Dan Greitz

Radiological assessment of hydrocephalus: new theories and implications for therapy

It is almost a century since Dandy made the first experimental studies on hydrocephalus, but its underlying mechanism has been unknown up to now. The conventional view is that cerebrospinal fluid (CSF) malabsorption due to hindrance of the CSF circulation causes either obstructive or communicating hydrocephalus. Analyses of the intracranial hydrodynamics related to the pulse pressure show that this is an over-simplification. The new hydrodynamic concept presented here divides hydrocephalus into two main groups, acute hydrocephalus and chronic hydrocephalus. It is still accepted that acute hydrocephalus is caused by an intraventricular CSF obstruction, in accordance with the conventional view. Chronic hydrocephalus consists of two subtypes, communicating hydrocephalus and chronic obstructive hydrocephalus. The associated malabsorption of CSF is not involved as a causative factor in chronic hydrocephalus. Instead, it is suggested that increased pulse pressure in the brain capillaries maintains the ventricular enlargement in chronic hydrocephalus. Chronic hydrocephalus is due to decreased intracranial compliance, causing restricted arterial pulsations and increased capillary pulsations. The terms “restricted arterial pulsation hydrocephalus” or “increased capillary pulsation hydrocephalus” can be used to stress the hydrodynamic origin of both types of chronic hydrocephalus. The new hydrodynamic theories explain why third ventriculostomy may cure patients with communicating hydrocephalus, a treatment incompatible with the conventional view.

Pulsatile brain movement and associated hydrodynamics studied by magnetic resonance phase imaging

SummaryBrain tissue movements were studied in axial, sagittal and coronal planes in 15 healthy volunteers, using a gated spin echo MRI sequence. All movements had characteristics different from those of perfusion and diffusion. The highest velocities occurred during systole in the basal ganglia (maximum 1.0 mm/s) and brain stem (maximum 1.5 mm/s). The movements were directed caudally, medially and posteriorly in the basal ganglia, and caudally-anteriorly in the pons. Caudad and anterior motion increased towards the foramen magnum and towards the midline. The resultant movement occurred in a funnelshaped fashion as if the brain were pulled by the spinal cord. This may be explained by venting of brain and cerebrospinal fluid (CSF) through the tentorial notch and foramen magnum. The intracranial volume is assumed to be always constant by the Monro-Kellie doctrine. The intracranial dynamics can be viewed as an interplay between the spatial requirements of four main components: arterial blood, capillary blood (brain volume), venous blood and CSF. These components could be characterized, and the expansion of the arteries and the brain differentiated, by applying the Monro-Kellie doctrine to every moment of the cardiac cycle. The arterial expansion causes a remoulding of the brain that enables its piston-like action. The arterial expansion creates the prerequisites for the expansion of the brain by venting CSF to the spinal canal. The expansion of the brain is, in turn, responsible for compression of the ventricular system and hence for the intraventricular flow of CSF.

Unraveling the riddle of syringomyelia

The pathophysiology of syringomyelia development is not fully understood. Current prevailing theories suggest that increased pulse pressure in the subarachnoid space forces cerebrospinal fluid (CSF) through the spinal cord into the syrinx. It is generally accepted that the syrinx consists of CSF. The here-proposed intramedullary pulse pressure theory instead suggests that syringomyelia is caused by increased pulse pressure in the spinal cord and that the syrinx consists of extracellular fluid. A new principle is introduced implying that the distending force in the production of syringomyelia is a relative increase in pulse pressure in the spinal cord compared to that in the nearby subarachnoid space. The formation of a syrinx then occurs by the accumulation of extracellular fluid in the distended cord. A previously unrecognized mechanism for syrinx formation, the Bernoulli theorem, is also described. The Bernoulli theorem or the Venturi effect states that the regional increase in fluid velocity in a narrowed flow channel decreases fluid pressure. In Chiari I malformations, the systolic CSF pulse pressure and downward motion of the cerebellar tonsils are significantly increased. This leads to increased spinal CSF velocities and, as a consequence of the Bernoulli theorem, decreased fluid pressure in narrow regions of the spinal CSF pathways. The resulting relatively low CSF pressure in the narrowed CSF pathway causes a suction effect on the spinal cord that distends the cord during each systole. Syringomyelia develops by the accumulation of extracellular fluid in the distended cord. In posttraumatic syringomyelia, the downwards directed systolic CSF pulse pressure is transmitted and reflected into the spinal cord below and above the traumatic subarachnoid blockage, respectively. The ensuing increase in intramedullary pulse pressure distends the spinal cord and causes syringomyelia on both sides of the blockage. The here-proposed concept has the potential to unravel the riddle of syringomyelia and affords explanations to previously unanswered clinical and theoretical problems with syringomyelia. It also explains why syringomyelia associated with Chiari I malformations may develop in any part of the spinal cord including the medullary conus. Syringomyelia thus preferentially develops where the systolic CSF flow causes a suction effect on the spinal cord, i.e., at or immediately caudal to physiological or pathological encroachments of the spinal subarachnoid space.

