Nomenclature

Fig. 20.1

Hydromyelia, syringomyelia or syringohydromyelia? In these sections of the spinal cord from a dog, it can be seen that some segments appear to have a syringohydromyelia (top row, short white arrows), some a separate syringomyelia cavity (middle row, short black arrows) and some a hydromyelia (bottom row, long arrows). The ependyma is disrupted in all sections on microscopy, and the apparently separate syrinx cavity is actually connected to the central canal more caudally or cranially, i.e. it is not truly separate. It can be argued that, without the aid of post-mortem and microscopic examination, it is impossible to categorise cavities on the basis of ependymal lining and central canal connection. It can also be argued that categorisation is completely unnecessary from a clinical point of view (Picture courtesy of Dr Fernando Constantino-Casas)

Table 20.1

Should syringomyelia be used in preference to syringohydromyelia?

Yes

1. Syringomyelia is already overwhelmingly used in preference to syringohydromyelia,^a,b especially by those authors considered expert in this field

2. The classification of syrinx cavity by central canal connection and ependymal lining is unworkable (Fig. 20.1). Concurrent involvement of the central canal and disruption of the ependymal limiting is almost universal, so only one term is actually required

3. It is correct from a historical point of view. Ollivier d’Angers originally defined syringomyelia as a fluid-filled tubular dilatation within the spinal cord

4. Adding ‘hydro’ and an extra two syllables is superfluous and complicates a word that is already difficult for patients to understand, pronounce or remember

1. Syringohydromyelia may be defined as the coexistence of hydromyelia (fluid contained within the ependyma of the central canal) and syringomyelia (fluid forming a cavity within the white matter). Using the term syringohydromyelia makes it clear to the reader that a syrinx also involves the central canal

2. Differentiation between hydromyelia and syringomyelia is difficult to make by radiological, clinical or pathological methods. Use of the combined term, syringohydromyelia (or hydrosyringomyelia), acknowledges this reality

3. Adding a component, meaning ‘water’ or ‘liquid’, to a ‘tube’ in the ‘marrow’ makes it clear that this is a fluid-containing cavity, emphasising that it is a disorder of CSF circulation

^aA PubMed search performed on 24th June 2012 revealed 136 articles when the key word ‘syringohydromyelia’ was used. In comparison 3,584 articles were found using the keyword ‘syringomyelia’

^bOnline searches performed on 8th July 2012 in Wikipedia, Encyclopaedia Britannica, Oxford Dictionary, American Heritage Dictionary, Collins Dictionary and Merriam-Webster failed to find a listing for syringohydromyelia. Search results returned either ‘not found’ or defaulted to ‘syringomyelia’

Table 20.2

Should we use hydromyelia to describe a small central dilation?

Yes

1. There is a need for a term to describe this MRI appearance. An ependymal-lined distension or dilation of the central canal has already been defined as hydromyelia; therefore, it is the most appropriate term to use

1. Many clinicians use the term inappropriately when syringomyelia would be more correct^a

2. Using the term central canal dilation or distension is descriptive and less open to misinterpretation

3. Before using this term a clearer definition is required, e.g. the size of central canal dilation. In veterinary medicine central canal dilation with a transverse width of two millimetres or more is categorised as syringomyelia. If less than two millimetres, then it is described as central canal dilation (Knowler et al. 2011)

4. Use of the term hydromyelia implies pathology when it is not clear that this is always the case. By analogy, ventricular dilation (or ventriculomegaly) does not necessarily equate with clinically active hydrocephalus

^aA PubMed search performed on 14th July 2012 using the criteria hydromyelia [Title] revealed 78 publications; however, only one was found (Jinkins and Sener 1999) which used the term hydromyelia to describe a central canal dilation. The remainder were referring to an extensive syrinx

20.2 Abnormalities of the Craniovertebral Junction

The first known illustration of hindbrain herniation was in an early nineteenth-century anatomy atlas by Jean Cruveilhier (Cruveilhier 1829; Pearce 2000). There was also a brief description of a single case in a paper on encephalocoele and spinal bifida by John Cleland in 1883 (Cleland 1883). It is, however, Hans Chiari (1851–1916), a pathologist working in Prague, who is honoured with the medical eponym because he provided the first detailed description of four types of cerebellar pathology, observed during post-mortem studies of infants with congenital hydrocephalus (Chiari 1891, Chiari 1896). Descriptions of the various types of Chiari malformation have, however, evolved since (Table 20.3). Chiari was careful to acknowledge the work of others, and in his conclusion in the second paper, he noted the observations of Cleland and also of Arnold , who had described an infant with spina bifida and hindbrain herniation (Arnold 1894). Two loyal students of Arnold, Schwalbe and Gredig, subsequently inserted his name when describing four patients with the type II malformation (i.e. in association with myelomeningocoele) coining the term Arnold-Chiari malformation (Schwalbe and Gredig 1907). Consequently, some refer to the spectrum of disorders of hindbrain herniation as Arnold-Chiari syndrome (Bejjani 2001), albeit to the indignation of others (Solt 2011; Sarnat 2007). The majority now use the term Chiari malformation to describe hindbrain herniation. The arguments for and against the terms hindbrain herniation, Chiari malformation and Arnold-Chiari malformation are presented in Tables 20.4 and 20.5.

