result of infection, trauma or neoplasia. in dogs hydromyelia may be accidentally diagnosed during ... diographic findings were nonspecific and the dog was dis-.
HYDROMYELIA IN THE DOG ROBERTM. KIRBERGER, BVSc, MMEDVET(RAD),LINDAS. JACOBSON, BVSc M M E D V E ' r ( M E D ) , JEREMYV. DAVIES,BVETMED,PHD, DVR, JEAN ENGELA MBCHB MMED(RADD)
Hydromyelia is a dilation of the spinal cord central canal. In man this may be due to congenital malformations such as Dandy-Walker syndrome and Chiari malformations or may be acquired as result of infection, trauma or neoplasia. in dogs hydromyelia may be accidentally diagnosed during routine cisterna magna myelography. Hydromyelia, and its possible etiology, may be confirmed by means of computed tomography or magnetic resonance imaging. Three dogs with hydromyelia due to differing etiologies are described. Veterinary Radiology & Ultrasound, Vol. 38, N o . I , 1997, p p 30-38. Key words: hydromyelia, canalogram, myelography, Chiari malformation, Dandy-Walker syndrome.
Introduction
Case History Reports Dog I
I
cavities have been divided into hydromyelia and syringomyelia. Hydromyelia is a dilation of the central canal which is lined by ependyma and often communicates with ventricular system. Syringomyelia arises primarily outside the central canal and is usually not lined by ependymal cells.3 Communicating syringomyelia may occur secondary to hydromyelia.' Noncommunicating syringomyelia may occur secondary to intramedullary tumours, myelitis, meningitis, trauma and h a e m ~ r r h a g e , ' - ~or , ~after obliteration of the subarachnoid space by i n f e ~ t i o n Intramedullary .~ cavities are often large and it is usually impossible to differentiate syringomyelia from hydromyelia by imaging techniques. ,2,6 Eccentric cavity location within the cord may be more indicative of syringomyelia but this is not pathognomonic.6 Some authors prefer to use the term syringohydromyelia or syringomyelia as all-encompassing terms.6 The term hydromyelia has been used in this paper because in each of the patients, the cord cavitation communicated with the ventricular system, indicating true hydromyelia. During routine myelography , contrast medium may accidentally enter the dilated central canal and this has been termed a ~ a n a l o g r a m . ~ NTRAMEDULLARY SPINAL CORD
A 3-year old Maltese poodle bitch (3.5 Kg) was presented with a history of non-localized pain during movement and mild pelvic limb hypermetria. This dog has been briefly reported upon previously.' Clinical and survey radiographic findings were nonspecific and the dog was discharged. Three weeks later the dog was re-admitted with hypermetria and hyperreflexia of all limbs and some neck pain. Survey radiographs were unremarkable except for a slight concavity of the dorsal vertebral canal margin of C2. A cisterna magna puncture was performed and cerebrospinal fluid (CSF) flowed easily. Three millilitres of Iohexol (300 mg Iiml) was administered over 60 seconds with no adverse effects and radiographs made. A distended, up to 8 mm diameter, mildly sacculated, canalogram was present and extended from C2 to L4 (Fig. 1A). The canalogram was difficult to see in the C1 region, indicative of no dilation. A small amount of subarachnoid contrast medium was visible in the region of C1 and C2. The cerebral ventricles contained contrast medium with the lateral ventricles moderately enlarged (up to 14 mm dorsoventral diameter) and the third and fourth ventricles appearing normal (Fig. IB). The canalogram persisted for 3 hours and only towards the end of this period did contrast medium appear in the subarachnoid space. The dog had a delayed recovery from anesthesia but returned to its pre-myelographic status the next day. Analysis of CSF was normal. The dog was treated with acetazolamide (30 mg per 0 s once per day for 4 days per week) and improved initially but after 3 months was readmitted with pelvic limb ataxia and absent conscious proprioception in all limbs. Lumbar myelography was performed and a narrowed subarachnoid
'
_____
From the Radiology Section, Department of Surgery (Kirberger) and Dcpartment of Medicine (Jacobson), Onderstcpoort Veterinary Academic Hospital, Univcrsity of Pretoria, Republic of South Africa; Ridgway Radiology (Davies), Greenfield, Bedford, United Kingdom; and Labuschagne & Partners (Engela), Capital Park, Pretoria, Republic of South Africa. Address correspondence and reprint requests to Robert M Kirberger, Department of Surgery, Onderstepoort Veterinary Academic Hospital, University of Pretoria, Private Bag X04, Onderstepoott, 0110 Republic of South Africa. Received July, 1995; accepted for publication December 14, 1995.