Syringomyelia: Current Concepts in Pathogenesis, Diagnosis, and Treatment

Syringomyelia is a condition that results in fluid-containing cavities within the parenchyma of the spinal cord as a consequence of altered cerebrospinal fluid dynamics. This review discusses the history and the classification of the disorder, the current theories of pathogenesis, and the advanced imaging modalities used in the diagnosis. The intramedullary pulse pressure theory (a new pathophysiologic concept of syringomyelia) also is presented. In addition, the current understanding of the painful nature of this condition is discussed and the current trends in medical and surgical management are reviewed.

Paradigm shift in hydrocephalus research in legacy of Dandy’s pioneering work: rationale for third ventriculostomy in communicating hydrocephalus

ObjectiveThis study aims to question the generally accepted cerebrospinal fluid (CSF) bulk flow theory suggesting that the CSF is exclusively absorbed by the arachnoid villi and that the cause of hydrocephalus is a CSF absorption deficit. In addition, this study aims to briefly describe the new hydrodynamic concept of hydrocephalus and the rationale for endoscopic third ventriculostomy (ETV) in communicating hydrocephalus.CritiqueThe bulk flow theory has proven incapable of explaining the pivotal mechanisms behind communicating hydrocephalus. Thus, the theory is unable to explain why the ventricles enlarge, why the CSF pressure remains normal and why some patients improve after ETV.Hydrodynamic concept of hydrocephalusCommunicating hydrocephalus is caused by decreased intracranial compliance increasing the systolic pressure transmission into the brain parenchyma. The increased systolic pressure in the brain distends the brain towards the skull and simultaneously compresses the periventricular region of the brain against the ventricles. The final result is the predominant enlargement of the ventricles and narrowing of the subarachnoid space. The ETV reduces the increased systolic pressure in the brain simply by venting ventricular CSF through the stoma. The patent aqueduct in communicating hydrocephalus is too narrow to vent the CSF sufficiently.

A method for MR quantification of flow velocities in blood and CSF using interleaved gradient-echo pulse sequences

The aim of this study was to establish a rapid method for in vivo quantification of a large range of flow velocities using phase information. A basic gradient-echo sequence was constructed, in which flow was encoded along the slice selection direction by variation of the amplitude of a bipolar gradient without changes in sequence timings. The influence of field inhomogeneities and eddy currents was studied in a 1.5 T interleaved sequences for calibration and in vivo flow determination were constructed, and flow information was obtained by pairwise subtraction of velocity-encoded from velocity non-encoded phase images. Calibration was performed in a nongated mode using flow phantoms, and the results were compared with theoretically calculated encoding efficiencies. In vivo flow was studied in healthy volunteers in three different areas using cardiac gating; central blood flow in the great thoracic vessels, peripheral blood flow in the popliteal vessels, and flow of cerebrospinal fluid (CSF) in the cerebral aqueduct. The results show good agreement with results obtained with other techniques. The proposed method for flow determination was shown to be rapid and flexible, and we thus conclude that it seems well suited for routine clinical MR examinations.

The hydrodynamic hypothesis versus the bulk flow hypothesis.

Dear Editor, I would like to thank Dr Egnor [1] and Dr Raybaud [2] for their great interest and thoughtful commentaries on the review article of hydrocephalus published in Neurosurgical Review [3]. The commentaries consider the two opposing conceptual frameworks on the pathophysiology of communicating hydrocephalus described in the review, i.e., the CSF bulk flow hypothesis and the hydrodynamic hypothesis. The main essence of the hydrodynamic theory, i.e., that increased arterial pulse pressure is a major cause of communicating hydrocephalus, was to some extent questioned in the commentaries. I am grateful for the commentaries since they have given me the opportunity to delimit the hydrodynamic theory against diseases other than hydrocephalus, to strengthen the arguments for the theory and to further demonstrate its applicability on neonates and children. Dr Egnor is a leading proponent of the hydrodynamic theories of communicating hydrocephalus. In an electrical circuit model of communicating hydrocephalus, Dr Egnor demonstrated that during increased ventricular pulsations, a voltage between the ventricular system and subarachnoid space is building up, which corresponds to the transmantle pressure gradient [4]. This model strongly supports the view that increased intraventricular pulse pressure may cause hydrocephalus. In the commentary, it is interesting to note that Dr Egnor is able to summarize the hydrodynamic theory in a few sentences. The only remark would be: Is cerebral edema really a general and prominent feature of human chronic hydrocephalus? This question is raised in spite of the fact that both increased water diffusion in brain tissue and CSF malabsorption are present in hydrocephalus. Except for the commonly occurring periventricular edema, it seems that the capillaries are capable of maintaining fluid homeostasis in other parts of the brain. As mentioned in the review, the very existence of infusion tests indicates that there always is a CSF absorption capacity reserve in chronic hydrocephalus. If the CSF absorption capacity were exceeded, the mean intracranial pressure would spontaneously increase instead of remaining stable. Dr Raybaud is a distinguished scientist in pediatric neuroradiology and has studied the pathophysiology of hydrocephalus in the fetus, neonate and child. I would like to quote a statement from Dr Raybaud:

