It is important to explore the mechanisms of these imaging abnormalities in the setting of decreased CSF volume. In doing so, the principles of Monro-Kellie doctrine[37] need to be considered. In the core of this doctrine exists the following principle: “with intact skull, sum of volume of brain plus volume of CSF plus volume of intracranial blood click here is constant, and therefore decrease or increase in one will result in increase or decrease in one or both of the remaining two.” In decreased CSF volume such as CSF leaks

(Fig. 5), given that the brain is essentially nonexpandable, it is the increase in intracranial blood volume that has to compensate for decrease in CSF volume. With meningeal venous hyperemia, there is diffuse pachymeningeal enhancement (leptomeninges, in contrast to pachymeninges, have blood brain barriers and therefore do not enhance). Engorgement and enlargement of cerebral venous sinuses and pituitary gland are also part of this compensatory hyperemia. Another volume compensatory phenomenon is collection of subdural fluids (Figs. 6 and 7). Similar changes are noted in spine MRI (Table 4) including dural enhancement and extra-arachnoid fluid collections. However, at the spine STI571 level,

in contrast to the skull, there exist the epidural space with adipose and soft connective tissue and the epidural venous plexus. Therefore, with CSF volume depletion the dural sac can collapse somewhat, and this will result in engorgement and prominence of epidural venous plexus, yet another spine MRI abnormality of CSF leaks (Fig. 8). Sinking of the brain is another consequence of CSF leak. On head MRIs, this is manifested by a decrease in size of the ventricles (“ventricular collapse”), descent of the cerebellar tonsils, descent and distortion of the brainstem, obliteration of some of basal cisterns, flattening of the optic chiasm, or crowding of the posterior fossa. Descent of iter below the incisural line,

an indication of descent of the brainstem, may be seen in the absence of any obvious descent of the cerebellar tonsils.[9] Iter is the MCE cephalad opening of the aqueduct of Sylvius as seen in the midline sagittal MRI views. Incisural line is the line drawn from anterior tuberculum sellae on midline sagittal image to the junction of straight sinus, inferior sagittal sinus, and the great vein of Galen. In reviewing head MRIs of patients with spontaneous CSF leaks, this author has been helped the most (although not exclusively) by T1-weighted midline sagittal image and gadolinium (Gd)-enhanced coronal image through sella and pituitary. The former is helpful to look for descent of cerebellar tonsils, descent and deformity of brainstem, and location of the iter. The latter typically shows the pachymeningeal enhancement well and enables assessing the size of pituitary, the appearance of the optic chiasm, and the perichiasmatic cistern.