I could assume from your writing that you are of very normal intelligence. :)
With what appears to be symptoms in the normal range, I would say go with the monitoring and if you have a radical change, see a doctor asap.
As far as the brain development, i have OCD and anxiety issues but (fortunately) not mental retardation!
Yes back in 2004 doctors said, it's nothing you've had it since birth, an anatomical variant and i guess if it didnt grow larger in 10 years maybe i shouldnt worry too much and just watch it with mri's. I have an app in a neurosurgery department in two weeks and i will show both the old ct and the mri. I don't have symptoms except a headache 2 or 3 times per month.
Do you have symptoms?
I found this abstract:
"A septum pellucidum cyst is defined as a cystic structure between the lateral ventricles, whose walls exhibit lateral bowing and are 10 mm apart or greater. This communication presents computerized tomography (CT) and magnetic resonance imaging (MRI) findings in six patients with septum pellucidum cysts, ages ranging from 1.5 to 47 yrs. The width of the cysts ranged from 13 to 23 mm, and their walls were laterally bowed. Only one patient had hydrocephalus, who was surgically treated. The remaining five patients only had intermittent headaches. It appears that further studies will be required to establish satisfactory diagnostic and therapeutic criteria for symptomatic or presumably symptomatic septum pellucidum cysts."
I also found this:
"Recent studies have shown that the persistence of the cavum septi pellucidi beyond the neonatal period is a marker of cerebral dysgenesis. It has been suggested that the finding of a persistent cavum vergae is also a marker of disturbed brain development. In order to investigate this hypothesis we reviewed 161 brain magnetic resonance imaging scans from normal individuals for the presence of cavum septi pellucidi or cavum vergae, or both. In the 34 prospectively obtained normal adults, there were no individuals with either a cavum septi pellucidi or cavum vergae. In the "defined" normal subjects 3 of 127 individuals (2.4%) had a cavum septi pellucidi whereas a cavum vergae was noted in 26 of 127 (20.5%). We next reviewed the neuroimaging studies of 249 children and adults evaluated for mental retardation or developmental delay. A cavum septi pellucidi was found in 38 of 249 (15.3%) and a cavum vergae in 48 of 249 (19.3%) of these patients. A cavum septi pellucidi and cavum vergae were found together in 19 of 249 (7.6%). We interpret these data as showing that the cavum septi pellucidi is rarely seen in normal individuals although the cavum vergae is seen with the same frequency in normal and retarded populations. Thus we conclude that the cavum septi pellucidi serves as a significant marker of cerebral dysfunction manifested by neurodevelopmental abnormalities while the cavum vergae alone does not identify individuals at risk for cognitive delays."
My guess-read is that they don't treat unless symptomatic. I would assume that as time goes on and the cyst remains the same, the MRI interval can also get longer.