Introduction

Chiari malformation type I (CM-1) is a congenital disorder resulting from a faulty development of the paraxial mesoderm [16, 17]. It is characterized by a reduced volume of the posterior fossa and by the caudal displacement of the cerebellar tonsils through the foramen magnum into the cervical canal [6, 8]. Syringomyelia has been associated with many intracranial and spinal anomalies. CM-1 malformation and syringomyelia present with clinical manifestations of posterior fossa or foramen magnum involvement, i.e., head and neck pain, retro-ocular pain, cerebellar signs, cranial nerve dysfunction, sleep apnea, and respiratory difficulty, or with those of spinal cord compression such as loss of fine movements, dysesthesias, dissociated sensory loss, sensory level, spasticity, scoliosis, or sphincteric disturbances. Nonetheless, supratentorial and posterior fossa mass lesions, such as tumors or arachnoid cysts, may also result in tonsillar herniation with or without syringomyelia [5]. Obstruction of cerebrospinal fluid (CSF) flow may lead to an increased pulse pressure within the spinal canal that may force the entry of CSF into the spinal cord [19]. There have been several reports of large arachnoid cysts in the posterior fossa associated with syrinx [2, 4, 7, 9, 11, 18–22, 24, 25].

Arachnoid cysts are benign collections of CSF that develop as the result of an abnormal duplication of the arachnoid membranes during brain development. They may lead to symptoms according to their location, but more often, their symptoms are unspecific [10]. Whether these two malformations are concomitant rather than caused by the presence of one of them is not clear. Obstruction of CSF flow is most commonly caused by the Chiari malformation and by crowding of the posterior fossa structures. A different mechanism has been proposed that attributes the development of tonsillar herniation and syringomyelia to the push exerted by a retrocerebellar arachnoid cyst [4, 14, 15]. The aim of this paper is to report a series of adult patients with CM-1 associated with the presence of an extra-axial cerebellar cyst, six of them with associated syringomyelia, and discuss its management.

Clinical material and methods

Included in this survey were ten patients (seven male and three female), with mean age of 33 (range, 23–49) years, who presented with clinical and imaging findings of a tonsillar descent together with a posterior fossa arachnoid cyst. We reviewed these individuals’ medical records in regard to symptoms and signs, neuroimaging findings, and operation charts (n = 8) when applicable. The degree of tonsillar herniation, arachnoid cyst location and size, and the presence of hydrocephalus and of associated brain and osseous anomalies were investigated in the neuroimaging studies. Preoperative cranial and spinal magnetic resonance (MR) findings were reviewed and compared with those obtained along the patients’ follow-up. Evolution and outcomes of the study group were obtained from the patients’ medical records or by clinical revision at the outpatient clinic and classified in worsened, unchanged, improved, and complete recovery.

Results

Patients’ characteristics and management

The patients’ most common clinical complaint was headache, which was limited to the occipito-cervical region in two of them. Retro-ocular pain was seen in two instances. Individuals with upper-extremity weakness had, in addition, dizziness or strabismus. One patient, diagnosed with a cervicothoracic syrinx and hydrocephalus, evolved with nuchalgia. Other symptoms included vertigo, nystagmus, acuphenos, and loss of hands’ fine movements. No patient had a relevant medical history, such as prenatal infections, head trauma, or postnatal meningitis. The patients’ main characteristics are summarized in Table 1 (Figs. 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10).

Table 1 Clinical characteristics of ten adult patients with posterior fossa arachnoid cysts and Chiari type 1 malformation
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Fig. 1
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a Patient 1. preoperative T1 sagittal MRI of a quadrigeminal posterior fossa arachnoid cyst compressing the upper vermis of the cerebellum and provoking secondary CM1 and b the same patient’s CT scan made in the 1980s because of headaches without evidence of an arachnoid cyst

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Fig. 2
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Patient 2. MRI T1 sagittal shows in a patient with achondroplasia, hydrocephalus, and platybasia, a midline posterior fossa arachnoid cyst and associated Chiari I malformation. This patient had resolution of hydrocephalus, cervical syrinx, and Chiari symptoms after insertion of a VP shunt

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Fig. 3
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a Patient 3. Preoperative T2 sagittal MRI image showing a midline posterior fossa arachnoid cyst associated with Chiari I malformation and syrinx from C2 to T1. This patient had a third ventriculostomy for hydrocephalus, before treating the posterior fossa with surgery (b)

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Fig. 4
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Patient 4. T2 sagittal MRI of a preoperative image of a hemispheric posterior fossa arachnoid cyst, cervical presyrinx state, and CM1

