Venous Sinus Stenosis with Prominent Emissary Veins: A New Common Cranial MRI Finding of Mucopolysaccharidosis I

DISCUSSION

MPSIH is associated with varying degrees of CNS involvement and a resultant impact on neurodevelopment. Previous studies have summarized various types of CNS pathology, including WM signal-intensity abnormality, ventricular dilation, hydrocephalus, enlarged perivascular spaces, a J-shaped sella turcica, an enlarged pituitary, and recently described posterior fossa horns (hypertrophy of the occipitomastoid sutures).^2⇓⇓-5,7 Several case reports have reported large occipital emissary veins in healthy cohorts, and the presence of emissary veins potentially improves cerebellar venous outflow and thus prevents the increase in intracranial hypertension.^7,8 To our knowledge, prominent emissary veins in MPSIH have not been reported in the literature. Recently in the literature, a high prevalence posterior fossa horns due to internal hypertrophy of the occipitomastoid sutures and the subsequent regression of posterior fossa horns have been described in MPSIH.^3,4,12 We propose a possible interaction between the posterior fossa horns, the impaired venous outflow, and intracranial hypertension. More specifically, the prominent posterior fossa horns, which develop possibly due to GAG deposits, just inferior to the transverse sinuses and posterior to the sigmoid sinuses narrow the posterior fossa. The posterior fossa horns compress the cerebellar hemispheres, which, in turn, compress the sigmoid and transverse sinuses, resulting in venous hypertension and poor CSF resorption.

In adult patients with idiopathic intracranial hypertension, various degrees of venous sinus stenosis are seen, and alleviation of intracranial hypertension via venous sinus stent placement in those patients supports the hypothesis that elevation in venous pressure restricts CSF resorption.¹³ Furthermore in those patients, alternative cerebral venous drainage has been reported, and occipital emissary veins and extrajugular venous drainage have been described as hallmarks of idiopathic intracranial hypertenion.^9,10,14,15 We, therefore, postulated that the compression of the sigmoid and transverse sinuses drives the development or enlargement of emissary veins in patients with MPSIH to provide alternative venous outflow, alleviate the venous hypertension, and ultimately improve CSF resorption and decrease the likelihood of developing hydrocephalus.

Although the exact incidence of hydrocephalus in MPSIH is unclear, 1 large multicenter study reported 30.6% of patients with hydrocephalus before HSCT and 5.9% with persistent hydrocephalus after HSCT.^11,16,17 In our study of 12 patients, we discovered that 3 of the 12 patients had VP shunt placement. All 12 patients had some degree of venous stenosis and posterior fossa horns compared with only 2 patients with posterior fossa horns and 1 patient with bilateral emissary veins in the control group. Almost all of them (11 of the 12) had prominent emissary veins from the sigmoid sinus or torcula. The mean OP among 9 of the 12 patients was 24 cm H₂O, which was the reported average OP in 25 patients with MPSIH in a previous study.¹¹ Only 3 of those 9 patients had an OP of >28 H₂O cm, which is the recently established upper normal limit of OP in the pediatric population.¹⁸ In our study of 12 patients, at the same time that the venogram was obtained, all 3 patients with elevated OP had venous stenosis and emissary veins, but none of them ultimately required shunt placement. Three of the 12 had VP shunts placed. One of the 3 patients developed acute ventriculomegaly and hydrocephalus on MR imaging 22 days after HSCT and required VP shunt placement, so the patient did not have OP measured and the patient’s MRV was obtained as part of the pretransplantation evaluation. The other 2 patients with VP shunt placement had undergone the procedure 4 years after HSCT due medical refractory papilledema with normal OP, and their MR images were obtained 5 years after shunt placement for evaluation of hormonal deficiency. There was no apparent association of elevated OP or the incidence of VP shunting with the severity of posterior fossa horns or the degree of venous sinus stenosis. Emissary veins are equally prevalent, but they are found in patients with MPSIH with both normal and elevated OP, contrary to the patient group of similar ages that did not have MPSIH. Only one of them had bilateral emissary veins without venous sinus stenosis. The authors postulated that the various venous stenoses and subsequent development of emissary veins, in fact, might be a common finding in patients with MPSIH.

The retrospective nature of this study and the small size are 2 main limitations. Not all patients had a dedicated venogram, and not all patients had OP measurements. Several patients had venography and OP measurements at different stages of their treatment, ie, some of the studies were performed before HSCT and some were performed after HSCT. Multiple studies have reviewed the effects of HSCT in patients with MPSIH and found improvement in the incidence of hydrocephalus, possibly due to improvement in GAG deposits and CSF flow.^17,19 Therefore, the OP may be affected by the intrinsic characteristics of the CSF and is not a surrogate for venous hypertension. However, our study illustrated the high prevalence of venous stenosis and emissary veins, which was not previously described. Whether posterior fossa horns or venous sinus stenosis contributes to the development of hydrocephalus in MPSIH requires further investigation with a larger cohort.