Hyperlipidemia Resulting in Abnormal Density and Signal Intensity of Blood in a Neonate with Lipoprotein Lipase Deficiency

Discussion

Chylomicrons are responsible for transport of all dietary lipids into the circulation. Clearance of chylomicrons from the blood is rapid: The half-life of disappearance is <1 hour. LPL is located on the walls of the capillaries in many organs and tissues, including the heart, adipose tissue, spleen, lung, and renal medulla. LPL is not found in blood but is released after injection of heparin and can be quantified thereafter. Phospholipids and apo C-II are cofactors for LPL activity. LPL is the rate-limiting enzyme for hydrolysis and removal of triglycerides from chylomicrons and VLDLs.²

Patients with FCS may present with acute pancreatitis when the triglyceride level is >1000 mg/dL. The massive elevation of plasma triglycerides may be clinically silent. On physical examination, eruptive xanthomas may be seen. Two different genetic defects may result in FCS: LPL deficiency and apo C-II deficiency. Mutations in either the LPL gene or the apo C-II cause functional inability to hydrolyze chylomicrons and subsequent hyperchylomicronemia. Of these 2 genetic defects, LPL deficiency is more common, with approximately 1 in 1 million persons being affected.¹ In FCS, in addition to elevation of triglyceride levels, the cholesterol level is also increased because cholesterol is transported in chylomicrons.

Wilson et al³ reported 2 siblings with FCS due to apo C-II deficiency. The older sibling presented with massively elevated triglyceride levels at 5 weeks of age. MR imaging showed multiple linear and globular foci of T1 shortening in the brain, corresponding to intravascular and extravascular fat. Despite dietary intervention and normalization of the abnormal lipid panel, this patient subsequently developed macrocystic encephalomalacia, cerebral atrophy, and subdural collections and became neurologically impaired. The younger sibling, reportedly, had less pronounced imaging findings; fat signal intensity was limited to the intravascular compartment, as was the case in our patient. This child had a normal neurologic outcome following dietary intervention.

CT findings in our patient were noteworthy because it is almost impossible to record negative Hounsfield units in the dural venous sinuses in the absence of marked lipemia. In fact, in the neonate and infant, the attenuation of dural venous sinuses is generally higher than it is in older children, due to hemoconcentration. In severe anemia, the dural venous sinus attenuation may decrease and approach that of CSF attenuation.

On MR imaging, lack of normal signal voids in the dural venous sinuses may raise the possibility of thrombosis. In the absence of a CT scan, this could potentially have caused diagnostic difficulty. In a dural venous sinus, observing a fluid-fluid level, particularly with a T1-weighted hyperintense nondependent component, is very unusual. We hypothesize that this T1 shortening of the anterior half of SSS and anterior deep medullary veins is due to the increased fat concentration in these vessels compared to the posterior venous vasculature. The relatively slow flow in the veins and dural venous sinuses may allow for this “fatty supernatant” due to the lesser specific gravity of fat. There is probably a threshold in plasma fat concentration to generate T1 shortening. Otherwise all the venous structures would have demonstrated increased T1 signal.

FCS or other hyperlipidemic conditions should be entertained when abnormal low attenuation is identified in the vasculature on CT. The presence of fat should be considered as a possibility in addition to venous thrombosis, when T1 shortening is present in the dural venous sinuses and cerebral veins.