As a result of the increased use of diagnostic imaging procedures in recent years, there have been larger numbers of patients diagnosed with filiform enlargement of the central spinal canal. Sometimes, these patients are quickly given the diagnosis of syringomyelia and thereby stigmatised. In these cases, it is useful to perform a careful diagnostic evaluation in order to differentiate between true early-stage syringomyelia and a congenital enlargement of the central canal or terminal ventricle [24]. True hydromyelia does not have a progressive clinical-neurological course and does not change on serial imaging and hence does not constitute a pathological finding.

In the pre-MRI era, the prevalence of syringomyelia was estimated to be about 8.4 per 100,000 [28] patients treated in a neurological clinic, but since the introduction and widespread use of MRI, this estimate has risen to 24.5 per 100,000 [16]. In those forms of syringomyelia associated with cerebral malformations, symptoms are often quite delayed and typically occur in the third to the fifth decades of life (the most frequently associated cerebral defect: Chiari I malformation). Frequently, the initial symptom is pain and paraesthesia in the arms [34] and, subsequently, nuclear atrophy. Spasticity primarily occurs in the lower extremities. With a progressive course, the expansion of the cavities results in paresis and atrophy, often unilateral at first, and scoliosis due to paresis of the paravertebral musculature. In children, scoliosis is often the predominant initial clinical symptom of an underlying syringomyelia. Neurological abnormalities only develop after certain period has elapsed. In these cases, one clinical sign indicating the presence of a syrinx may be unilaterally diminished or absent abdominal skin reflexes [35]. The syrinx is predominantly located in the cervical or upper thoracic spinal segments. However, rostral extensions to the brainstem (syringobulbia) or caudal extension to the sacral segments is not unusual.

The spontaneous course of the illness depends upon the underlying pathology. In cases with tonsillar herniation or a mass in the area of the posterior cranial fossa, persistent disturbances in cerebrospinal fluid circulation may result in progressive syringomyelia, which can only be alleviated by relieving the disturbed CSF drainage in the posterior fossa. Likewise, in the case of spinal tumours, the clinical course of the secondary syrinx arising from the tumour depends upon treatment of the mass. Post-traumatic, post-inflammatory and post-chemical syrinxes frequently show an unpredictable course. Overall, approx. 60 % of patients with syringomyelia have a chronic and progressive course, while 25 % have a fluctuating course with periods of progression and periods when symptoms remain stable. Only approx. 15 % of patients have no progression in their deficits [31]. Generally, it is not possible to predict the course of the illness without knowing the individual aetiology. Case reports repeatedly remind us that any physical manoeuvres involving a sudden increase in intraspinal or intracerebral pressure may precipitate sudden deterioration. These would include vigorous sneezing or straining. Clinically, this often results in increased pain or the initial occurrence of pain. Secondary problems may also occur from infections, autonomic dysregulation and trophic disturbances. In most cases restoration of free cerebrospinal fluid flow leads to stabilisation of existing neurologic symptoms. In some cases, existing neurological deficits may even improve, although once neuropathy has been present for a long time, it will tend to persist. In cases of the Chiari malformation, symptoms caused by brainstem compression will often regress, whereas symptoms directly caused by the syrinx may remain unchanged.