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Fig. 5
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a Patient 5. T2 sagittal showing a midline posterior fossa arachnoid cyst, Chiari I malformation, and significant cervicothoracic syrinx. b T2 coronal sequence of the same patient. Although the cyst was relatively small, after posterior fossa decompression and cyst fenestration, the patient improved his syrinx state

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Fig. 6
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a Patient 6. T1 axial MRI of an hemispheric posterior fossa arachnoid cyst. Note the important mass effect compressing right cerebellar hemisphere. b Same patient, an T2 sagittal image showing a secondary CM1

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Fig. 7
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a, b Patient 7. Preoperative T1 and T2 sagittal MRI image showing a midline posterior fossa arachnoid cyst associated with Chiari I malformation and syrinx from C2 to C6

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Fig. 8
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a Patient 8. T2 axial image showing an arachnoid cyst in the right cerebellopontine angle and in sagital T1 sequence (b), the associated Chiari malformation with no syrinx. This patient was treated conservatively

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Fig. 9
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a Patient 9. T2 MRI axial left cerebellopontine angle arachnoid cyst compressing brainstem and b its asssociated C1 Chiari malformation in T1 sagittal sequence

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Fig. 10
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a Patient 10. Preoperative T2-weighted MRI showing a cerebellopontine angle arachnoid cyst (b), tonsillar descent, and significant cervical syrinx formation on sagittal sequence. The patient had hydrocephalus (not shown). After a third ventriculostomy (c), it is evident the shrinking of the cervical syrinx along with upward movilization of the tonsils, with the PFAC remaining intact

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Cranial MR imaging disclosed cerebellar tonsil descent into the cervical canal that reached the posterior arch of C-1 in four patients, the lamina of C-2 in other three cases, and the tectorial membrane in three remaining cases. Six patients had syringomyelia that involved the cervical cord (n = 3) or the cervicothoracic region (n = 3). Cine phase-differential mode MR imaging was not performed in our patients.

Of the three patients with hydrocephalus, two initially underwent endoscopic third ventriculostomy and the other ventriculoperitoneal shunting. Surgery for the tonsillar herniation included suboccipital decompression with duraplasty (n = 5), with additional C1 posterior arch resection (n = 3) and limited suboccipital decompression with tonsillectomy and duraplasty (n = 1). Fenestration with ample resection of the cyst wall was done in five cases. No syringo-subarachnoid shunting was performed. The initial surgical technique employed was dictated from the main clinical manifestation and, regarding the posterior fossa, according to the surgeon’s particular preference. Two patients with chronic headaches had a small cerebellopontine angle cyst and were treated conservatively with oral analgesics. Tonsillar descent in these two patients reached the posterior atlanto-occipital ligament in one case and the posterior arch of C1 in the other, and both had no syrinx. They had normal somatosensory and visual evoked potentials, normal ocular fundi and audiograms. Diffusion-weighted mode and contrast-enhanced MR and computed tomography (CT) imaging excluded the diagnosis of an epidermoid cyst. The patients’ mean clinical evolution was 12 months with a range from 6 months to 3 years.

Posterior fossa lesions in our patients included vermian/mega cisterna magna (n = 4), cerebellopontine angle (n = 3), quadrigeminal cistern (n = 1), and hemispheric cerebellar (n = 2) arachnoid cysts. Other malformations included temporal primary arachnoid cyst, achondroplasia with platybasia, and craniofacial dysmorphism in the context of trichorhinophalangeal syndrome.

Mean follow-up time was 2 years (range, 6 months to 5 years). One patient with hydrocephalus and achondroplasia was initially treated with a ventriculoperitoneal shunt. Imaging follow-up, including CT and MR, in this patient, performed 8 months after surgery, showed resolution of hydrocephalus. The cerebellar tonsils returned to a normal position, which was accompanied by improvement of clinical symptoms. In another patient, who initially underwent a third ventriculostomy, the cerebellar tonsils returned to a normal position; her syrinx improved along with her clinical symptoms (Fig. 10). In another patient, firstly submitted to third ventriculostomy, the cerebellar tonsils failed to return to a normal position, and this was accompanied by a lack of clinical improvement. Accordingly, this patient underwent surgery for his posterior fossa arachnoid cyst and tonsillar descent.

Clinical evolution and outcomes

The patients’ clinical outcomes are summarized in Table 2. Five patients experienced a total clinical recovery: two with cerebellar hemispheric cysts, two with a vermian pouch and one with a quadrigeminal plate cyst. One of these patients had hydrocephalus and achondroplasia, while the other had a temporal arachnoid cyst. Three of them referred resolution of their headaches, while the other two referred to have ameliorated substantially.

