According to literature report, intracerebral migration of silicone oil may take place from 2 months to 12 years after the initial intraocular endotamponade with silicone oil [6, 9]. Extraocular silicone oil is mainly distributed in subconjunctival space, optic nerve, optic chiasm, lateral ventricle, as well as the third and the fourth ventricles [3, 7, 10, 11]. Specifically, most of the silicone oils are found accidentally, and they are relatively fixed in unrestricted locations; however, long-term tracking records of intraventricular silicone oil are lacking. It is clinically unclear about whether silicone oil that migrates to extraocular region should be treated surgically or not, and the patients are usually asymptomatic. In our case, ventricular migration of silicone oil was found at 30th month postoperatively, but the initial migration might be earlier. The patient was followed up at different intervals, and the imaging characteristics of intraventricular silicone oil were recorded at various time periods. Our research suggested that the morphology or density/signal intensity of intraventricular silicone oil was changing inconstantly, which had displayed a process of migration, aggregation, separation and re-aggregation. In addition, the density of intraocular silicone oil had undergone impressive changes after surgery, which had a CT value range from 73HU to 94HU (one thousandth of the difference between water and air in density as one HU).
The mechanism of silicone oil migration to the subarachnoid space or ventricle remains unclear so far, although it has been discussed in many studies from the point of pathophysiology or anatomy [11, 12].Here are some local risk factors that are likely to cause leakage of intraocular silicone oil:1 silicone oil emulsification, the emulsified silicone oil can be easily separated into small droplets and migrate out of the eyeball, which is related to the silicone oil filling time, silicone oil molecular weight and application of surfactants during perioperative period ; 2 increased intraocular pressure, which is considered to be the main factor leading to migration, but not a separate one ;3 pre-existing glaucoma or abnormalities of the optic nerve and intraocular structure, such as optic pits, internal limiting membrane incomplete, and etc. [6, 14];4 changes in local inner environment and macrophage phagocytosis of emulsified oil bubbles, which may be the potential migration factors . As discovered from this case report and other literature reviews, the migration pathway of intraocular silicone oil might be shown below: silicone oil droplets in vitreous cavity could migrate to the retina and/or the optic nerve sheath through the fissures, which could then enter the intracranial regions along the optic nerve sheath and the optic chiasm, including lateral ventricle, the third and the fourth ventricles [6, 7, 10, 14,15,16]. However, It is not clear where the silicone oil will eventually migrate to.
The imaging findings of intraventricular silicone oil were basically consistent with those reported previously, which included high density on CT, slightly high signal on T1WI, and slightly high signal on T2WI, accompanying with chemical shift artifacts, as well as markedly low signal on T1WI or T2WI lipid suppression images (on which the lipid components could be easily identified) [11, 16, 17]. We noticed that the silicone oil both in intraocular and brain were changing in terms of CT density and MRI signal intensity (as shown in Fig. 5). The ADC maps suggested that, the above changes were partially determined by accumulating degree of silicone oil droplets in cerebrospinal fluid (CSF), which could also confirm the separation and reunification process of silicone oil in the ventricle. It is temporarily called “small steps”, which can also explain its difficulty to detect a small amount of silicone oil migration on CT/MRI image. Typically, the body position and specific gravity of silicone oil are the key factors determining this process. Silicone oil gengrally used has a slightly lower specific gravity than that of CSF . Normal ventricular walls are smooth, and silicone oil can accumulate in the non-dependent part of the ventricle, so it is always located at the apical ventricle.
However, why did the silicone oil stay in the middle of left lateral ventricle for a period of time in this case? The key factor was the new cerebral infarction in left radiation corona, which made local ependyma non-smooth and restricted the movement of silicone oil.The silicone oil-water clusters were more likely to stop at the unsmooth area of ependyma. In theory, the migration location of intraventricular silicone oil depends on patient position and CSF circulation, which may be the cistern magnum, spinal subdural space, and extraspinal canal through the intervertebral foramen, or even the surgical eyeball. Typically, intraventricular silicone oil is not invariable in the brain, instead, it will change with time, position and other factors affecting its movement. It could be seen from Fig. 5 that the increased density of silicone oil in brain always be accompanied by the decreased density or disappeared of another silicone oil. Finally, due to the lacking of sufficient volume data, we were unable to accurately assess the volume of silicone oil in the brain. Further study will benefit from MRI 3D sequences (e.g. 3D-T1WI, from which the precise volume of silicone oil in the brain can be obtained).
It is important to recognize the extraocular silicone oil, especially in brain in emergency circumstance; more importantly, it is also of vital significance to recognize whether the silicone oil stays in the unrestricted areas of the brain or not, and the occurrence of other lesions may be alerted. This requires to understand the detailed medical history, recognize its imaging characteristics, prevent the covering of real lesions and identify the” real murderers”, including intracerebral hemorrhage, metastasis, lymphoma, choroid plexus papilloma/carcinoma, meningioma, subependymoma, ependymoma and etc. Meanwhile, the migration of silicone oil between eyeball and brain may be bidirectional, which may provide an alternative to the removal of silicone oil in the brain compared to surgery(if necessary).