Massive supratentorial lesions, bilateral deep-seated cerebral lesions, and mild metabolic disturbances give rise to altered patterns of breathing, particularly periods of waxing and waning hyperpnea alternating with a shorter period of apnea (Cheyne-Stokes respiration). This phenomenon has been attributed, on uncertain grounds, to isolation of the brainstem respiratory centers from the cerebrum, rendering them more sensitive than usual to carbon dioxide (hyperventilation drive). It is postulated that as a result of overbreathing, the blood carbon dioxide drops below the concentration required to stimulate the centers, and breathing gradually stops. Carbon dioxide then reaccumulates until it exceeds the respiratory threshold, and the cycle then repeats itself. Alternatively, the periodicity has been attributed to the stimulating effect of a low arterial PO2 on a depressed respiratory center. In either case, the presence of Cheyne-Stokes breathing signifies bilateral dysfunction of cerebral structures, usually deep in the hemispheres or diencephalon, usually from intoxication or a metabolic derangement or from bilateral structural lesions such as subdural hematomas. Careful observation will disclose that the apneic part of the cycle corresponds to a diminished level of arousal. In itself, Cheyne-Stokes breathing is not a grave sign. It may occur during sleep in elderly individuals and can be a manifestation of a variety of cardiopulmonary disorders in awake patients. Only when it gives way to more irregular respiratory patterns that reflect structural damage of the brainstem is the patient in imminent danger, as discussed below.
A number of other aberrant breathing rhythms occur from brainstem lesions (these are reviewed in Chap. 25), but few are specifically localizing. The more conspicuous respiratory arrhythmias are associated with lesions below the level of the reticular-activating system and are therefore found in the late stages of brainstem compression or with destructive brainstem lesions such as infarction, hemorrhage, or infiltrating tumor.
Lesions of the lower midbrain-upper pontine tegmentum, either primary or secondary to transtentorial herniation, may give rise to central neurogenic hyperventilation (CNH). This disorder is characterized by an increase in the rate and depth of respiration to an extent that produces advanced respiratory alkalosis. The pattern must be distinguished from compensatory overbreathing caused by systemic acidosis, particularly diabetic ketoacidosis (Kussmaul breathing). In addition, mild degrees of hyperventilation are common after a number of acute neurologic events, notably head injury. The neurologic basis of central neurogenic hyperventilation is uncertain. It is theorized to represent a release of the reflex mechanisms for respiratory control in the lower brainstem. It has been observed with tumors of the medulla, lower pons, and midbrain. However, North and Jennett, in a study of respiratory abnormalities in neurosurgical patients, found no consistent correlation between tachypnea and the site of the lesion. A rare but noteworthy cause of central hyperventilation is primary brain lymphoma without brainstem involvement (Pauzner et al).
Low pontine lesions, usually caused by basilar artery occlusion, sometimes cause apneustic breathing (a pause of 2 to 3 s in full inspiration) or so-called short-cycle Cheyne-Stokes respiration, in which a few rapid deep breaths alternate with apneic cycles. With lesions of the dorsomedial part of the medulla, the rhythm of breathing is chaotic, being irregularly interrupted and each breath varying in rate and depth (Biot breathing; also called “ataxia of breathing”). This pattern progresses to one of intermittent prolonged inspiratory gasps that are recognized by all physicians as agonal in nature, and finally to apnea. In fact, respiratory arrest is the mode of death of most patients with serious central nervous system (CNS) disease.