Table 2 Surgical and conservative treatment and evolution of clinical characteristics in patients with posterior fossa arachnoid cysts associated with Chiari type 1 malformation
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A partial clinical improvement occurred in the remaining five individuals. These patients improved in regard to dizziness, ataxia, upper limb weakness, hand fine movements, and tremor. One of them had persisting strabism with resolved hydrocephalus and syrinx. All of these cases referred persisting headaches, which constituted their main clinical symptom at presentation. There were no surgical complications among our patients.

Regarding neuroimaging findings, syringomyelia was resolved in four patients and improved in two, as was evidenced at their first or second postoperative spinal MR performed 5 to 8 month after surgery. Patient 5, who had trichorhinophalangeal syndrome, presented with a significant cervicothoracic syringomyelia. This patient’s motor deficits improved immediately after surgery, although a decrease in syrinx size was not appreciated until the MR study performed 1 year later.

Illustrative case report

A 38-year-old man presented with headaches, right arm weakness, and sensory loss to pain and temperature on his right hand of 24 month evolution. He also referred having had three episodes of drop attacks. On physical examination, the patient had a bilateral sixth cranial nerve paresis and loss of hearing on the left side. Motor strength and ability were diminished on his upper extremity, but there was no atrophy. Deep reflexes could not be elicited on the right arm. There was also dissociated hypoesthesia on his right arm and shoulder. MR showed a retrocerebellar arachnoid cyst, descent of the cerebellar tonsils, and a syringomyelic cavity extending from C2 to T6 together with hydrocephalus (Fig. 3). The patient underwent an endoscopic third ventriculostomy, after which the headaches markedly improved. A 6-month postoperative cerebral MRI study showed marked reduction of hydrocephalus but his posterior fossa cyst, cerebellar tonsil descent, and syringomyelia were unchanged. The patient underwent a suboccipital craniectomy and Cl laminectomy with ample removal of the arachnoid cyst’s walls. The dura mater was closed with a dural graft. Histopathological study of the excised tissues showed flattened cells of meningothelial origin on a loose fibrous wall with normal vessels. The patient made an uneventful recovery and, at 10-month follow-up visit, his objective neurological examination remained unchanged, although he had gained some strength on his right hand. A control MR showed normalization of the cerebellar tonsils’ position and a significant shrinkage of the cervicothoracic syringomyelic cavity.

Discussion

We report on ten patients in whom a posterior fossa arachnoid cyst was found associated with clinical and imaging findings of tonsillar heniation and syringomyelia in more than half of them. Two of our cases have been previously reported [14, 15]. Hans Chiari [8] described in 1891 this anomaly as one of the four types of hindbrain malformations. Evidence supports the hypothesis that the main problem in the Chiari I malformation is a posterior cranial fossa of smaller than normal volume [19]. The posterior fossa originates by endochondral ossification of the cartilaginous scaffold that forms the cranial base as well as the first four somites that model the occipital bone [4]. Marin-Padilla and Marin-Padilla [13] attributed the origin of the Chiari malformation to a primary defect in the development of the basicranium. Based on dynamic MRI studies, Oldfield et al. [19] proposed that the descended tonsils obstruct the CSF flow to and from the spinal compartment at the foramen magnum, thus explaining the appearance of syringomyelia. Several reports have documented tonsillar herniation and syringomyelia in a variety of conditions, including supra- and infratentorial tumors, pseudotumor cerebri, craniosynostosis, cranio-encephalic disproportion, overdrainage of CSF, or spinal CSF drainage [1, 2, 6]. Hence, in cases of so-called “acquired” Chiari malformation, it seems that there is a pressure gradient across the cranial and spinal compartments. According to one theory, syringomyelia results from obstruction to CSF flow at the foramen magnum and at the outlets of the fourth ventricle [1–3, 12].

We may assume that most, if not all, of our patients have an “acquired” CM-1. We cannot affirm that tonsillar descent is acquired, rather than we suggest it might constitute a combination with a developmental origin. The posterior fossa cyst in most of the presented cases appears to be the precipitating factor, as it presses down the lower part of the cerebellum through the foramen magnum. On the other side, we had cases with relatively small cysts of the cerebellopontine angle (cases 8–10), in whom the pressure exerted by the pouch, probably, was not sufficient to lead to CM-1.

