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an intracranial arachnoid cyst ( iac ) is a developmental brain disorder in which fluid accumulates within a split of the arachnoid membrane .
these structures do not fit the anatomical definition of a cyst and should be referred to as pseudocysts or diverticula .
however , we have adopted the widely used term  arachnoid cyst in this article .
the cavity does not usually communicate with the subarachnoid space (  non - communicating cyst ) .
however , iacs communicating with the subarachnoid space have been reported in humans . in humans , iacs are often incidental findings , but their clinical incidence seems to be more frequent than previously thought in humans , as well as in dogs , although this is hard to confirm .
iacs represent 1% of all intracranial masses in humans , and they have been sporadically reported in dogs , where they occur most commonly in the quadrigeminal cistern .
only three cases have been reported in cats ; all were located in the caudal fossa at the level of the quadrigeminal cistern .
this case describes an iac with an unusual location treated conventionally by cystoperitoneal shunt ( cps ) placement .
a 1-year - old male neutered european cat was presented to the small animal neurology department at vetagro sup , lyon , after a 24 h history of focal seizures and one generalised seizure .
the cat had about 12 complex partial seizures characterised by orofacial motor signs ( chewing , licking , and eye and facial twitching ) , hypersalivation , obtundation , tremors of the thoracic limbs and urination .
routine biochemistry ( glucose , urea , creatinine , alkaline phosphatase , alanine aminotransferase , total proteins , total cholesterol and triglyceride ) and bile acid stimulation were normal . magnetic resonance imaging ( mri ) of the brain was performed using a 0.2-tesla mri system ( e - scan xq ; esaote ) . a large , well - demarcated , extra - axial cystic lesion was present in the left temporal area , mainly beneath the piriform lobe .
it measured 1.6 cm in width , 1.7 cm in height and 2.1 cm in length at its largest points .
the signal intensity of the content was identical to cerebrospinal fluid ( csf ; hypointense on t1-weighted and fluid - attenuated inversion recovery images , hyperintense on t2-weighted images ) ( figures 1 and 2 ) .
this lesion did not enhance after administration of gadolinium ( dotarem ; guerbet ) . t1-weighted transverse magnetic resonance image ( left ) demonstrating a hypointense extra - axial lesion in the left temporal lobe ( open arrowhead ) .
the lesion is hyperintense in t2-weighted images ( right ) t1-weighted parasagittal magnetic resonance image demonstrating a hypointense lesion in the left temporal lobe ( open arrowhead ) the degree of parenchymal compression of the forebrain was calculated with a method previously described by matiasek et al for iac in dogs .
the mri features were consistent with the diagnosis of a supratentorial left arachnoid cyst with left hemisphere ( and particularly temporal lobe ) compression .
the seizures were controlled with anticonvulsant treatment started after admission ( phenobarbital 2.5 mg / kg q12h ) .
the catheter can be observed as a hypointense signal on the right ( filled arrowhead ) a dorsolateral approach to the skull was chosen , lateral to the sagittal crest .
a peritoneal catheter was tunneled to the left hypochondrium and introduced into the peritoneal cavity by mini - laparotomy .
it was anchored to the temporal bone and fixed to a low - pressure valve in the neck , under the muscular fascia .
however , persistent modifications of the brain parenchyma were observed in the temporal lobe , including decreased thickness of the temporal lobe compared with the contralateral side and slight signal abnormalities in t2-weighted images ( figure 3 ) .
four months after treatment discontinuation , epilepsy recurred and the cat presented two focal and one generalised seizures over 24 h. mri was performed again and showed the same lesions as described 3 weeks after surgery ( figure 3 ) .
phenobarbital treatment was restarted at the initial dose ( 2.5 mg / kg q12h ) ( figure 4 ) .
( a , c ) t2-weighted image signal abnormalities ( diffuse hyperintensity ) in the former cyst location ( filled arrowhead ) at ( a ) 3 weeks and ( c ) 21 months after surgery .
( b , d ) t1-weighted image showing parenchymal atrophy in the former cyst location ( open arrowhead ) compared with the contralateral side at ( b ) 3 weeks and ( d ) 21 months
differential diagnoses for intracranial cystic lesions are extensive but histological analysis of the lesions can give an accurate diagnosis . in our case ,
the lesions considered in the differential diagnoses list in cats , according to veterinary literature , are parasite cysts , cystic neoplasms , abscesses , cysts with a non - neural epithelial lining or ex vacuo - type cysts . the lack of contrast enhancement , extra - axial position and homogenous csf - like aspect of this cyst makes all other possibilities than an iac less likely . to our knowledge
, this is the first case of a supratentorial arachnoid cyst reported in a cat .
