Alireza Mansouri MD1 and Travis Tierney MD2
1
2
University of Toronto, Division of Neurosurgery
University of Toronto, Division of Neurosurgery, Toronto Western Hospital
Case
C.R. was a 51 year old, right hand dominant female who presented to clinic with a 2 week
history of mid back pain and spasms that started when she initiated mild intensity back exercises.
Within a week, she developed bilateral leg pain (ranging from 7-8/10 in intensity, L>R). Two
days prior to presentation, she developed progressive leg weakness, subjectively L>R, to the
point that she was unable to ambulate independently. She had been afebrile and denied weight
loss, chills or rigors.
She did not report any history of congenital abnormalities or past surgeries. C.R. had been
generally healthy with the exception of two episodes of seizures for which she was taking
trileptal. There was no family history of cancers or neurological issues.
On examination, she had significant difficulty ambulating secondary to back spasms and
shooting pain in the left leg. As a result, it was not possible to assess her gait. Straight leg raise
(SLR) on the left was positive at about 10º and negative on the right side. Motor examination
revealed a significantly diminished left-sided strength (1/5) in the L4-5 nerve root distribution
and a mild decrease in the L1-2 (4/5) and L5-S1 (3/5) nerve root distributions. Strength on the
right side was normal (5/5). Pin-prick sensation was diminished in the L5 dermatome on the left
side but otherwise intact. Light touch and proprioception were also intact. There was no saddle
anaesthesia and rectal tone was normal. Babinski was down-going bilaterally. Reflexes were 2+
bilaterally and symmetrical.
MR imaging (Figure 1) revealed a diagnostically-challenging lesion. A saggital view of the T1weighted MRI without Gadolinium enhancement (Fig. 1a) revealed an obvious L1-L2 disc
herniation. However, in addition to the herniated disc, a hypo-intense lesion extending from T12L2 was also evident on the posterior portion of the spinal canal. The lesion did not enhance postgadolinium infusion (Fig. 1b). On T2-weighted imaging the lesion appeared to be iso-intense to
the cerebrospinal fluid (Fig. 1c). In the axial view of the T2-weighted image (Fig. 1d) the nerve
roots were eccentric secondary to the compression caused by the lesion. Within this view, the
lesion appears to be divided (arrow). This was believed to be either the dural sac, with the lesion
surrounding it, or a septum which had developed within the lesion. Based on the radiographic
images, it was difficult to ascertain whether the lesion was intra- or extra-dural; hence the
challenge in diagnosis. The patient was taken to the operating room for L1-L2 laminectomy
without discectomy (for decompression of the spinal cord). Final diagnosis of the unknown
lesions was only possible intra-operatively…
(A)
(C)
(B)
(D)
Figure 1. MR imaging of C.R.’s spine. (A) Saggital view, T1-weighted image, no Gadolinium (B) T1weighted image, post Gadolinium (C) T2-weighted image (d) Axial T2-weighted image, arrow points to
septation
What is the diagnosis?
Septated arachnoid cyst with arachnoiditis
Definition and epidemiology
The extension of an arachnoid-lined diverticular membrane, from the subarachnoid space, and
the subsequent creation of a pouch-like entity that communicates with the rest of the CSF space,
results in the creation of Arachnoid cysts (AC)1. It has been estimated that approximately 4% of
the population has an AC2. However, due to the slow growing nature of these cysts, many are
often asymptomatic. Symptoms such as back pain, spasms, and radicular pain due to
compression of the spinal cord or nerve roots3 are often difficult to distinguish from other neural
compressive disorders. ACs can occur either intra-cranially or less frequently within the spine.
Data in the literature indicate that spinal cysts extend an average length of 4.6 vertebral bodies
with the ventral ones spanning almost double that of dorsal ones (6.2 vs 3.6). Post- traumatic
cysts are often significantly larger than non-traumatic ones. In terms of location within the spine,
the thoracic spine is the most common area to be affected followed by the cervical region;
lumbosacral cysts are the least common. While symptoms can arise at any age they are
frequently observed in individuals in the 30-50 age range4.
Classification
ACs can be classified into three groups based mainly on their relationship to the dural sac and
nerve roots5. Type I cysts are those that are extra-dural and do not involve the nerve root fibers;
extra-dural meningeal cysts are further classified as IA while Sacral meningoceles are identified
as type IB. Type II cysts, commonly found in the sacral region, are extra-dural and do involve
the nerve root fibers. Type II cysts are also known as Tarlov’s cysts and are located between the
endoneurium and the perineurium of the nerve root sheath near the dorsal root ganglion but
outside the dural sac. Occasionally, the cyst may extend along the nerve root fibers into the dural
sac. Hence, there is controversy within the literature as to whether type II cysts should be
classified strictly as extra-dural or whether a sub-classification should be devised. Type III ACs
are intra-dural. In the case presented here an intra-dural cyst was observed, implying a type III
AC; this was definitively confirmed on histopathology. Basis of diagnosisMR imaging, often
with contrast, is often helpful in diagnosing ACs. However, as described above, intra-operative
assessment may also be necessary in certain cases. ACs can be identified on MRI as welldemarcated masses that are iso-intense with the CSF on T2-weighted sequences4. Intraoperatively, the presence of a cystic lesion containing clear fluid of similar consistency to the
CSF would suggest the diagnosis of an AC but pathological analysis may also be required.
