Recurrent spinal arachnoid cyst in a cat

Abstract

Spinal arachnoid cysts (SACs) are uncommon expanding lesions in the spinal canal. They are rarely diagnosed in dogs, and there are only four published cases in cats. We report a case of a 12-year-old cat with recurrent signs of intermittent urinary incontinence and hind limb ataxia 2 years after surgical marsupialisation of a spinal arachnoid cyst at T11/12. Recurrence of a cyst was diagnosed by myelography. Repeated marsupialisation after laminectomy was successful and the cat recovered satisfactorily although intensive physical therapy was necessary. SACs are very rare in cats and seem to occur mainly as a secondary lesion to spinal and meningeal trauma or irritation due to bony changes of the vertebrae.

Arachnoid cysts represent subarachnoidal cerebrospinal fluid-containing diverticula, leading to neurological dysfunction of adjacent neurological tissue. They are incidentally found in the intracranial vault in companion animals (Milner et al 1996, Saito et al 2001, Vernau et al 2002) as well as in the spinal meninges.

Nabors et al (1988) have simplified the classification of spinal meningeal cysts into three major categories: extradural cysts without nerve root fibres (type 1), extradural cysts with nerve root fibres (type 2) and intradural cysts (type 3). Histological examinations of SACs in companion animals suggest the exclusive occurrence of type 3 cysts in animals (Nabors et al 1988, Grevel et al 1989, Dyce et al 1991, Rylander et al 2002). As no inner epithelial lining is present in the cysts examined in human and veterinary medicine they are more cerebrospinal fluid-filled diverticula of the subarachnoid space than true cysts (Schachenmayr and Friede 1979, Rengachary and Watanabe 1981, Skeen et al 2003). The aetiology of this lesion is unclear; however, congenital, traumatic and inflammatory aetiologies have been postulated. A small number of arachnoid cysts in people are associated with spinal dysraphism, meningomyeloceles (Rabb et al 1992, Kotil et al 2004), or occur as complications of adhesions following arachnoiditis, haemorrhage, surgery or any kind of trauma to the arachnoidea (Honda et al 1998, Shibata et al 2001). Theories of primary cyst formation have been abundantly discussed in human and veterinary medicine; however, the pathogenesis of primary cysts has not been clearly determined.

A 12-year-old Persian crossbreed was presented to the Small Animal Clinic (Surgery) of the Justus-Liebig-University (JLU), Giessen. Prior to presentation the owner had noticed signs of an abnormal gait in the hind limbs and urinary incontinence, which was initially mistaken for a urine marking problem. The gait abnormalities became progressively worse. No history of trauma was reported. Whereas no conscious proprioceptive deficits were present on the neurological examination, however, the cutaneous trunci reflex was absent all over the thoracic and lumbar vertebral column. Plain radiographs of the thoracic and lumbar vertebral column in laterolateral and ventrodorsal projections revealed spondylarthrosis of the facet joints at T11–13 (Fig 1). A T11/12 spinal arachnoid cyst was diagnosed on myelography and thereafter surgically treated with marsupialisation after hemilaminectomy. After 4 weeks of strict cage rest the cat recovered fully and showed no signs of neurological deficit on follow-up examination.

Fig 1.

Laterolateral plain radiograph of the thoraco-lumbar transition of the spine of a 12-year-old Persian crossbreed cat. Note the spondylarthrosis of the articular facets from T11/12 to T13–L1.

Two years after surgery, the signs of ataxia and incontinence recurred and the cat was presented again at JLU. Neurological examination revealed ambulatory paraparesis with severe hyperextension on both pelvic limbs. Patellar and tibialis cranialis reflexes were increased, the withdrawal was normal. The cutaneous trunci reflex was markedly reduced caudal to the thoraco-lumbar transition. The cranial nerves as well as the thoracic limbs were unaffected and there was no sign of spinal pain. The bladder was tight, enlarged, and difficult to express. Based on the clinical signs an upper motor neuron lesion, localised to T3–L3 spinal cord segments was suspected.

A complete blood count, serum biochemistry profile, electrolytes and urinalysis were within normal limits, feline leukaemia virus and feline immunodeficiency virus tests were negative. After injection of iopamidol (0.3 ml Solutrast/kg; Bracco Althana Pharma) into the cisterna magna under general anaesthesia a drop-shaped dilation of the dural sac at T11/12 could be identified. This finding was consistent with a subarachnoid cyst (Fig 2). Cerebrospinal fluid analysis was unremarkable.

Fig 2.