Posterior fossa arachnoid cyst, syrinx, and hydrocephalus

The average delay in diagnosis of a Chiari malformation is 5 years after the onset of symptoms, leaving ample time for the development of syrinx formation [7]. Although an extensive discussion regarding the origin of syrinx is beyond the scope of this paper, we found 15 reported cases of syringomyelia in association with diverse cystic conditions of the posterior cranial fossa. Cystic lesions included congenital primary arachnoid cysts, Dandy–Walker malformation, Blake’s pouch cyst, and posttraumatic CSF pouches [2, 4, 7, 9, 11, 18–22, 24]. However, all these instances were case reports. Tubbs et al. [23] reported two cases of mega cisterna magna in a series of pediatric CM-1. In previous documented cases, the obstruction to normal CSF flow through the foramen magnum was due to blockage caused by the cyst wall itself, and all manifested with coexistent syringomyelia [2, 4, 7, 9, 11, 18–22, 24, 25]. The reported cases with both CM-1 and posterior fossa cysts have been managed with a variety of procedures, including foramen magnum decompression, with or without cyst removal [4, 7, 9, 11, 20, 22, 24, 25], cysto-peritoneal shunting [21], or endoscopic cyst fenestration [18]. In addition, ten of these patients had hydrocephalus of varied severity that might have contributed to the formation of the syrinx. In our series, six cases had syringomyelia, and three of them hydrocephalus. After posterior fossa decompression along with cyst wall resection, insertion of a ventriculoperitoneal shunt (VPS), or third ventriculostomy, the syringomyelic cavity had improved or completely resolved.

Milhorat et al. [16] reviewed a series of 364 symptomatic patients with CM-1, and 43 of these patients had a familial history of CM-1 or syringomyelia, which suggests a genetic transmission. We had two patients with a strong genetic trait, namely, trichorhinophalangeal syndrome type I and achondroplasia. Patients with achondroplasia in all age groups generally have a small cranial base, whereas the rest of the skull is normal in size. This volume discrepancy may predispose patients to anomalous formation of the posterior fossa and of the foramen magnum, thus creating a smaller than normal posterior fossa. In our patient with achondroplasia and hydrocephalus, VPS was sufficient to resolve both the CM-1 and the syringomyelia. In another case with achondroplasia reported by Bauer et al. [4], the obstacle to CSF circulation through the foramen magnum was due to acquired Chiari malformation that, in turn, seemed to originate from the cerebellar displacement exerted by the push of the retrocerebellar arachnoid cyst. Chiari malformation and syringomyelia in our other cases must have been the consequence of the pressure exerted by the retrocerebellar cyst, as demonstrated by the resolution of the tonsillar descent and the syringomyelia following osseous decompression and surgical excision of the cyst walls.

As in other instances of hydrocephalus-associated syringomyelia, the initial treatment for hydrocephalus, in our patients, was aimed at alleviating the raised intracranial pressure and to prevent the risk of acute cerebellar herniation that can occur during the posterior fossa procedure, while the second operation consisted of a foramen magnum decompression with ample fenestration of the cysts.

We had included headaches as a clinical relevant factor for these patients. In the series by Helland and Wester [10], 11 out of 13 surgically treated posterior fossa arachnoid cyst presented with headaches, and only one of them had a poor clinical outcome after surgery. Two of our patients complained of retro-ocular pain—a symptom that could be attributed to stretching of the inferior portion of the tentorium by the crowded cerebellum. This feature resolved after surgery in one case, and in another, it was improved after conservative treatment. Certainly, chronic and persisting headaches hardly disappear totally after posterior fossa decompression.

We acknowledge that a shortcoming of our paper is the lack of preoperative CSF flow studies. However, in all our operated cases, we could verify the restoration of the CSF flow by direct inspection during the procedure through the open dura mater. In addition, the decrease in the volume of the syrinx after removal of the cyst walls and duraplasty, as shown by repeat neuroimaging studies, seems to indicate improvement in the CSF flow at the foramen magnum.

The presence of syringomyelia, the level of tonsillar descent, and the cyst size constitute key factors at the time of deciding surgical treatment. Syringomyelia alone constitutes a main indication for surgery. Interestingly, we have also documented the progressive enlargement of a quadrigeminal arachnoid cyst in one of our patients, until reaching a clinically significant size, during a 25-year follow-up time (Fig. 1). Three cases of relatively small cerebellopontine angle arachnoid cysts are being treated conservatively. We believe that a wait-and-see conduct can be indicated in selected cases that evolve with subtle clinical symptoms and have no syringomyelia. From our literature review and from our observations, it is obvious that patients with syringomyelia or CM-1 must be managed in an individualized manner. The pathogenetic factors, such as the presence of a posterior fossa arachnoid cyst and the coexistence with hydrocephalus, should be taken into account. This should tailor surgical treatment accordingly.