Table 20.3

Classification of Chiari malformations

Chiari malformation	Original description by Chiari (Batzdorf 2001b)	Modern description
Type I	Elongation of the cerebellar tonsils and the medial part of the inferior cerebellar lobes into cone-like projections, which accompany the medulla into the spinal canal	Volumetrically small posterior cranial fossa with hindbrain overcrowding (Milhorat et al. 1999)
Type I		The classic radiographic description is tonsillar herniation of at least 3 mm below the foramen magnum
Type II	Displacement of portions of the vermis and also of the pons and medulla into the spinal canal and elongation of the fourth ventricle into the spinal canal	Downward displacement of the cerebellar vermis, brainstem and fourth ventricle, associated with a myelomeningocoele (Geerdink et al. 2012)
Type III	Displacement of virtually the entire hydrocephalic cerebellum into a cervical spinal bifida	Hindbrain herniation into a high cervical or occipital encephalocoele (Castillo et al. 1992)
Type IV	Hypoplasia in the region of the cerebellum without displacement of portions thereof into the spinal canal	Obsolete term describing cerebellar hypoplasia unrelated to the other Chiari malformations
Type 0		Overcrowding of the posterior fossa with abnormal brainstem anatomy (posterior pontine tilt, downward displacement of the medulla, low-lying obex) but with normally placed cerebellar tonsils (Markunas et al. 2012)
Type 1.5		Cerebellar and brainstem herniation through the foramen magnum. Similar to type II malformation but not associated with spinal dysraphism (Tubbs et al. 2004)
Canine		Disparity in volume between the caudal cranial fossa and its contents so that the cerebellum and brainstem are herniated into or through the foramen magnum

Table 20.4

Should hindbrain herniation be used in preference to Chiari malformation?

Yes

1. In common with most eponymous terms, Chiari malformation tells us nothing about the nature of the condition

2. There are many causes of hindbrain herniation including cranial constriction, spinal cord tethering, cranial settling, intracranial hypertension, intraspinal hypotension, basilar impression/invagination and craniosynostosis. Nomenclature should identify the underlying subgroup and cause, for example, ‘hindbrain herniation secondary to intracranial hypertension’

3. Using eponymous terms for diseases is to be discouraged especially when there is a simple alterative

1. The eponym Chiari malformation is by far the most commonly used term^a

2. Some cases with overcrowding of the posterior fossa and disrupted CSF flow (Chiari 0) do not actually have a hindbrain herniation

3. The classification of various Chiari malformations is commonly used and well understood. For example, the term ‘Chiari type II’ is much simpler to say or read than ‘hindbrain herniation associated with spinal myelomeningocoele’

^aA PubMed search performed on 14th July 2012 using the criteria (Chiari [Title]) not Budd [Title]), not Arnold [Title], revealed 1,422 publications. A search using the criteria Chiari [Title]) and Arnold [Title] revealed 509 publications. A search using the criteria hindbrain herniation [Title] revealed 22 publications

Table 20.5

Should the term Arnold-Chiari malformation be dropped?

Yes

1. Arnold’s work does not merit an eponym , and it was an injustice that his name was added. Chiari provided a detailed and careful description of the pathology, whereas Arnold’s observation in a single case was ‘a ribbon of tissue that protruded through the foramen magnum’. Arnold did not even acknowledge previous work by Cleland and Chiari (Solt 2011; Sarnat 2007)

2. Arnold-Chiari is a confusing term because there is not universal agreement as to the definition. Some use it for all types of hindbrain herniation and others use it exclusively to refer to hindbrain herniation associated with a myelomeningocoele^a

3. The eponym Arnold-Chiari malformation is not the most frequently used term^b

4. Many patients think that Arnold-Chiari was one person

1. This term is useful to distinguish the type II malformation, i.e. hindbrain herniation associated with myelomeningocoele, and should be reserved exclusively for that

2. If we are going to use an eponymous term, then the contribution of this clinician should be recognised as well as that of Hans Chiari

^aA PubMed search performed on 14th July 2012 using the criteria (Chiari [Title]) and Arnold [Title]) and spina bifida revealed 42 publications. A search using the criteria (Chiari [Title]) not Arnold [Title]) not Budd [Title]) and spina bifida however revealed 44 publications

^bA PubMed search performed on 14th July 2012 using the criteria (Chiari [Title]) not Budd [Title]), not Arnold [Title], revealed 1,422 publications. A search using the criteria Chiari [Title]) and Arnold [Title] revealed 509 publications. A search using the criteria hindbrain herniation [Title] revealed 22 publications

20.3 Communicating and Noncommunicating Syringomyelia

Syringomyelia and hydromyelia were traditionally classified into communicating and noncommunicating types (Milhorat et al. 1995). The term communicating indicated the presence of a connection between the syringomyelic cavity and the fourth ventricle. Communicating syringomyelia, or hydromyelia, is associated with hydrocephalus, especially of the fourth ventricle and conditions which cause obstruction to the outflow of this chamber, for example, post-meningitic or posthaemorrhagic hydrocephalus or complex hindbrain malformations such as Dandy-Walker malformation. It was at one time widely assumed that the pathogenesis of communicating syringomyelia is due to a water-hammer effect of cerebrospinal fluid from the fourth ventricle, being forced into the central canal (Williams 1980; Gardner 1965). Indeed this theory is still accepted by some authors (Hagihara and Sakata 2007). However, this theory was challenged, not least because communicating syringomyelia is sometimes seen with distal spinal cord pathology, such as caudal, cervical, spinal and intradural subarachnoid cysts (Yamashita et al. 2012) and thoracic spinal cord tumours (Mock et al. 1990). Communicating hydrocephalus, however, is now known to be present in fewer than 10 % of affected human patients, so this categorisation now tends to be disregarded (Batzdorf 2001a).

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