Letter Re: Internal jugular and vertebral vein volume flow in patients with clinically isolated syndrome or mild multiple sclerosis and healthy controls: results from a prospective sonographer-blinded study

In the discussion section of the paper “Internal jugular and vertebral vein volume flow in patients with clinically isolated syndrome or mild multiple sclerosis and healthy controls: results from a prospective sonographer-blinded study”, the authors face the potential criticism regarding the validity of the method for estimating volume flow, and conclude that the method they followed to estimate the blood flow by using vein diameter instead of using the cross sectional area (CSA) introduces a systematic error. ¹ We believe that, for collapsible tubes, this assertion is true only when certain conditions are met. In fact, the relationship between the area at equilibrium (p_i = p_e) A₀ and the actual area A of the internal jugular vein (IJV) depends on the Young modulus (E) of the tissue, on the Poisson's ratio (ν) and on the external pressure (p_e) acting on the vein, and it is expressed by the following equation^2,3

where p_i is the internal vein pressure, R is half of the vein diameter when supposed of circular shape ad n and m are real values. ³ If the parameters E, ν and p_e are within the normal range of variability, the ratio (A/A₀) does not change and then the use of A₀ instead of A as CSA only introduces a systematic error, as stated by the authors. The theory of Prof. Zamboni asserts that venous abnormality can be a cause of chronic cerebrospinal venous insufficiency (CCSVI) and such abnormality principally consists of luminal obstructions and wall stenoses of the IJVs. ⁴

In both cases the conditions assuring the normal A/A₀ ratio of the IJV are not met.

Indeed, the anatomy of the vein can be significantly different from the normal one and thus affecting both the Young modulus and the Poisson's ratio. Besides, in case of stenoses an abnormal external pressure p_e occurs. ⁵

This means that, flow measurement calculated deriving the CSA from diameter and then multiplying CSA for the average blood velocity has to be limited to subjects having a normal A/A₀ ratio. Indeed, such approach is not applicable to CCSVI patients whose IJV A/A₀ ratio can be abnormal. In such case the CSA has to be measured directly in the horizontal plane, as already reported by other authors.^6,7

Therefore, the conclusion that there is no significant differences in volume flow between patients with CCSVI associated to multiple sclerosis and normal subjects is obtained using an erroneous assumption and therefore such differences has still to be established.