 l/mm Hg; apnoea index, 52 events/hour of sleep, with a total number of 212 obstructive sleep apnoeas, four central apnoeas, and 39 hypopnoeas. Maximal duration of obstructive sleep apnoeas was 125 seconds and oxygen saturation values during apnoeic episodes lower than 50%. There was also evidence of severe autonomic dysfunction with orthostatic hypotension, arterial hypotension at rest, and complete loss of sinus arrhythmia. Despite the severity of the respiratory abnormalities recorded, the patient refused to receive any respiratory support, and to date has not developed any cardiorespiratory complication during a follow up of 9 years.
It seems that extension of the syrinx to ventral and rostral medullary areas may lead to more severe respiratory and cardiovascular abnormalities. Three stages in the progression of syringobulbia may be described in involvement of autonomic and respiratory structures: (1) initial involvement of the caudal and dorsolateral medulla with damage to the nucleus tractus solitarius, vagal motor nucleus, and nucleus ambiguus which may impair cardiovascular reflexes; (2) ventral extension with involvement of the intermediate reticular formation; and (3) further ventral and rostral extension to the anterolateral surface of the rostral medulla, involving vasomotor neurons and central chemoreceptors. These last two stages may be accompanied by severe respiratory abnormalities and arterial hypotension.
We have encountered similar difficulties to those described by Morrellet al 1 in outlining small lesions in the medulla by using MRI. Unless the cervicomedullary junction is studied with thin slices, small cavities may be easily overlooked. In such patients, chest and abdominal movements due to respiratory difficulties contribute to poor MRI definition.
The authors reply:
We were interested to read the paper by Nogueset al,1-1 and we are grateful to the authors for bringing it to our attention. One of the aims of our study was to investigate the effect of unilateral lesions in humans at sites previously defined as being important for chemosensitivity in animals. For this reason we studied patients with relativity small focal lesions in the rostrolateral medulla.1-2 Nogueset al have investigated the effect of syringomyelia and syringobulbia on chemosensitivity and breathing during sleep; the MR images of their patients show the lesions to be more extensive than in our study. Nevertheless, both papers highlight the gross sleep apnoea in these patients and the importance of carrying out nocturnal polysomnography to identify any abnormalities in breathing during sleep; this point is emphasised in an excellent editorial by Malow.1-3
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