CSF pulsatility in hydrocephalus: respiratory effect on pulse wave slope as an indicator of intracranial compliance.

The effect of inspiration and expiration on the systolic slope of the cerebrospinal fluid (CSF) pulse wave has been studied in 83 shunted and non-shunted patients undergoing diagnostic tests for suspected hydrocephalus. A ratio of the systolic CSF pulse slope on inspiration to the same in expiration (I/E ratio or index) has proved statistically valid in identifying non-hydrocephalic patients from hydrocephalic patients and in separating hydrocephalic patients into arrested, communicating and aqueductal stenosis hydrocephalus. The I/E ratio depends on the comparative damping effect of intracranial venous venting on the systolic CSF pulse slope during inspiration (I) when venous volume is evacuated from the cranium by negative mediastinal pressure, and during expiration (E) when cranial venous volume flow to heart is minimal due to positive mediastinal pressure. The low cranial venous outflow on expiration produces little effect on the normal damping of the systolic CSF pulse slope. The high venous outflow on inspiration produces a loss of damping, causing a high systolic CSF pulse slope. Therefore, exhausted cranial venous volume, or exhausted intracranial compliance, produces an I/E ratio approaching 1.0, whereas a normal I/E ratio is between 2.0 and 3.0. The I/E ratio can presumably be used to assess intracranial compliance changes occurring before the dangerous late intracranial pressure (ICP) upward surge related to the volume-pressure curve in all clinical problems of increasing ICP. The I/E ratio may be used likewise to assess the urgency of treatment for any hydrocephalus and increased intracranial pressure problem, i.e. the closer to unity the greater the urgency.

Symptomatic low intracranial pressure in shunted hydrocephalus.

Fourteen patients with ventricular cerebrospinal fluid shunts in place for chronic hydrocephalus presented with a history and neurological deficits usually associated with high intracranial pressure (ICP) caused by an obstructed shunt system. However, the symptoms were characteristically present when the patient was upright and active, and were usually relieved by lying down. The symptoms of intermittent headache, nausea, emesis, lethargy, and diplopia were associated with paresis of upward gaze or minimal strabismus. Measurement of ICP showed unexpected dramatically low levels with a marked drop in pressure when the patient was in the upright position, whereas ICP was near normal when the patient was supine. The low ICP was corrected by insertion of a high-pressure Flo-Control valve into the shunt system already in place. Postoperatively, the immediate clinical improvement and more normal ICP measurements were striking. The important clinical finding in this group of patients was the presence of disabling symptoms which occurred when the patients were up and active and which were relieved by lying down. Measurements of ICP with the patient in the supine and then in the upright position were critical in establishing an accurate diagnosis of symptomatic low ICP in these hydrocephalic patients with indwelling shunts. With the patient in the Trendelenburg position, ICP showed a marked increase, as expected; in some patients this position was prescribed as treatment for several days before surgery.

Hydrocephalus: increased intracranial pressure and brain stem auditory evoked responses in the hydrocephalic rabbit.

The auditory evoked response (AER) was used to study the effect of increased intracranial pressure (ICP) on the auditory pathway in normal New Zealand rabbits and in those made hydrocephalic by intracisternal injections of kaolin. AERs were studied: (a) in the normal and then in the hydrocephalic animal; and (b) in the hydrocephalic animal during further ICP elevation by cerebrospinal fluid infusion. The AER was obtained from ongoing electroencephalographic activity after rarefaction auditory clicks presented at 90 dB sound pressure equivalent. In comparing base line normal AERs to those found in hydrocephalic conditions, a statistically significant increase in latency for AER components N2, P2, and P5 was noted in hydrocephalic rabbits. Increased ICP in the hydrocephalic model showed an increase in the latencies of AER components for P0 and P1 at 250 mm H2O, and a prolongation of P3-P5 central conduction time at 700 mm H2O above base line cerebrospinal fluid pressure. In addition, a decrease in the P4/N5 amplitude and an increase in P1-P3 central conduction times at 700 mm H2O was observed. The differences between normal and hydrocephalic rabbit AER base lines may be the result of the chronically increased ICP and presumed chronic anatomical changes within the auditory pathway due to kaolin itself. The differences in the AER from base line hydrocephalus to acute increased ICP may indicate that the hydrocephalic system is more sensitive to acute neuropraxic pressure effects on the brain stem auditory structures than is the normal brain.

Acute negative intracranial pressure effects on the auditory evoked response in rabbits.

Recent clinical experience has shown that significant neurological symptoms and deficits occur in patients who have been shunted for hydrocephalus when "overshunting" produces unusual negative intracranial pressure (ICP). Therefore, the effect of acute negative ICP .on the early auditory evoked response (AER) was studied in the normal New Zealand rabbit. ICP was reduced to -50, -100, and -150 mm H2O below base line pressure. The AER after rarefaction auditory stimulation was obtained from ongoing electroencephalographic activity at the base line ICP and at each of the three negative ICP levels. Off-line statistical evaluation of the AER showed minimal changes in the absolute and interpeak latencies of N-0 to P-5 at some negative pressures. However, no statistically significant changes were observed for any of the measures for grouped data. At the negative ICP levels studied, the cerebrospinal fluid (CSF) pulse pressure was considerably augmented over base line measures. Such augmentation of the CSF pulse pressure may be the cause of the minimal effects observed on the AER, which may be due to a neuropraxic effect rather than ischemia from cerebral perfusion pressure changes.

New control algorithms via brain theory.

New concepts and viewpoints are needed to develop control algorithms for large scale systems. It is hypothesized that brain functions in human beings clearly demonstrates the existence of superior techniques for controlling complex systems with multiple objectives. The theory of compacta is investigated as a basis for learning, memory, and perception.