the cats were three persian - breed males ranging in age from 10 weeks to 4 years .
the cysts were diagnosed with mri in two cases or during necropsy in one case .
the cyst was in the caudal fossa at the level of the quadrigeminal cistern in all cases , and in one case it extended rostrally to the cerebral hemisphere .
canine iac tends to occur mainly in the quadrigeminal cistern , with other infratentorial locations sporadically reported ( eg , cerebellomedullary angle ) . in humans ,
iac are mainly supratentorial , with 5060% of them occurring in the middle cranial fossa .
they occur less often in the caudal fossa ( 10% ) , wherein they occur most frequently in the cerebellopontine angle and not the quadrigeminal cistern . in our case ,
the cystic lesion was mainly in the forebrain region , spreading slightly to the brain stem , and it looked more similar to human than canine iac .
iac is often an incidental finding in humans , and it has recently been suggested that this may also be true for dogs ; however , numerous canine symptomatic cases have been reported .
matiasek et al evaluated brain compression in 28 dogs with quadrigeminal cysts and concluded that occipital lobe compression of > 14% was always associated with clinical signs ( focal seizures , generalised seizures or both ) .
no such statistics exist in cats because only three cases of quadrigeminal cysts have been reported , and their clinical significance is difficult to assess . the first case was a 1-year - old cat with a history of uncontrollable seizures .
anticonvulsant therapy ( diazepam and phenobarbital ) stopped the seizures for 5 months , but the cat relapsed and was euthanased .
the second reported case was a 4-year - old cat with a 12 month history of episodic collapse and altered mentation that was presented after a severe episode of non - ambulatory tetraparesis and cranial nerve abnormalities .
the third case was a 10-week - old kitten that died after a 24-h period of lethargy with decreasing levels of consciousness . in our case , the clinical significance of the cyst , located in the left temporal area , is difficult to assess .
the type of seizures ( complex partial seizures with predominant orofacial motor signs ) is similar to those observed with hippocampal necrosis .
this value is given on an indicative basis and can not be compared to the percentages found in the study by matiasek et al ( 12 23% ) because the location of the cyst was not the same .
recurrence of seizures after resolution of the cyst can be explained by the persistence of lesions in the temporal lobe ( atrophy and diffuse hyperintensity in the hippocampal structures in t2-weighted images ) ( figure 3 ) .
management of iac in dogs is usually achieved by either cyst fenestration or , more commonly , by cps placement . in humans , it is reported that cps reduces the cyst permanently , while fenestration may require secondary intervention .
arachnoid cysts do not have an epithelial lining and should be described as pseudocysts or diverticula , but the inner membrane is believed to have secretory activity . in veterinary medicine , both fenestration and shunting are described as treatments , but no study reports the therapeutic superiority of any of them . in our case , cps placement resulted in no complications during surgery and lead to total resolution of the cyst 21 months after surgery ( mri assessment ) .
shunting was chosen because it is considered as an efficient and low - morbidity method , and is reported in the literature more frequently .
although this technique does not allow histological analysis of the cyst , to avoid catheter leaking , it is considered a less invasive method than fenestration .
to our knowledge , this is the first reported case of a supratentorial arachnoid cyst in a cat , as all other iacs mentioned in the literature were quadrigeminal cysts .
the location of the cyst in the temporal lobe is unusual in cats and dogs but is consistent with the predominant location in humans .
the semiology of the seizures suggests that the cyst had clinical significance , even if this can not be definitively proven .
shunting could be considered a reference method in iac surgical therapy but too few cases are reported in veterinary medicine to assess the therapeutic superiority of either cps or fenestration .
arachnoid cysts are defined as an accumulation of fluid within the arachnoid membrane . 
 feline intracranial arachnoid cysts are seldom reported , with only three cases in the veterinary literature . 
 a 1-year - old male neutered european cat with a 24 h history of seizures was presented to the small animal neurology department at vetagro sup , lyon . magnetic resonance imaging ( mri ) revealed a large intracranial arachnoid cyst ventral to the brain in the left temporal area . 
 cystoperitoneal shunt placement resulted in complete resolution of the cyst without recurrence ( follow - up mris 3 weeks and 21 months after surgery ) . 
 anticonvulsant treatment ( phenobarbital 2.5 mg / kg q12h ) was initiated at presentation and gradually stopped after 17 months . 
 seizures recurred 4 months after ending treatment , and seizure therapy was therefore restarted at the initial dose . 
 we report a case of an intracranial arachnoid cyst in an unusual location not previously described . 
 a cystoperitoneal shunt resolved the cyst without complications . 
 maintenance anticonvulsant treatment was required to control symptomatic epilepsy .