Centrally mediated hypoventilation causes respiratory failure without respiratory distress. We present a case of recurrent acoustic neuroma at the cerebellopontine angle causing acute and chronic respiratory failure. Tumor resection eliminated recurrence of respiratory failure.
Respiratory failure has not been associated with acoustic neuromas, which typically present with hearing loss, tinnitus, dizziness, headache, and facial paresthesias.[1,2] We describe a patient with recurrent acoustic neuroma without increased intracranial pressure who presented with episodes of hypoventilatory respiratory failure. The patient's ventilatory drive was altered by tumor compressing the brain stem, and was restored with removal of the tumor.
CASE REPORT
A 74-year-old woman presented with fatigue, cyanosis, leg edema, and intermittent confusion. Eight years prior to hospital admission, she had undergone partial resection of a right acoustic neuroma. Six months prior to admission, her family noted the onset of hypersomnolence. She had never smoked cigarettes and had no history of lung disease. She was 155 cm and weighed 85.5 kg. She was alert and oriented, in no distress, but became confused at night. Her lungs were clear on auscultation, and cardiac examination showed no gallop or right ventricular lift. There was mild pedal edema. The only focal neurologic findings were longstanding, postoperative right facial and cochlear nerve defects. Arterial blood gases on room air showed a pH of 7.38, [PCO.sub.2] of 65 mm Hg, and [PO.sub.2] 25 mm Hg. The hematocrit value was 48 percent. Thyroid function was normal. Chest radiograph was normal. Echocardiogram revealed right ventricular hypertrophy and tricuspid regurgitation, with good left ventricular systolic function. Spirometry demonstrated only mild obstructive lung disease ([FEV.sub.1], 1.36 L [75 percent predicted]; FVC, 1.88 L, [81 percent predicted]; FEF25-75, [54 percent predicted]). The patient breathed shallowly during sleep, but never became apneic. Despite diuretics, bronchodilators, and oxygen therapy, over the next 3 months she had repeated episodes of respiratory failure with hypersomnolence, slurred speech, fluid retention, and asterixis. Multiple observers noted the lack of distress despite respiratory acidosis and hypoxemia. A magnetic resonance scan revealed residual tumor adjacent to the medulla causing midline shift (Fig 1). There was no hydrocephalus.
The patient underwent a subtotal resection of a well-encapsulated acoustic neuroma (schwannoma). Blood gases prior to operation were a pH of 7.38, [PCO.sub.2] of 64 mm Hg, and [PO.sub.2] of 107 mm Hg on 50 percent Venturi mask. Postoperatively, the patient immediately required less oxygen, and blood gas values at hospital discharge showed a pH of 7.48, [PCO.sub.2] of 43 mm Hg, and [PO.sub.2] of 70 mm Hg on room air. Two months postoperatively, the patient's mental status was normal, room air blood gas values were near normal (pH, 7.43, [PCO.sub.2] 48 mm Hg; [PO.sub.2] 65 mm Hg), and she was independent in all activities.
DISCUSSION
We believe this case illustrates tumor compression of the brain stem causing central hypoventilation. The patient's lack of dyspnea despite severe hypoxia and acidosis delayed medical attention until mental status changes had occurred.
Respiratory neurons initiating inspiration and expiration have been localized to the medulla: the so-called dorsal respiratory group within the solitary nucleus, and two ventral respiratory groups, lying within the retroambigualis and ambigualis nuclei.[3,4] In our patient, motoneuron functions originating in the ventral respiratory group were probably intact because ventilation could be restored transiently by medical treatments. Therefore, we postulate that the recurrent acoustic neuroma interfered primarily with dorsal respiratory group functions responsible for the ventilatory responses to hypercapnea and hypoxemia. It is notable that the tumor, despite its size and location, did not interfere with other voluntary or autonomic functions.
To our knowledge, this is the first description of an acoustic neroma causing central hypoventilation. Brainstem tumors are known to cause hypoventilation, but do so in the setting of increased intracranial pressure and in association with cranial nerve abnormalities.[5] Our patient had none of these findings, only an apparently isolated defect in central respiratory drive. Our patient's global neurologic findings were entirely attributable to hypercapnea and hypoxemia, as they resolved with removal of her tumor. A brain-stem tumor should be considered among the reversible causes of centrally mediated respiratory failure, even in the absence of focal neurologic findings.
REFERENCES
[1] Harner SG, Laws ER. Diagnosis of acoustic neurinoma. Neurosurgery 1981; 9:373-79
[2] Traquina DN, Guttenberg I, Sasaki CT. Delayed diagnosis and treatment of acoustic neuroma. Laryngoscope 1989; 99:814-18
[3] Berger AJ, Mitchell RA, Severinghaus JW. Regulation of respiration (part 2). N Engl J Med 1977; 297:138-43
[4] Mitchell RA, Berger AJ. Neural regulation of respiration. Am Rev Respir Dis 1975; 111:206-24
[5] Hughes JMB. Central respiratory failure reversed by treatment. Brain 1967; 90:675-80
COPYRIGHT 1994 American College of Chest Physicians
COPYRIGHT 2004 Gale Group
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