Occipital Artery

Overview:  The occipital artery, most commonly, originates from the external carotid carotid artery, and sweeps a posterior superior course under the skull base, towards the back of the head and upper neck.  It is mainly concerned with supply of upper neck muscular elements.  This somewhat humble disposition, however, belies its important embryologic and functional roles as a conduit between the anterior (carotid) and posterior (vertebral) systems.  This is aptly illustrated in frequent connections between the occipital and vertebral arteries at C1 and C2 levels, and in the proatlantal variant (which is, simply, an occipitovertebral anastomosis with hypoplasia of the more proximal vertebral artery). The longitudinal functional nature of the vessel is further demonstrated by its frequent association with the ascending pharyngeal artery where, perhaps 20-30% of time, both vessels originate from a common pharyngo-occipital trunk.  Another potentially important contribution of the occipital artery can involve structures related to the facial nerve and mastoid bone.  From an interventional standpoint, the occipitovertebral anastomoses must be kept in mind, particularly during embolizations.  Aggressive small particle occipital artery embos can also produce skin necrosis, or result in postoperative wound healing issues.  On a different note, its a very good place to stick a stiff exchange wire into — no better.
Cross-sectional Imaging — Select Axial Images of the neck, demonstrating the course of the occipital artery (in this patient), and its particular relationship to the digastric muscle.  For a full set of CT angiographic images, consult CT Angio Vascular Atlas section.
The Occipital Artery ostium projects posterior and typically lateral in relation to the parent vessel
Some anatomical landmarks; the occipital artery passes deep (along the medial aspect) of the digastric muscle
Notice proximity of the occipital, deep cervical, and vertebral arteries at the skull base, where anastomoses via the 1st segmental artery are common.
A shallow groove can be seen on the undersurface of the mastoid bone sometimes, made by the occipital artery as it curves posteriorly.  The occipital artery stays with the digastric muscle until its insertion.
Vertebro-occipital anastomoses
The vertebral artery communicatates with the occipital via C1 and C2 (segmental) branches, which typically supply adjacent muscular and ossesous elements.  In this sense, the occipital is part of a larger network of cervico-vertebral connections between the occipital at C1 and C2, the deep cervical at C3 and C4, and ascending cervical at C5 and C6 (for example, the C5 level entry of vertebral artery into its foramen is, in fact, proximal vertebral hypoplasia with ascending cervical reconstitution via the C5 segmental artery).
For a full discussion of the longitudinal system and segmental vascular organization, see Vertebrobasilar Artery,Vertebral Artery, Basilar Artery sections.
1) Vertebral origin of occipital artery — relatively commmon (~1%) — a large C1 segmental artery supplies occipital territory, in setting of relative proximal occipital hypoplasia.
Unlabeled (top) and labeled (bottom), AP (left) and lateral (right) injections of left vertebral artery (red), demonstrating a prominent C1 segmental branch (white) supplying the occipital artery (yellow).  The anastomosis between the C1 and occipital is marked in blue.  Contrast reflux demonstrates hypoplastic occipital proximal to the C1 branch (light blue).
Pharyngo-occipital trunk (red) origin from the internal carotid artery. Occipital is yellow, and AP is pink.
Mastoid Branch
The occipital artery almost invariably participates in supply of posterior fossa dural structures via transosseous channels originating from its mastoid branch.  When the transosseous branch is large enough (i.e. gives rise to supply of posterior meningeal region), its corresponding hole in the skull is recognized as the “mastoid foramen”.  The territory supplied is usually posterior to petrous pyramid base, in region of sigmoid sinus.  Consequently, dural fistulas of the sigmoid sinus usually demonstrate a component of occipital supply.  Usually, middle meningeal and ascending pharyngeal arteries represent more fruitful venues for transarterial embolization.
Mastoid branch supply of the posterior meningeal artery.  Non-pathologic role of occipital artery (via its mastoid branch) in supply of dural structures is demonstrated in this patient, where the OA (red) gives gives rise to posterior meningeal artery (green and pink).   The mastoid branch consists of a short extracranial segment (black), followed by transosseous segment (purple).  The intracranial portion consists of a short anteroinferior dural segment (yellow), and a long medial segment (green) towards the lateral aspect of the foramen magnum, and then heading superiorly (pink) as the posterior meningeal artery.
Mastoid Foramen. Same patient.  The extracranial (black) and intracranial (white) openings of the mastoid foramen are defined by the transosseous segment (purple) of the mastoid artery.