Occipital myelography performed in 2003 (A) and the second one performed 2 years after primary surgery (B). The recurrent subarachnoid cyst is located at a similar site to cyst seen before primary marsupialisation. The outlines of the cyst are more irregular on the second myelogram. The dorsal contrast column extends dorsally due to the former hemilaminectomy on this site.

A dorsal T11/12 laminectomy disclosed a pinkish glistening cystic mass originating from the meninges. When the dura was opened, the cyst was also entered, yielding a gush of colourless liquid, presumed to be cerebrospinal fluid. After excision of a specimen for histopathology (Fig 3), the fully opened cyst wall was sutured to the adjacent dura or to the paraspinal musculature, respectively, with 5–0 Monocryl to avoid reformation of a cyst. Closure was routine. To support voiding of the bladder 1 mg bethanechol (Myocholine Glenwood, Glenwood GmbH, Starnberg, Germany) three times a day and 0.25 mg phenoxybenzamin (Dibenzyran, esparma, Osterwedding, Germany) once a day were administered and the bladder was manually expressed if necessary. Medication was discontinued 4 weeks after surgery and the cat regained voluntary micturition control.

Fig 3.

Histological slice of a specimen of the cyst wall excised on the transition to the normal dura. The trichrome stain helps to highlight collagenous stroma (A). As the name implies, the tissue is stained in three colours, each colour being selective for the tissue element demonstrated. Collagen fibres stain intense green, nuclei of cells black or brown, cytoplasm stains red. The arachnoidea itself (a) is thickened and is appositioned to glioneural tissue. The silver impregnation method (reticulin stain, B) identifies the reticulin fibres in the wall of blood vessels. The abundant capillaries found in the glial trabecular tissue show fibrous thickening of their wall (B, arrows). There are no reticulin fibres apart from blood vessels (as found in peripheral nerve tissue). Fibrillary gliosis in central nervous tissue was confirmed by immunohistochemistry with antibodies to gliofibrillary acid protein (GFAP); nerve cells and their processes were identified with autoantibodies to neurofilament protein (both reactions not shown here). These changes are consistent with a meningomedullary scar as a residual lesion of a surgical procedure at the meninges.

Postoperative rehabilitation of motor function was considerably longer after the second surgery and intensive physical therapy was necessary to improve the neurological status of this patient. After 6 months, the cat was able to walk, but remained severely ataxic. Motor functions improved further with physical therapy but the hind limbs remained hyperextended.

According to the latest retrospective analysis by Jurina and Grevel (2004) only 54 cases of SACs have been published in dogs since 1968, whereas only four other cats with diagnosed SAC have been introduced to the literature to the author's knowledge (Grevel et al 1989, Shamir et al 1997, Galloway et al 1999, Vignoli et al 1999). There are insufficient feline cases to allow conclusions to be drawn regarding any breed predisposition, with cases reported in a Burmese, a Persian, a Domestic Longhair, a Domestic Shorthair as well as in a Persian crossbreed in our report. An intracranial arachnoid cyst at the cerebellopontine angle was also found in a Persian (Milner et al 1996).

The clinical signs going along with the cervical and thoraco-lumbar cysts were very consistent. Cats described in previous reports had clinical signs indicative of cervical or thoraco-lumbar disease. They showed paraparesis or tetraparesis with hyperextension of the limbs and exaggerated reflexes. Urinary incontinence has been identified in a number of dogs with caudal thoracic spinal arachnoid cysts (Skeen et al 2003) and similarly all the cats with described spinal arachnoid cysts had evidence of urinary incontinence. None of the cats exhibited signs of any pain during neurological examination.

The localisation of the cyst was variable, but three out of five cysts, including the cyst in our cat, were found at the thoraco-lumbar transition one at L1–L2, another at C3–C4. The latter was the only bilobular SAC. Four of the cats had degenerative changes of the spine and one had a history of lumbar spinal trauma at the level of the cyst formation. The age of the cats ranged from 5 to 12 years. All of the cats had a history of trauma or changes of the vertebral canal, like kyphosis, lordosis, spondylarthrosis or at least an enlarged spinal canal at the level of the cyst formation. As not all SACs have been histopathologically examined, the occurrence of SACs as a primary or secondary entity in these cats cannot be determined. The histopathological appearance of the cyst wall in our case showed inclusion of neural tissue enclosed in highly vascularised connective tissue, outlined by arachnoidea. This might be a sign for a reactive lesion due to meningeal inflammation or irritation. It might also explain the long rehabilitation caused by invasion and damage of neural tissue after the second surgery. Meningioneural adhesions with a tethering effect on the spinal cord are a well-known complication in spinal surgery in human medicine (Zhang et al 2004) and might contribute to the neurological damage. Although surgical manipulation was minimal the influence of the surgical procedure itself and of a reperfusion injury after decompression of a chronically compressed spinal cord may affect the postoperative neurological status.

