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agenesis and hypoplasia of the internal carotid artery ( ica ) are very rare congenital anomalies , occurring in less than 0.01% of the population.[13 ] they are more often unilateral .
bilateral ica agenesis or hypoplasia is even rarer . of the slightly more than 100 cases of ica agenesis or hypoplasia
many of these patients are asymptomatic due to good collateral flow , and the anomaly may be detected only incidentally .
we report a case of asymptomatic bilateral ica hypoplasia in a young adult , which was detected incidentally while the patient was being investigated for a head injury .
a 36-year - old man was brought to the emergency department with a history of fall from a height of 8 feet .
the cause of his fall was not known as he had amnesia for the event .
he had had transient loss of consciousness , but there were no significant external injuries .
ct scan of the head revealed subarachnoid blood in the right sylvian fissure and right frontoparietal convexity , along with a small temporal hematoma [ figure 1 ] .
he was further investigated with digital subtraction angiography to rule out a distal middle cerebral artery ( mca ) aneurysm .
the proximal 1 cm of the ica , and the external carotid artery ( eca ) were normal bilaterally .
the cervical and petrous carotids on both sides were visualized as thin streaks , while the cavernous and the supraclinoid part of the icas up to the origin of the posterior communicating artery ( pcoa ) were not visualized [ figure 2 ] .
the anterior circulation was supplied by the basilar artery through bilateral dilated and tortuous pcoas .
the right ophthalmic artery was probably arising from the anterior carotid artery - anterior communicating artery ( aca - acoa ) complex .
there were no associated aneurysms or abnormal transcranial eca - ica anastomosis or any primitive persistent arteries .
ct scan showed bilateral diminutive carotid canals in the petrous bone [ figure 4 ] .
as there was no evidence of aneurysms or vessel wall pathology , the subarachnoid hemorrhage was attributed to trauma and the patient was managed conservatively .
he was discharged 9 days after admission at which he time he was conscious and alert with no neurologic deficits .
axial noncontrast ct scan shows a right sylvian fissure bleed ( arrow ) frontal digital subtraction angiograms show bilateral ica hypoplasia , with only the proximal ica being normal ( arrows ) lateral ( a ) and ap ( b ) digital subtraction angiograms show the aca and mca on both sides being filled by the basilar artery via dilated pcoas ( arrowhead in a and curved arrow in b ) . the right ophthalmic artery
acoa region axial ct scans ( bone windows ) show bilateral small carotid canals ( arrows )
bilateral ica hypoplasia is rarer still , with only around 20 cases reported till date .
the terms agenesis , aplasia , and hypoplasia of the ica are often used erroneously interchangeably .
agenesis refers to the complete failure of development of the ica ; hypoplasia indicates that the development of the ica has taken place , but the artery is of a very small caliber ; and the term aplasia is used when only vestiges of the ica are present .
the cause of incomplete development of the ica is probably due to some insult sustained by the growing embryo during the 48 weeks of gestational life .
the cause of the unilateral hypoplasia of the ica has been postulated to be exaggerated folding of the embryo to one side or constriction by amniotic bands .
the development of the aortic arch , great vessels , and the craniocerebral vasculature starts around the 24 day of embryonic life , with the appearance of the first aortic arch . the first two aortic arches involute by day 28 of embryonic life .
the third aortic arch , which is well formed by then , forms the cca and the proximal portion of the ica .
the rest of the distal ica arises from the cranial part of the dorsal aorta [ figure 5 ] .
the eca arises as a sprout from the cca ( i.e. , the third aortic arch ) and also receives contribution from the first and second aortic arches .
the stapedial artery from the second aortic arch contributes to the formation of the internal maxillary branches and the middle meningeal branch of the eca .
initially , during the 4 week of embryonic life , the aortic arches are connected to the dorsal aorta . as
development progresses , the connection of the first and second arches to the dorsal aorta regresses and they contribute to the formation of the eca .
nonregression of the communication with the dorsal aorta may persist as a transcranial eca - ica anastomosis . through this anastomosis
the internal maxillary artery and the middle and accessory middle meningeal arteries can supply the distal ica in cases of hypoplasia of the ica ( known as rete mirabile in the region of the cavernous sinus ) .
