Benign syringomyelia with the abortive type of the course

of the disease, lack of progression and/or development of myelopathic symptoms, and signs of syringomyelia cavity collapse according to magnetic resonance imaging findings. The authors designate this childhood-onset variant of the disease as abortive. The tendency towards collapse in the cavity in these patients may be due to a single patho-genetic mechanism, which is of interest for a further investigation.

Syringomyelia is a disease characterized by formation of progressive fluid-containing cavities in the spinal cord. The emergence of neuroimaging diagnostic methods empowered the research of this disease. It has been shown that primary cavity progression occurs in the first 5-10 years after the disease onset [1]. In the majority of cases, progression of symptoms decelerates with increased duration of the disease, which is associated with a certain degree of reduction of the cavity size due to its spontaneous collapse [2]. On magnetic resonance imaging (MRI) scans the cavity phenomenon can undergo changes from the initial round-oval shape on the axial section to its collapse in a long duration of the disease. It is characterized by various degrees of cavity flattening in the anterior-posterior direction (up to a threadlike shape) with concomitant signs of spinal cord atrophy [1,2,3]. Long-term follow-up of unoperated patients has demonstrated that 25.8% of them had a significant decrease in the cavity size over time [1]. This phenomenon, characterized by the full-scale myelopathy clinical picture and the collapsed cavity on MRI, was defined as a stage of prolonged syringomyelia and designated as "post-syrinx" syndrome [3].
According to recent publications, besides the described typical course of the disease, there are variants that have a benign course with the absence of spinal symptoms progression. Asymptomatic syringomyelia can be regarded as one of such variants. This variant is characterized by the presence of a cavity in the spinal cord, which is detected incidentally on MRI during an examination for other diseases, and has no clinical manifestations typical of long-term syringomyelia. The absence of symptoms in such patients does not correlate with the cavity size [4]. According to T.H. Millorat et al. (1995), symmetrical central cavities tend to have an asymptomatic course [5].
We observed a variant of syringomyelia with a spontaneous collapse of the cavity at the early stages of its development, which can lead to regression of the initial clinical symptoms or to the arrest of their progression. There are few clinical descriptions of such cases in literature [6]. We assume that they occur more frequently, but remain undiagnosed. Identification of such cases and their analysis allow us to determine the mechanisms of the reverse development of syringomyelia with childhood onset. We are presenting 4 clinical cases. The first one contains a complete evidence base for the dynamics of clinical and MRI data, while the others demonstrate a similar course of the disease. Cervicothoracic spine MRI showed a syringomyelia cyst up to 0.8 cm in diameter at the C2 level and lower, the cystic index (CI) was 0.5 (Fig. 1B). MRI scans of the posterior cranial fossa (PCF) revealed Chiari malformation (CM) (2.2 cm lower the foramen magnum), as well as the signs of narrow PCF and disturbance of bone indexes: Boogart's angle increased up to 144°. (Fig. 1A).
Given the lack of clinical manifestations other than the presence of scoliosis, the patient's parents decided not to have their child operated on by a neurosurgeon. The patient "dropped off the radar" for 10 years.
10 years later, patient S. complained of musculoskeletal pain in the interscapular area. The kyphoscoliotic deformation of the spine remained. The patient had growth delay. As for neurological status, there was CCN disease, muscle atrophy, while no disturbances of reflexes or sensitivity were revealed.
Cervical spine MRI revealed an almost complete collapse of the syringomyelic cavity (Figure 2A), the spinal cord was significantly atrophied and had a flattened anterior-posterior shape on the axial section (Fig. 2B). On the PCF MRI scans, cerebellar tonsils ectopia degree decreased to 0.9 cm below the foramen magnum line. Posterior subarachnoid space remained narrowed at the foramen magnum level. Boogart's angle increased to 147°. The remaining parameters of the PCF were within the normal range (Figure 2A) (Fig. 4.A). Spinal cord atrophy and syringomyelic cavity were revealed on the cervical spine MRI sagittal section at C2 level and lower with anterior-posterior size up to 2 mm, CI = 0.3 (Fig. 4A, B). The signs of spinal cord atrophy and collapsed cavity (elongated-threadlike shape in anterior-posterior direction) were seen on the axial section.