Skeletal Muscle Perfusion During Exercise Using Gd-DTPA Bolus Detection

The study was performed to evaluate if skeletal muscle perfusion can be determined during exercise using an IV bolus injection of Gd-DTPA. A fast spoiled gradient echo sequence (T1 weighted) was used with intermittent imaging during one-legged plantar flexion at different workloads. Between repetitive flexions, a 2-sec rest allowed magnetic resonance imaging (MRI) of the lower legs and measurements of the blood flow in the popliteal artery by ultrasonography for subsequent calculation of muscle perfusion. Maximal signal intensity, upslope and downslope of the bolus, mean transit time, and integrated curve area were measured within regions of interest bilaterally. The skeletal muscle perfusion estimated by ultrasonography increased in the exercising leg from 4 ml x 100 g(-1) x min(-1) at rest to 38 ml at low, 86 ml at medium, and 110 ml x 100 g(-1) x min(-1) at high workload. The SImax increased from 1.38 +/- 0.12 to 1.58 +/- 0.15 and the negative slope of the peak nonsignificantly from - 2.38 +/- 1.75 to - 12.05 +/- 9. 71. All obtained MRI parameters could visually separate the muscles into exercising, nonexercising, and presumably low active muscles. It is concluded that the signal intensity curve using a fast spoiled gradient echo sequence did not overall quantitatively mirror the perfusion, evaluated as the blood flow measured by ultrasonography. However, the signal intensity seemed to follow the blood flow velocity within a limited range of 15-60 cm x sec(-1), corresponding to 35-90 ml x 100 g(-1) x min(-1). Nonetheless, it might be useful when studying ischemia or endothelial dysfunction in skeletal muscles during exercise.

Changes in cross-sectional area in human exercising and non-exercising skeletal muscles

Abstract This research was performed to study how the cross-sectional area (CSA) changes in the skeletal muscles of exercising (E-leg) and contralateral non-exercising (N-leg) legs and to evaluate to what extent changes in CSA mirror changes in blood flow or extravascular water displacement. Seven healthy volunteers performed plantar flexion exercise at three different exercise intensities for 10 min each. Six plantar flexions followed by a 2-s rest in between allowed repeated measurement of the blood flow to the lower limbs by duplex ultrasonography in the popliteal artery and CSA by magnetic resonance imaging. The CSA was measured using manual planimetry at rest and after 3 and 9 min of the exercise periods. The CSA increased in the E-leg by 4.5% and decreased in the N-leg by −2.4%, from rest to highest exercise intensity. Post-exercise imaging of the E-leg showed a bi-phasic recovery of CSA with a rapid phase followed by a slower phase while the blood flow very rapidly returned almost to basal. The time course of the post-exercise decrease indicated that about 50% of the increase in CSA at the highest exercise intensity might have been a result of extravascular water displacement and 50% of an increase in the vasculature volume related to the flow increase. The CSA reduction in N-leg seems to have been related to vasoconstriction, probably mainly of the capacitance vessels since blood flow was not reduced.

A theoretical study of amplitude modulation and time shifting in quantitative MR measurements of motion in brain tissue

MR imaging pulse sequences can be made sensitive to motion by adding gradients with different strengths at different time intervals. In the well-known phase mapping method, such velocity encoding gradients are used to obtain phase information linear to the velocity of the studied object in the direction of the gradient. When very low velocities are studied, a long duration velocity-encoded gradient is required to obtain sufficient velocity sensitivity. In such cases, variation in the object velocity during the execution of the sequence may hamper the accuracy of the method. In this study, we have made a computer simulation of the performance of a phase mapping method sequence (TE = 46 msec) designed for quantitative studies of motion in brain tissue. Using a Gaussian-shaped velocity input function, the time shifting and the amplitude modulation properties of the sequence was studied for various values of the duration, defined as the full width of tenth of maximum (FWTM), of the input function. The time shift corresponded well to the center of the 180 degrees RF pulse, and the amplitude modulation was seen to decrease with increasing time duration of the velocity input function. Applied on in vivo data, where an approximately gaussian-shaped brain motion velocity pattern was assumed to have a duration of 150 msec, the amplitude modulation of the sequence was estimated to 2%.