Causes
Genetics play a key role in the pathogenesis of ACs. In fact, most cysts are congenital in
origin6,7. Congenital defects in the dural sac can result in outpouchings that serve as pockets for
the collection of arachnoid cysts. Arachnoiditis subsequent to spinal surgery, epidural injections,
or disc disease is another cause of ACs. Other inflammatory processes affecting the arachnoid
matter, such as subarachnoid bleeding or meningitis, can also result in the formation of these
cysts4.
Pathophysiology
The pathophysiological basis of spinal ACs involves the proliferation of fibroblasts that are
recruited for the repair of the arachnoid membrane post insult in a manner that is similar to the
repair of serous membranes. This involves very little in the form of inflammatory cellular
exudates but extensive fibrous exudate. As a result, adhesions are formed between the fibrin-coat
of the nerve roots and their meninges or the thecal sac. Consequently, CSF flow is obstructed and
pockets of CSF are created8. These cysts expand in size primarily via a valve-like mechanism4.
The breakdown of arachnoid trabecular cells into the cyst cavity has also been postulated to
result in expansion of the cyst secondary to osmosis-based influx of fluid 9. Other theories such
as active secretion of fluid from the cyst wall and passive diffusion of water into the cyst have
been suggested but not as well accepted as the ball-valve mechanism10. Symptoms arise either
when the cyst becomes large enough to cause compression or when a new lesion is created in the
presence of a long-standing cyst. Based on the extensive boneremodeling of the vertebral bodies
adjacent to the cyst in the present case, it is highly likely that our patient has had a long-standing
cyst and would have experienced symptoms as a result of the latter scenario.
Differential diagnosis
Among intra-dural cystic lesions of the spinal cord, ACs are the most common. However, it is
important to consider other possible diagnoses as part of the differential. Other cystic lesions that
can be found in the thoracic region include lipomas, neuroepithelial (ependymal), epidermoid or
dermoid, and neurenteric cysts4. Certain tumors, such as schwannomas and astrocytomas, can
also present as cystic lesions. The hypointensity of the lesion relative to the surrounding fatty
tissues on T1-weighted imaging argues against a lipoma4. Ependymal cysts have a low density
appearance on non-enhanced T1 images and are hyperintense on T2-weighted images11. Dermoid
cysts, rare developmental tumors, also tend to have fatty components, are usually midline, and
tend to present with other signs such as sacral dimpling or tufts of hair12. While they appear
heterogenous on MRI, homogeneity and calcifications can be observed within the cyst on CT.
Epidermoid cysts can be identified on diffusion studies as brightly-appearing lesions and are
hyperintense relative to the CSF13. Neurenteric cysts are another group of developmental
anomalies and they tend to be iso/ hyper intense relative to the CSF, depending on the protein
content within the cyst. Like other cysts secondary to a developmental anomaly, other
anatomical defects should be noted as part of the clinical exam14. It is noteworthy to mention that
the smoothness of the septation and the dural sac in our case argue against a surgical, traumatic
or inflammatory cause for the cyst6. Along with the observed changes on the adjacent vertebral
bodies, it is highly likely that C.R. had a rare form of a congenital AC, which was discovered
incidentally as a result of a mild disc herniation.
Treatment
Expectant management is a treatment option for asymptomatic patients. However, the presence
of neurologic signs dictates the need for surgical evaluation. Complete neurosurgical resection is
the treatment of choice for ACs. Partial resection or fenestration is an option for extensive spinal
cysts that require multiple laminectomies, ventrally located cysts that require significant cord
manipulation, and for multiple adhesions 4. In severe cases of recurrent cysts, shunting of the
cyst content to the pleural or peritoneal cavity may be considered.
Prognosis
Untreated, ACs may cause permanent severe neurological damage due to the progressive
expansion of the cyst(s) or hemorrhage3.
Recurrences of these cysts are probably the result of the cyst sealing after an insufficient primary
resection15. The incidence of recurrence varies, and they have recurred even after aggressive
removal of the cyst wall16. The results of surgery are usually excellent, with weakness,
hyperreflexia and incontinence more likely to improve than neuropathic pain and numbnes17. For
patients with a predominant radicular pain syndrome, lasting relief can be achieved after
surgery14. Neurological recovery is related to the duration and severity of cord compression and
to the associated atrophy or myelomalacia rather than the location of the cyst.
Back to the case …
After complete surgical resection of the cyst, C.R. remained in the hospital for 2 days for pain
control and to assess wound dehiscence. She was discharged home and follow-up with MR
imaging of the spine was arranged for 6 weeks later: aside from evidence of scarring, the cyst did
not appear to have recurred. On clinical assessment 6 weeks post discharge, C.R. was
asymptomatic as well. Repeat MRI will be performed in 1 year to reassess cyst recurrence.
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