Diploic Veins

Introduction

Diploic veins are valveless channels traveling intraosseously in the diploic space between the inner and outer tables of the calvaria (diploë). 

Diploic veins function as:

• Venous channels within the diploe with connecting to the intra- and extracranial venous systems.

Emissary veins link the dural sinuses to the diploic and extracranial veins including the veins of the vertebral venous plexus of the spine and spinal canal. The direction of venous blood flow in this vast system of interconnected valveless veins is determined by hydrostatic pressure gradients that are affected by body movements, respiration, Valsalva maneuvers, upright posture, and inversion (Flanagan 2015).

• Part of the brains cooling mechanism. Venous blood gets cooled at the surface by conduction, convection, radiation, and sweating. This venous blood travels internally to keep the brain 2-3 degs cooler than the rest of the body. The diploic veins are cooled by surface veins and provide an additional layer of thermal protection for the cranial vault. They also with this cool blood cool the suboccipital cavernous sinus (Flanagan 2015).

• CSF drainage pathways to the extracranial spaces (Tsutsumi et al 2014 & Lankar et al 2019).

The diploic veins and CSF drainage

The classic CSF drainage pathways are through:

• Lymphatic vessels including lymphatics in the nasal mucosa (Engelhardt 2016), perineural spaces (Koh et al 2005) and dural venous sinuses (Benveniste et al 2019).

• Dural venous sinuses: intracranial arachnoid granulations protrude into the dural venous sinuses. Miyajima and Arai (2015) described this as a secondary pathway but only if there is an increased CSF hydrostatic pressure.

In addition to this there is an intraosseous routes of CSF drainage into the diploe vein. In this model CSF is filtered through arachnoid granulations that protrude through defects in the dura mater into the diploë and drain into the diploic veins (Lachkar et al 2019).

These arachnoid protrusions seem to develop with age. Tsutsumi et al (2014) found the the number of arachnoid protrusions over eighteen years was, except for in the pterional region, more than three-fold higher compared with under eighteens. These authors hypothesised the maturation of the dura mater may be related to the development of these arachnoid protrusions.

A similar process may occur with spinal CSF whereby it is absorbed through spinal arachnoid villi and granulations into the spinal venous network.

Absorption of CSF through the spinal cord arachnoid granulations is less than through intracranial arachnoid granulations. This is possibly due to the much smaller size of the spinal arachnoidal structures compared with those in the intracranial cavity (Tsutsumi et al 2015). 

The diploic veins and venous drainage

Tsutsumi et al (2014) found arterial blood to reach the arterioles and capillaries of the dura mater before passing into the venules, and then finally pouring into the diploic veins.

The diploic veins serve as an important connection between the extracranial and intracranial venous systems. They drain either internally to the venous dural sinuses or externally to the veins of the head.

Lachkar et al (2019) thought it is possible that there is greater communication between the dural venous sinuses and the general circulation than previously believed.

Standring et al (2015) identified four main trunks of the diploic veins:

• Frontal diploic veins: drains into (i) supraorbital vein at the bottom of the supraorbital notch; (ii) superior sagittal sinus near the superolateral aspect of the frontal sinus.

• Anterior temporal diploic veins: situated near the pterion being mainly confined to the frontal bone. It drains into (i) sphenoparietal sinus; (ii) one of the deep temporal veins through an aperture in the greater wing of the sphenoid.

• Posterior temporal diploic veins: situated near the asterion in the parietal bone. They drain into (i) sphenoparietal sinus. Lachkar et al (2019) found most of the diploic veins in the parietal bone are characterized by directly draining to the sphenoparietal sinus; (ii) Transverse-sigmoid sinus either through an opening at the mastoid angle of the parietal bone or through the mastoid foramen; (iii) superior sagittal sinus; (iv) emissary veins connecting the extracranial and intracranial veins (Lachkar et al 2019).

• Occipital diploic veins: largest of the four diploic veins and is confined to the occipital bone. It can drain externally or internally. Its external drainage route courses inferiory in the medial occipital bone to drain into the occipital vein until it reaches the suboccipital venous channels. Internally it drains into either the transverse sinus or the confluence of the sinues. Lachkar et al (2019) found the diploic veins of the occipital bone only contributed minutely to venous drainage.

Tsutsumi et al (2019) identified the diploic veins of the cranial base. Whilst the veins were scarce over the posterior and medial-third regions of the middle cranial base [presumabley the occiput] they were plentiful elsewhere. The pterional veins (intersection of the frontal, temporal, parietal, and sphenoid bones) were generally well defined, as pivotal channels connecting the lateral parts of the anterior and middle cranial base.

The diploic veins in the frontal bone are more developed during adolescence, as peak growth of the frontal sinus is supported by increased vascularity. This allows for potential intracranial spread of frontal sinus infection (Lachkar et al 2019).

During the transition from adolescence to adulthood, diploic veins are mostly observed in the parietal bone.

In adults, as the diploic veins mature they articulate with intracranial dural sinuses most often, in the middle, rather than the anterior or posterior portions of the skull (Lachkar et al 2019)

References

The Diploic Veins: A Comprehensive Review with Clinical Applications (2019). Stefan Lachkar, Mary-Margaret Dols, Basem Ishak, Joe Iwanaga, and R. Shane Tubbs

Cerebrospinal fluid drainage through the diploic and spinal epidural veins (2015). Satoshi Tsutsumi, Ikuko Ogino, Masakazu Miyajima, Masanori Ito, Hajime Arai, and Yukimasa Yasumoto

Cranial Arachnoid Protrusions and Contiguous Diploic Veins in CSF Drainage (2014) S. Tsutsumi, I. Ogino, M. Miyajima, M. Nakamura, Y. Yasumoto, H. Arai, and M. Ito

Vascular, glial, and lymphatic immune gateways of the central nervous system (2016). Engelhardt B, Carare RO, Bechmann I, Flügel A, Laman JD, Weller RO.

The glymphatic system and waste clearance with brain aging (2019). Helene Benveniste, Xiaodan Liu, Sunil Koundal, Simon Sanggaard, Hedok Lee, and Joanna Wardlaw

The glymphatic system and waste clearance with brain aging (2019). Helene Benveniste, Xiaodan Liu, Sunil Koundal, Simon Sanggaard, Hedok Lee, and Joanna Wardlaw

Integration of the subarachnoid space and lymphatics: Is it time to embrace a new concept of cerebrospinal fluid absorption? (2005). Lena Koh, Andrei Zakharov and Miles Johnston

Evaluation of the Production and Absorption of Cerebrospinal Fluid (2015). Masakazu MIYAJIMA and Hajime ARAI

The Role of the Craniocervical Junction in Craniospinal Hydrodynamics and Neurodegenerative Conditions (2015). Michael F. Flanagan