unilateral agenesis of internal carotid artery ( ica ) is a rare congenital anomaly . in such cases , the anterior cerebral artery ( aca ) and middle cerebral artery ( mca ) on the side of agenesis are usually supplied by the basilar artery and the contralateral ica through the circle of willis . patients are usually asymptomatic but a few patients may present with headache , subarachnoid hemorrhage ( sah ) and transient ischemic attack ( tia ) . a 25-year - old female patient visited the neurology outpatient clinic with severe holocranial headache that had persisted for 2 months . computed tomography ( ct ) scan of the brain showed absent bony carotid canal on the left side [ figure 1a and b ] . magnetic resonance angiography ( mra ) [ figure 2a and b ] showed complete absence of the cervical and petrous part of the left ica . this vessel was passing across the floor of sella , reconstituting the cavernous and supraclinoid segments of the left ica , and continuing as the left mca . 25-year - old female presented with holocranial headache and was diagnosed with unilateral agenesis of internal carotid artery with intercavernous anastomosis . axial ct scan ( a and b ) two consecutive sections at the base of skull show absent bony carotid canal on the left side ( arrows ) . 25-year - old female presented with holocranial headache and was diagnosed with unilateral agenesis of internal carotid artery with intercavernous anastomosis ( a and b ) time of flight ( tof ) mra with maximum intensity projection ( mip ) of the neck vessels and the intracranial arteries show complete absence of the cervical and petrous part of the left ica with left common carotid artery continuing as external carotid artery ( arrow in a ) . mra with mip of intracranial arteries shows an abnormal vessel ( arrow in b ) seen arising from the cavernous segment of the right ica and reconstituting the cavernous and supraclinoid segments of the left ica , and continuing as the left mca . magnetic resonance imaging ( mri ) showed absent left ica flow void with an abnormal flow void in the sella [ figure 3a and b ] . 25-year - old female presented with holocranial headache and was diagnosed with unilateral agenesis of internal carotid artery with intercavernous anastomosis . ( a ) axial t2-weighted sequence shows an abnormal vessel running across the sella ( white arrow ) . ( b ) sagittal t1-weighted sequence shows abnormal flow void ( arrow ) in the floor of sella and mildly bulging into the sphenoid sinus , representing the intercavernous anastomotic vessel , which can cause devastating complication during a transphenoidal pituitary surgery . doppler imaging [ figure 4a  d ] showed complete absence of the left ica . the left common carotid artery ( cca ) the left cca and eca showed high resistance flow with narrow systolic peak and low diastole . since the right cca has to compensate for the absent left ica , it showed low resistance flow with broad systolic peak and high diastolic flow . 25-year - old female presented with holocranial headache and was diagnosed with unilateral agenesis of internal carotid artery with intercavernous anastomosis ( a - d ) duplex doppler scans with spectral waveform of the ( a ) right cca , ( b ) left cca , ( c ) right ica , and ( d ) left eca show high velocity and low resistance flow in the right ica because it has to compensate for the absent left ica . lateral radiograph of the skull [ figure 5a ] and the corresponding sagittal reformatted ct scan [ figure 5b ] showed a well - defined lytic area in the anteroinferior aspect of the sella due to the abnormal intercavernous vessel traversing across it . ( a ) 25-year - old female presented with holocranial headache and was diagnosed with unilateral agenesis of internal carotid artery with intercavernous anastomosis . ( a ) lateral radiograph of the skull ( zoomed image ) and ( b ) sagittal reformated ct scan ( bone window ) show a well defined lytic area ( arrows ) in the anteroinferior aspect of the sella due to the abnormal intercavernous vessel traversing across it . patients with agenesis of ica are usually asymptomatic due to the collateral circulation from the contralateral ica and the vertebrobasilar system via the circle of willis . in 1787 , tode documented the first case of carotid agenesis , identifying it on postmortem examination . in 1954 , the first case of ica agenesis at cerebral angiography was reported by verbiest . certain mechanical and hemodynamic stresses over the embryo like exaggerated folding of embryo toward one side and constriction by amniotic band have been postulated as hypothesis for agenesis . agenesis , aplasia , and hypoplasia of ica form a spectrum , with the agenesis showing complete absence of ica and the carotid bony canal , whereas hypoplasia shows a small bony carotid canal . absence of bony carotid canal on ct scan is a characteristic feature of agenesis of ica . the circle of willis is formed during the 7- to 24-mm stage of embryonic development . the pattern of collateral blood supply and intracranial vasculature is dependent on time at which insult happened . the ica is developed from seven distinct segments and agenesis of any one segment will lead to regression of all the segments proximal to it . according to lie , these are : 