The role of the surgical approach to the spinal cord referring to reformation of the cyst is unclear. Both hemilaminectomy and laminectomy have been successfully performed in dogs and cats and cysts have reformed after dorsal laminectomy and hemilaminectomy dogs and cats of previous reports. According to Rylander et al (2002) both approaches can be used. However, decompression alone is not sufficient without the removal of the cyst (Jurina and Grevel 2004). The method of choice is marsupialisation although full resection of the cyst wall seems to be equally effective. Significant predictors of good long-term outcome were not identified in dogs and cats, however, factors associated with a better outcome included the age (<3 in dogs) and the duration of clinical signs (<4 month; Skeen et al 2003). However, surgical removal or marsupialisation does not prevent cyst reformation in all cases.

The aetiology and pathogenesis of primary SACs is still a subject of debate in human and veterinary medicine. Some authors consider them to be congenital malformations in association with neural tube defects and vertebral column anomalies (Jena et al 1990, Rabb et al 1992, Lee and Cho 2001, Kotil et al 2004).

A common theory for the development of primary cysts in the human medical literature is the widening of the septum posticum of the spinal meninges (septum arachnoidale, septum posticum of Schwalbe, and septum cervicale medium) as it was first described by Perret et al (1962) and later by Lake et al (1974). They describe this septum as bundles of fibrous tissue interlacing with each other connecting the arachnoid with the pia mater. Local widening inside this structure was postulated to be the origin of SACs according to Perret, who defines SACs arising from the septum posticum as primary arachnoidal cysts. However, the formation does not explain the development of cysts in any other location (lumbar cord, ventrally to the cord, and intracranial vault).

The hypothesis that SACs could derive from this septum is often quoted in veterinary literature (Grevel et al 1989, Galloway et al 1999, Skeen et al 2003). However, the presence of a septum posticum in the subarachnoid space of dogs and cats is not mentioned in the veterinary anatomical textbooks (Kappers 1920, Ellenberger and Baum 1974, Schaller 1992, Nickel et al 1992, Evans 1993, King 1999) or specialised anatomical articles (Cloyd and Low 1974).

A related interpretation in human medicine believes that SACs originate from abnormal arachnoid proliferation of the trabecular system during the embryonic period (Agnoli et al 1982). This congenital abnormality could lead to the formation of arachnoid diverticula which gradually develop into cysts (Teng and Papatheodorou 1966). Thorough study of the ultrastructure of the spinal meninges may offer a synthesis of the two latter theories. Electron microscopy studies revealed detailed information about the morphology of the arachnoid trabeculae of the dog (Cloyd and Low 1974). Whereas some mammals develop only few arachnoid trabeculae others develop closely packed, fused bands, strengthened with fibrous tissue especially in the dorsal midline (Brauss 1932, Klika 1967, Nickel et al 1992). These individual anatomical findings may form the basis of the varying reports about the development of ‘abnormal’ arrangements of arachnoid trabeculae or the presence of a posterior septum in the subarachnoid space in some animals as mere individual physiological variations of the arachnoid trabeculae. Condensations of these ‘pillars’, especially web-like formations are found anywhere dorsally and ventrally in the subarachnoid space and may play a role in the development of SACs. Although no electromicroscopical study of the feline meninges exist to the authors' knowledge, cats could possess comparable anatomical structures.

Whereas the origin of primary SACs is controversial, there is common agreement that SACs can develop secondarily to any kind of irritation or trauma to the meninges leading to adhesive arachnoiditis and a blockage of the cerebrospinal fluid flow. Cases occur following inflammatory meningitis from viruses, bacteria, subarachnoid haemorrhage, spinal surgery, myelography, disc disease, trauma and any kind of degenerative changes of the vertebrae (Perret et al 1962, Sklar et al 1989, Dyce et al 1991, Shamir et al 1997, Kazan et al 1999, Jurina and Grevel 2004, Muthukumar 2004). These result in scar formation, which separates the subarachnoid space into diverticula. Haemosiderin containing macrophages (haemosiderophages) found trapped in cyst walls or in the cyst lumen support the theory.

An SAC is an uncommon entity leading to chronic spinal cord compression, but should be considered in differential diagnosis of spinal cord disease. Signs of spinal cord compression can recur after successful surgery due to reformation of the cyst. The aetiology of cyst formation is not certain but believed to be caused by meningeal irritation in cats. The prognosis after surgical decompression and removal of the cyst is good, however, clinical signs of paresis can recur because of reformation of the cyst and may be more severe due to scar formation or adhesions.