Gray’s  Anatomy Plate 135.  Although the mastoid foramen is drawn and labeled here, the posterior meningeal grooves arise from the condyloid (condylar) foramen which, when it exists, transmits the jugular branch of the ascending pharyngeal artery (in this specimen, the source of posterior meningeal artery supply)
Occipitocarotid anastomoses and Common Carotid Occlusion
As illustrated above, the occipital artery serves as a segmental artery to the skull base — where its connection with the vertebral artery via segmental C1 and C2 channels is often evident.  In this role, serves as the most common channel of cervical carotid reconstituion in setting of common carotid occlusion.  Far less prevalent than ICA occlusion, gradual (atheromatous, usually) CCA occlusion is usually very well tolerated.  In such instances, collateral circulation often comes from the external carotid artery to reconstitute the carotid bifurcation and subsequently the ICA.  The occipital is most commonly recruited to re-establish external carotid artery flow because of its extensive segmental connections to the three longitudinal cervical systems — the deep cervical, vertebral, and anterior cervical arteries. Typical routes include vertebral to occipital, deep cervical to occipital, or anterior cervical to occipital routes. In the example below, the left common carotid artery and proximal left vertebral artery are occluded.  Injection of the left subclavian artery demonstrates a large deep cervical artery (purple) which, through C2, C3, and C4 segmental collaterals (brown) reconstitutes the distal cervical vertebral artery (yellow) thus supporting the PICA component of the posterior circulation (not shown).  Additionally, the deep cervical, through a large connection to the occiptal artery (orange), reconstitutes the carotid bifurcation (light blue) and subsequently the markedly irregular internal carotid artery (red), maintaining antegrade ophthalmic artery flow and even opacifying briefly the left M1 segment (rightmost angiogram, above the top red arrow). 
Proatlantal Artery
An embryonic carotido-vertebral anastomosis, the proatlantal artery is, in fact, the occipital artery.  A C1 segmental connection between the occipital and vert is the Proatlantal type I; C2 level connection is Proatlantal type II (a different classification based on whether the proatlantal comes off the ECA or ICA exists also, to generate confusion, — embryologically, the classification according to cervical level of occipital-vertebral anastomosis makes more sense.)  Usually, the vertebral artery proximal to the proatlantal segment is hypoplastic.
Stereo AP and Lateral views of left ECA injection, opacifying the vertebrobasilar system(yellow) via the C1 segmental artery (purple) connection to the occipital artery (red).  This is the Proatlantal I type; the proximal occipital artery (red) is the proatlantal.  Occipital artery distal to the vertebral anastomosis is blue, and IMAX is black.
For diagrams and other related information you can also look at the PICA, Vertebrobasilar, and Vertebral Artery pages.
Below is diagram of the proatlantal artery, which is the occipital artery.
Occipital origin of PICA
Just as in the above diagrams the extradural origin of PICA is part of a connection between the vert and occipital artery at C1, in the same way the extradural PICA can originate from the occipital artery just as well as it can from the vert.  It is far less common, but the important point is that both dispositions are predictable based on the embryonic connections between the occipital and vertebral arteries.  In fact, both occipital and vert origins of PICA are, in a way, extremes of a continuum.  Functionally we often see reflux of contrast into the vertebrobasilar system during occipital injections, and more frequently C1 origin of the occipital artery from the vert. All of these are possible variations on the theme of transverse/longitudinal organization.
In this case, courtesy of Dr. Antonio López-Rueda from Hospital Clínic i Provincial de Barcelona (alrueda81@hotmail.com), an isolated PICA (red arrows) is opacified via injections of the left occipital artery (white arrows).
A diagram of this disposition is shown below:
Note please that the above case and diagram are exactly the same as that of the Proatlantal artery disposition, except that the PICA is isolated and therefore there is no occpital supply to the basilar.  The proatlantal artery is just an occipital artery with a persistent connection to the vertebrobasilar circulation, Type 1 at C1, and Type 2 at C2.  Below is a diagram of the C1 type.
How to make a proatlantal artery?
Answer: Get a vertebral fistula, and keep it for several years.  Remember that proatlantal artery (I or II) is simply an occipital artery which, via the first (type 1) or second (type 2) segmental artery, is connected to the vertebral artery (in setting of vertebral hypoplasia proximal to the segmental artery in question).  So, in effect, we all have small proatlantals.  There are several ways to make one larger — one is to occlude the proximal vertebral artery (with no effective contralateral vertebral), or (much less often) increase demand on the ipsilateral vert.
In this case, a very high flow C3 vertebral artery fistula has been present for several years.  At this time, the vertebral artery essentially falls apart out into the perivertebral venous plexus.  The resulting fisula sumps flow from the contralateral vert, as well as from the ipsilateral occipital, ascending, and deep cervical arteries.  Shown below is the occipital component, whereby contrast within the occipital artery (red), via the first segmental artery (yellow), is directed retrograde (down) into the vertebral artery (purple) towards the site of the fistula (brown).  The top AP stereo view is looking at the head from the back.  Notice also a small contribution of the ascending pharyngeal artery, via the muscular branch, towards the odontoid branch (white)
Embolization of the fistula has been carried out, with coils in the vert (black).  The occipital (red), via the first segmental artery (yellow), now opacifies the vertebral (green) in antegrade fashion, a la Proatlantal I artery.