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spinal cord atrophy (threadlike, flattened shape of the spinal cord) were seen on the axial section, CI was 0.4. Discussion. The analysis of the presented cases showed almost complete cavity regression and spinal atrophy on sagittal and axial sections in Case 1. Spinal cord atrophy is the result of brain tissue compression by a cavity of considerable size and integration of remaining parenchyma volume after cavity regression (complete or partial). Clinical and MRI dynamics of Case 1 indicate the presence of a wide cavity at the initial stages of the disease, accompanied by scoliosis, which is the most frequent debuting symptom of syringomyelia in children. At a distant stage of the patient's illness, cavity collapse and cerebellar tonsils elevation were not associated with any new symptoms. We have designated such type of interruption of the further development of syringomyelia as abortive type of the disease. The other three cases are similar. In our opinion, they also represent an abortive type of the disease. In all these cases the disease began in childhood (with initial symptoms of scoliosis in 2 cases and symptoms of CM in 4 cases). Clinical manifestations of spinal cord damage were minimal in cases 2 and 3 and had no progression or were absent in case 4. In cases 2-4 the cavities were revealed at a mature or elderly age during an examination for other reasons. MRI data in all cases (2)(3)(4) revealed narrow cavities (2 mm) that had the sign of collapse on axial sections (a "flattened" threadlike cavity). There were also signs of spinal cord atrophy on sagittal and axial sections. All these data allow us to assume that the arrest of the disease progression at the early stage in childhood is associated with spontaneous collapse of the cavity. A significant factor is the cavity collapse at the early stages of the disease before the development of irreversible damage to the spinal cord or severe myelopathic symptoms due to prolonged pressure on the spinal cord parenchyma by the cavity. The influence of cavity long existence on the course of syringomyelia was confirmed by the results of surgical treatment of syringomyelia at the early and late stages of the disease. It is known that in long-lasting syringomyelia the possibility of significant neurologic regression after surgery is minimal [7].
To date, dozens of cases have been described along with the assumption of the mechanism of cavity spontaneous collapse in syringomyelia [6]. Most often, collapse of the cavity was verified in childhood and was accompanied by cerebellar tonsils elevation [8][9][10][11][12]. This is supposed to be related to the skeleton growth in children, to the increase of the PCF volume and, as a consequence, tonsil elevation, restoration of normal CSF circulation in the foramen magnum area and subsequent disappearance of the syringomyelic cavity [8]. Unlike children, spontaneous cavity collapse was accompanied by cerebellar tonsils elevation only in 1/3 of cases in adult patients, indicating different mechanisms of cavity resolution in adults and children. In 1991 C.R Jack et al. explained the adult case of a cavity collapse without regression of CM degree by an increase in intracavitary pressure and rupture of the spinal cord tissue, which led to the development of drainage between the syringomyelic cavity and spinal subarachnoid space resulting in the leak of the cavity contents [13]. Spontaneous cavity collapse, accompanied by cerebellar tonsil elevation in adults, could not have occurred because of the PCF growth. In this connection, J. A B C D Klekamp et al. (2001) suggested the presence of arachnoid adhesions at the level of the foramen magnum and foramen of Magendie, which could be the cause of subarachnoid space obstruction. Their rupture led to the restoration of liquor outflow [14]. As for the cases of spontaneous isolated CM regression in adults, a hypothesis has been put forward on the change in the ratio between the PCF and cerebellum volume due to the agerelated cerebellum atrophy [15]. This theory is applicable to elderly patients with syringomyelia associated with CM. In 1998 F. Fukutake at T. Hattori [16] published an observation that exclusion of physical exertion and other Valsalva-like maneuvers resulting in increased piston movement of the cerebellar tonsils [17] may lead to regression of syringomyelia associated with CM. Taking into account the mechanisms proposed in literature, it is possible to distinguish 2 main groups of theories of spontaneous cavity collapse: 1) CSF flow restoration through the foramen magnum (cerebellar tonsil elevation in response to the PCF growth [8][9][10][11][12]; arachnoid adhesion rupture at the foramen magnum level [14,18]; Valsalvalike maneuvers elimination [16,[19][20][21], cerebellar tonsils elevation due to the brain atrophy [15]); 2) development of drainage between the cavity and the spinal subarachnoid space [13,22,23].
It is important to note that it is not always cerebellar tonsils elevation in connection with the PCF growth that leads to spontaneous cavity collapse in childhood. In addition to this mechanism, syringomyelic cavities may collapse in children, as in adults, as a result of drainage into the spinal subarachnoid space, as well as after elimination of Valsalva-like maneuvers provoking syringomyelia development in pathological conditions at the foramen magnum level [24,25].
We assume that the basis for spontaneous collapse described in Cases 1-3 was the cerebellar tonsils elevation leading to the reversal of CSF circulation block which was related to the patients' growth and PCF distention, as evidenced by the presence of small PCF size on the initial MRI in Case 1, as well as preservation of minimal signs of PCF shallowness in Case 3. Cerebellar tonsils elevation was verified in the first case. In Cases 2 and 3 cerebellum was already in a physiological position at the time of the examination, which suggests the possibility of cerebellar tonsils elevation in childhood. In the last case, the cerebellar tonsils had a pronounced ectopia with preserved CSF circulation blockage at craniovertebral junction level. Therefore, the most likely mechanism of cavity collapse in this case is development of spontaneous drainage between the cavity and the spinal subarachnoid space.
The natural course of syringomyelia is poorly understood to date. It raises multiple questions for clinicians in choosing further patient management strategy in children, especially in those having asymptomatic course or minimal manifestations. The cases of spontaneous resolution of syringomyelia in childhood provide the opportunity to consider the ambiguity of surgical intervention in patients with minor symptoms and a cavity of small or medium size on MRI. The choice of conservative strategy involves a thorough neurologic examination and repeated MRI with a frequency of at least once in every 6 months in order to monitor neurological symptom progression and MRI indices to the age of skeleton and PCF full growth.
Despite the existence of numerous hypotheses, the specific mechanism leading to spontaneous syringomyelia resolution remains unknown. Further research is needed to explain the mechanism of spontaneous syringomyelia resolution and to discover new directions of treatment.