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A
FIG. I . Dog 1 . Radiographs made after cisterna magna myelogram. (A) Lateral ccrvical view with widcned saccular canalogram indicative of hydromyelia. Note concavity of dorsal vertebral canal of C2. (B) Ventro-dorsal view of caudal skull and cranial cervical vertebrae. Normal sized mesencephalic aqueduct (long solid arrow) and lateral recesses of the fourth ventricle (short solid arrow). Thc circular filling defect in 4th ventricle between the 2 arrows is believed to be part of the cerebellar vermis pressing on the floor of the fourth ventricle. Note distended contrast-filled lateral ventricles. The dog was in left latcral recumbency during the procedure resulting in more content in the left lateral vcntricle. Distended central canal visible between open arrows
space identified in the cranial thoracic region, presumably secondary to the effect of the dilated central canal. A further 3 months conservative therapy resulted in no improvement. A ventriculoperitoneal shunt was surgically installed in an attempt to limit any further nervous system damage from increased intracranial pressure. Three weeks postoperatively, magnetic resonance imaging (MRI) was performed with a 1.5 T superconducting magnet* to evaluate the success of the surgery and determine the possible etiology of the hydromyelia. On midsagittal images, hyperintense tissue was seen dorsal to the C1 cord substance and caudal to the foramen magnum (Fig. 2A). This was believed to be caudally displaced caudal velum of the fourth ventricle and associated choroid plexus or a caudally displaced ventral lobe of the cerebellar vermis. Mild hydrocephalus was visible (Fig. 2B). In the cervical region hydromyelia was evident but this narrowed to a very small canal in the C l region (Figs. 3A & B). The shunt tubing had been placed too deeply (Fig. 2B) resulting in adequate drainage. The MRI findings were believed to be indicative of a possible Chiari I-type malformation. The caudally displaced tissue was believed to be similar to the elongation of the cerebellar tonsils found in this condition in
*Magnetom SP4000, Siemens Medical, Johannesburg, Republic of South Africa.
man.2 A week later the dog deteriorated rapidly and was euthanazed. Permission for necropsy was refused. Dog 2
A 2%-year old male Maltese poodle (6.9 Kg) was referred for myelography for a suspected cervical intervertebra1 disc prolapse. The dog had initially presented with neck pain which 10 days later progressed to dragging the pectoral limbs and hypermetria of the pelvic limbs. Survey radiographs were unremarkable. A cisterna magna puncture was made and CSF appeared to flow under increased pressure. Three milliliters of lohexol (300 mg liml) was administered over 60 seconds with no adverse effects. Cervical radiographs were made immediately and after 5 minutes. On ventro-dorsal and lateral radiographs a distended sacculated canalogram, up to 7 mm wide and extending to T5, was visible (Figs. 4A & B). Radiographs were not made caudal to this. On the lateral view in the region of C1 the canalogram narrowed to 3 mm and cranial to this the dilated central canal widened to 11 mm in the region of the cisterna magna puncture site. This dilation extended into the cranium where the fourth ventricle appeared markedly dilated (Figs. 4A & B). This gave the impression that the fourth ventricle was dilated into the base of the cerebellum and appeared to protrude through the foramen magnum into the cranial cervical canal. A poorly
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5). Hydromyelia was visible. The MRI findings were believed to be indicative of Dandy-Walker-type syndrome. Telephone follow-up 15 months later disclosed that the dog appeared to be normal. Dog 3 A 4-year old castrated King Charles spaniel ( 1 7 Kg) with a history of chronic intermittent pain around the neck and head, particularly of the submandibular region, was referred for radiologic examination. There was no history or clinical evidence of lameness but a complete neurologic examination was not performed. Results from routine laboratory tests were normal. Survey radiographs of the head and neck were unremarkable. Cisterna magna puncture resulted in rapid release of CSF and the needle was repositioned but CSF continued to flow under increased pressure. The CSF was grossly normal but was not submitted for laboratory evaluation. No twitching was observed during needle passage. Three millilitres of lohexol (300 mg Iiml) was administered over 20 seconds with no adverse effect. In lateral cervical radiographs made
0
A
FIG. 2. Dog I . T1 weighted MRI images. (A) Sagittal midline image with short thick arrows indicating normal cerebellar vermis and long thin arrow indicating caudally displaced brain tissue (see tcxt). (€3) Parasagittal image in which hypointense hydroccphalic fluid is visible in the lateral ventricle. Arrow indicates shunt placed too deeply. Spin echo, TR - 450 mscc, TE - 15 mscc.
defined 7 nim wide area of contrast medium positioned in a caudodorsal direction was visible just cranial to the tentorium cerebelli (Fig. 4A). The subarachnoid space and lateral ventricles contained no contrast medium on radiographs made up to 5 minutes after the procedure. Additional radiographs were not made. The dog made an uneventful anesthetic recovery. Analysis of CSF was normal. Three days later MRl was performed using a 1.5 T superconducting magnett with a circular polarized head coil. Marked dilation of the lateral and fourth ventricles was present. A large CSF containing diverticulum was visible cranial to the cerebellum and appeared to communicate with the mesencephalic aqueduct. Caudal to this cystic dilation the rostra1 half to third of the cerebellum was absent (Fig. ~
tMagnetom SP 4000, Siemens Medical, Johannesburg, Republic of South Africa.