the first two aortic arches largely involute ( dark lines ) the basilar artery develops as two longitudinal vascular plexuses dorsal to the third and fourth arches during the 5 week of intrauterine development .
a number of primitive vessels connect the developing basilar artery with the ica , all of which involute and disappear except for the cranial - most one , which persists as the pcoa [ figure 6 ] . in cases of unilateral or bilateral ica agenesis or hypoplasia ,
bilateral absence of the ica , with dilated bilateral pcoas supplying the anterior circulation ( our case ) , comes under type c. in types e and f also there is bilateral ica hypoplasia with bilateral pcoas supplying the mca , the hypoplastic ica supplying the aca in type e and bilateral rete mirabile in type f. in cases of unilateral hypoplasia , the distal ica can also receive collaterals from other persistent primitive arteries or through intercavernous anastomoses .
the dotted lines indicate the multiple primitive ica  basilar artery anastomoses , which later regress the origin of the ophthalmic artery in cases of ica agenesis has not been commonly reported .
the ventral artery anastomoses with the ica in the supracavernous portion ( the region where the artery arises in adults ) and loses its proximal attachment to the aca .
another anastomosis takes place in the orbit between the ventral and dorsal ophthalmic arteries , and the distal portion of the dorsal artery loses its proximal connection .
the proximal part of the dorsal ophthalmic artery persists as the inferolateral trunk of the cavernous ica .
anomalous origin of the ophthalmic artery in cases of ica agenesis is most commonly from the aca .
, the exact site of origin of the right ophthalmic artery could not be identified .
hypoplasia of the ica must be differentiated from acquired stenosis ( especially when unilateral ) .
unilateral narrowing of the ica is more likely to be due to chronic dissection , severe atherosclerosis , fibromuscular dysplasias or , sometimes , moya - moya disease .
it is well known that the carotid canal gives the clue to the presence of hypoplasia of the ica .
the carotid canal is closely linked to the development of the ica during embryonic life .
the skull base develops between the 5 and 6 weeks of embryonic life and is temporally associated with ica development .
the presence of a developing ica is essential for the formation of the carotid canal .
thus the carotid canal is absent in cases of agenesis and is small and not well developed in cases of hypoplasia , as in our case . in acquired stenosis of the ica
, the carotid canal will be of normal size as the icas are developmentally normal .
many patients are asymptomatic due to good collateral flow and the condition is usually diagnosed incidentally .
the presence of a well - perfused brain in such cases has been shown using single - photon emission computed tomography ( spect ) studies .
however , patients can present with transient ischemic attacks , infarct , seizures , aneurysmal subarachnoid hemorrhage , or parenchymal bleeds . in our case ,
the ct scan showed a right sylvian fissure and a cortical surface subarachnoid bleed , which prompted us to get an angiogram done to rule out a ruptured distal mca aneurysm .
however , as the angiogram did not reveal any aneurysm or vascular pathology to account for the cortical bleed , the patient was managed conservatively .
there is an increased risk of aneurysm formation in patients with aplastic or hypoplastic icas ( 23%45% as compared to 2%4% in normal individuals ) .
these aneurysms are more common along the basilar artery and the posterior cerebral artery ( pca )
. this could be due to increased hemodynamic stress as a result of increased flow through the collaterals and altered direction of blood flow and/or may be due to a genetic disorder that has led to both hypoplasia of the ica and vessel wall weakness , predisposing to aneurysm formation .
many cases are probably asymptomatic , being identified only incidentally . the exact cause of this developmental anomaly is not known . a small or an absent carotid canal indicates the congenital nature of the pathology .
ica hypoplasia has to be distinguished from acquired stenosis as the management of the two conditions is different .
agenesis and hypoplasia of the internal carotid artery ( ica ) are rare congenital anomalies , occurring in less than 0.01% of the population . 
 we report a rare case of bilateral hypoplasia of the ica in a patient with post - traumatic subarachnoid hemorrhage . 
 we describe the embryological development of the cerebral vasculature and present a review of literature .