 type a  unilateral absence of the ica with collateral circulation to the ipsilateral aca through a anterior communicating artery ( acom ) and to the ipsilateral mca from the hypertrophic posterior communicating artery ( pcom)type b  unilateral absence of ica with collateral flow to the ipsilateral aca and mca across a patent acom arterytype c  bilateral ica agenesis with supply to the anterior circulation via carotid - vertebrobasilar anastomoses and through hypertrophic pcomtype d  unilateral agenesis of the cervical portions of the ica with collateral from an intercavernous communication from the cavernous segment of contralateral icatype e  bilateral hypoplastic icas supplying aca and the mcas are supplied by enlarged pcomstype f flow to ica across transcranial anastomoses from the branches of the eca system , the so - called rete mirabile . type a  unilateral absence of the ica with collateral circulation to the ipsilateral aca through a anterior communicating artery ( acom ) and to the ipsilateral mca from the hypertrophic posterior communicating artery ( pcom ) type b  unilateral absence of ica with collateral flow to the ipsilateral aca and mca across a patent acom artery type c  bilateral ica agenesis with supply to the anterior circulation via carotid - vertebrobasilar anastomoses and through hypertrophic pcom type d  unilateral agenesis of the cervical portions of the ica with collateral from an intercavernous communication from the cavernous segment of contralateral ica type e  bilateral hypoplastic icas supplying aca and the mcas are supplied by enlarged pcoms type f flow to ica across transcranial anastomoses from the branches of the eca system , the so - called rete mirabile . the simplified form of lie 's original six collateral pathways are : collateral flow through the circle of willis ( most frequent ) , collateral flow via persistent fetal circulation , and reconstitution of the ica through skull base collaterals from the eca . hypertrophy of normally present embryologic vessels or their persistence can explain the collateral circulation in types a , b , c , and e. however , in lie 's type d intercavernous communication between the icas could not be explained as there is no in utero communication observed between these arteries . various theories have been put forward regarding the development of abnormal intercavernous vessel . according to lie , the collateral vessel is thought to arise from a union between two primitive trigeminal arteries that have lost their communication with the basilar artery . another theory states that this may be due to hypertrophy of few embryologically developing vessels , like plexiform network surrounding rathke 's pouch or remnants of the primitive maxillary artery . these differing theories may answer the variable course of intercommunincating artery in relation to the sella turcica . the intercavernous channel can pass along the floor of sella , above the sella or posterior to clivus . also according to lie in type d collateral pathway , supply to the ipsilateral aca and mca the reported cases demonstrate that when an intercavernous communication exists , there is associated hypoplasia or aplasia ( aplasia in our case ) of the ipsilateral a1 segment of the aca with acom supplying the ipsilateral distal aca territory . thus the intercavernous communication supplies the majority of the ipsilateral mca supply making the hemisphere isolated from alternate routes of collateral flow . nearly , 18 cases of unilateral agenesis of ica with intercavernous anastomosis are reported in the literature . in adults presenting with thromboembolic disease , diagnosis of this anomaly is important as emboli in one cerebral hemisphere can be explained by atherosclerotic disease in the contralateral carotid artery or vertebrobasilar system . it is also important when planning for carotid endarterectomy , as both cerebral hemispheres may be dependent upon the atheromatous carotid artery . on imaging , the transsellar course of this vessel has been known to mimic a pituitary microadenoma . . they can be aberrant ica , persistent carotid vertebrobasilar anastomosis , or retropharyngeal tortuous ica . in aberrant ica , due to the agenesis of the cervical segment of ica the inferior tympanic branch of the eca and the carotico - tympanic branch of petrous ica are enlarged and the ica is reformatted from the petrous segment . multidetector computed tomography ( mdct ) show hypoplasia / aplasia of the vertical segment of carotid canal as well as reduced caliber and lateralization of the ica with protrusion of the artery into the middle ear . clinical examination of the ear can mimic glomus tumor and other vascular lesions like dehiscent jugular bulb , cholesterol granuloma , petrous carotid aneurysms , pseudoaneurysms , and hemangiomas . persistent carotid vertebrobasilar anastomosis include persistent trigeminal artery arising from the proximal cavernous ica , persistent otic artery arising from the petrous ica , persistent hypoglossal artery arising from the cervical ica , and the persistent proatlantal artery . more importantly , the ica can be tortuous and course medially bulging into the retropharyngeal space . on clinical examination it appears as a pulsating submucosal mass along the posterior pharyngeal wall . potential life - threatening complications can occur during intubation , endoscopy , and invasive procedures in the oropharynx like tonsillectomy or retropharyngeal abscess drainage . the finding of absent ica flow void on routine mri and absence of bony carotid canal on routine ct should suggest the diagnosis . it is important to identify the transsellar intercavernous anastomosis on imaging to avoid devastating complications during transsphenoidal pituitary surgery . it is also important in the management of cerebrovascular accidents as the single ica supply both the cerebral hemispheres .
unilateral agenesis of internal carotid artery ( ica ) with intercavernous anastomosis is a rare congenital anomaly . 
 we present a case of a 25-year - old female with 2-month history of holocranial headache . 
 neurological examination was unremarkable . 
 magnetic resonance imaging ( mri ) with magnetic resonance angiography ( mra ) showed absence of left ica with an abnormal intercavernous vessel in the sella . computed tomography ( ct ) showed absence of the left carotid canal . 
 doppler ultrasonography ( usg ) showed high resistance flow in the left common carotid artery ( cca ) . 
 since no hemorrhage or aneurysm was seen , patient was managed conservatively and is on regular follow - up . 
 based on our knowledge , this is the first case to demonstrate the features of unilateral agenesis of ica with intercavernous anastomosis in x - ray , doppler usg , ct , and mri scans of the brain .