The diploic venous system is connected with both the intra- and extracranial venous systems via the dural sinuses and the emissary veins, respectively. It is involved in various vascular anomalies. Although its occurrence is rare, abnormal arteriovenous shunts to the diploic vein can be lethal. For instance, dural arteriovenous fistulas can cause intracerebral hemorrhage (Yako et al. 2016) and subdural hematoma (Rivera-Lara et al. 2015). Scalp arteriovenous malformation, which causes scalp swelling, headache, and recurrent bleeding (Chowdhury et al. 2013), can also drain to the diploic vein (Bekelis et al. 2011). On the other hand, the most common diploic venous anomaly is sinus pericranii. It is considered as an abnormal communication between the intra- and extracranial venous systems (Sakai et al. 1997) and typically presents as a soft scalp swelling. In some cases, this is accompanied by local pain, headache, nausea, and vertigo (Akram et al. 2012). In regards to other venous anomalies, there have been reports of cases of subepicranial varix with connections to diploic veins, but these varices had no connections to intracranial venous sinuses (Asano et al. 2000; Mori et al. 1976).
The patient in this case report also had a varix-like venous sac in the diploe that was continuous with the diploic veins, but it was covered by the outer table of the skull and isolated from the extracranial structures. Because of this distinct lack of communications with the extracranial venous system, it belonged to neither sinus pericranii nor subepicranial varix. Accordingly, it was diagnosed as another entity of venous anomaly that should be termed “intradiploic varix”. This entity has not reported previously maybe because it should be asymptomatic in general.
On the other hand, from a pathological viewpoint, as our histological study showed the endothelial lining, which is typically observed in sinus pericranii (Bollar et al. 1992), their etiological backgrounds may be similar. Although the etiology of sinus pericranii is still unknown, two possible etiologies have been postulated: congenital and traumatic. As sinus pericranii is frequently associated with intracranial developmental venous anomalies or other anomalies, it suggests a congenital cause, such as transient venous hypertension during the late embryonic period (Nomura et al. 2006). On the other hand, some patients developed sinus pericranii without congenital anomalies after head trauma. In this case, it suggests that sinus pericranii has an acquired pathophysiology (David et al. 1998), but the causal trauma is frequently too slight to be noticed (Bollar et al. 1992). The patient in our case did not have any other venous anomalies but may have suffered minor trauma in the past.
Another significant point in our case was the expansion of the lesion during the 5-year period and the surrounding hyperosteosis. Assuming that the pathology is similar to that of sinus pericranii, the intravarix pressure would increase and decrease intermittently in response to the intracranial pressure. For sinus pericranii, this intermittent pressure change leads to a change in the size of the scalp swelling. However, in our case, because the venous sac was covered by the cortical bone, it may have gradually eroded the diploe and simultaneously induced reactive hyperosteosis.
In regards to the treatment, the patient opted for surgery despite the lack of symptoms because the lesion had been expanding and it was necessary to differentiate it from neoplastic diseases. Intradiploic varices that have a low risk of being lethal do not always need to be treated, but in cases of expanding lesions such as in our patient, treatment is recommended to avoid further erosion of the skull. Aside from direct surgical interventions such as resection or venous occlusion, endovascular occlusion may also be an option, based on the history of sinus pericranii treatment. However, there is still insufficient evidence to support it (Brook et al. 2009).