Spontaneous intracranial hypotension (SIH) is now a fairly well known syndrome. Leakage of CSF is considered the pathogenetic factor, even in cases in which it is not demonstrated.

The loss of CSF volume explains the usual complaint of orthostatic headache, relieved by lying down, and the characteristic MRI findings: 1) thickening of the dura, enhancing after contrast medium administration, 2) subdural fluid collections, 3) sagging of the brain, 4) dilatation of the venous structures, which includes enlargement of the dural sinuses and veins, enlargement of the pituitary gland, and, in the spinal canal, engorgement of the epidural plexuses. All these features are explained by the Monro-Kellie doctrine: in a closed compartment, such as the intracranial cavity and spinal canal, which contains nervous tissue, blood, and CSF, the loss of one component is compensated by the equivalent increase of the other ones. Therefore, if a dural leakage causes a loss of CSF, an increase in nervous tissue or blood must compensate for that loss to re-establish the equilibrium. Obviously, the easiest compensation comes from an increase in blood, and specifically venous blood because the veins may dilate passively more than the arteries. In peculiar cases, the nervous tissue may participate in the compensation through swelling of the brain (Savoiardo et al. Brain 2007).

Most of the authors who have published papers on SIH, have shifted their emphasis from the loss of pressure in the closed system (intracranial hypotension) to the actual loss of volume of CSF. We agree that the loss of volume of CSF rather than its decreased pressure should be emphasized and pointed out in the denomination of this condition because it is more correct in terms of pathophysiology. However, the term “CSF hypovolemia”, that has been used by most authors, is wrong. We would like to point out again why this is so.

The suffix “emia” in “hypovolemia”, indicates blood, as in glycemia, uremia, and so on. Therefore, “CSF hypovolemia” means “decreased (hypo) volume (vol) of the blood (emia) of the CSF” which is a total nonsense. There is no “blood of the CSF”; moreover, we have seen that venous blood increases (“hypervolemia”) to compensate for the loss of CSF.

According to dictionaries, “hypovolia” exists and should be the correct term. However, since “hypovolia” has never been used and is unknown to most of us, we propose using “CSF loss of volume” or “decreased volume of CSF” rather than “CSF hypovolia”. In our opinion, “CSF hypovolemia” remains a misnomer and should be banned. This is probably a lost cause, but we think it’s worth using the precise terms.

We thank Dr. Neeraj Kumar and Dr. Mauricio Castillo for discussing this matter.

Mario Savoiardo and Marina Grisoli
Department of Neuroradiology
Foundation IRCCS Istituto Neurologico Carlo Besta
Milan, Italy

E-mail:
msavoiardo@istituto-besta.it
mgrisoli@istituto-besta.it

Savoiardo M, Minati L, Farina L, et al. Spontaneous intracranial hypotension with deep brain swelling. Brain 2007;130:1884-93.

Kumar N. Neuroimaging in superficial siderosis: an in-depth look. AJNR Am J Neuroradiol 2010;31:5-14.

Savoiardo M, Grisoli M. Further in-depth look at superficial siderosis (and intracranial hypotension). AJNR Am J Neuroradiol Published June 25, 2010 as DOI 103174/ajnr.A2172

Kumar N. Reply. AJNR Am J Neuroradiol Published June 25, 2010 as DOI 103174/ajnr.A2187