0
FIG.3 . Dog 1. T1 weighted transverse MRI images o f cranial cervical region. (A) Region of C2 with widened hypointense central canal visible (arrow). (B) Region of C1 with small slightly eccentric hypointense central canal (arrow). Spin echo, TR - 500 msec, TE - 15 msec.
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DOG
A
B
FIG. 4. Dog 2. Radiographs made after cisterna magna myclogram. (A) Latcral cervical view with widened saccular canalogram indicative of hydromyclia. Cranially thc central canal becomes thinner in the region of C 1, then dilates in the cisterna magna area, n~irr~iws slightly and then dilates in the region of the 4th vcntricle. Some poorly-defined contrast medium cranial t o this (arrows) indicative of filling in the region of the tcntorium ccrcbclli. (B) Ventro-dorsal view of caudal skull and cranial cervical vertebrae. Notc widened saccular canalogram and enlargcd 4th ventricle (arrows).
2 minutes after contrast medium administration there was a non-sacculatcd canalogram that varied in diameter from 1-3 mm with accompanying filling of the ventricular system (Fig. 6A). The lateral ventricles appeared slightly enlarged and the fourth ventricle was dilated into the base of the cerebellum with its caudal tubular portion appearing to straddle the foramen magnum, extending to the level of the atlanto-axial joint. Only streaks of contrast medium were visible in the subarachnoid space on the cervical radiograph. In follow-up radiographs made after 15 minutes the canalogram extended to the sacral region, again with only occasional opacification of the subarachnoid space (Fig. 6B). In radiographs made at 80 minutes, contrast medium persisted in the ventricles and central canal with faint opacification of the cervical subarachnoid space. No contrast medium was visible on radiographs made at 18 hours. A diagnosis of suspected Dandy-Walker-type syndrome was made. Magnetic resonance imaging was not performed. The dog recovered routinely from anesthesia and was sent home without treatment. Telephonic follow-up 16 months later revealed that the dog had been normal since the myelographic examination.
Discussion Embryology . .__ ‘loThe tube is embryologically formed by sure of the neuroectoderm to form the neural canal. This
FIG. 5. Dog 2. T2 weighted sagittal midline MRI image with arrows indicating caudal border of cerebellum. Note markedly distended hyperintense lateral and fourth ventricles. laree hvDcrintense CSF accumulation v in the region of the tentorium cerebelli, and absence of cranial aspect of cerebellum. Hydromyelia only partially visible in cervical area due to incorrect positioning. Spin ccho, TR - 2500 msec, TE - 90 msec. .I.
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fourth ventricle (foraminae of Luschka). * The dog does not have a foramen of Magendie.” Flow of CSF is thought to be due to the pulsation of blood in the choroid plexuses. With each pulsation the CSF pressure rises and surges towards the lateral apertures. The cilia of the ependymal cells may contribute to the flow.* The existence of caudally directed CSF flow in the central canal of the rabbit spinal cord has been proven and the escape of this CSF into the sacral subarachnoid space demonstrated. l o In the same study CSF flow in the central canal of the guinea-pig, rat, rhesus monkey and cat could not be demonstrated. In an experimental study, ventriculography in normal cats resulted in contrast leaving the ventricular system via the lateral apertures to opacify the subarachnoid space, but filling of the central canal by contrast medium was never observed. l 2 Whether, in the dog, CSF flow occurs from the ventricular system to the central canal, flows within the latter, or whether or not there is a communication between the central canal and sacral subarachnoid space could not be ascertained from the literature. In experimental canine hydrocephalus/hydromyelia, a communication was occasionally found between the central canal and sacral subarachnoid space. l 3
A
Etiopathogenesis
FIG. 6 . Dog 3. (A) Lateral cervical radiograph. Note presence of contrast medium in caudal aspect of lateral ventricles (long arrows) and dilated fourth ventricle (short arrow) with its distended caudal tubular aspect appearing to straddle the foramen magnum. Distended cervical canalogram and minimal subarachnoid contrast medium visible. (B) Lateral lumbar radiograph in which a variable diameter canalogram cxtending to L5 is visiblc.
closure progresses rostrally and caudally from the site of development of the rhombencephalon. The rostra1 opening closes and forms the brain vesicles while the caudal opening at the caudal extremity of the spinal cord closes later or not at all.’ The neural canal is a single fluid-filled cavity consisting of the ventricles and central canal. The fluid starts to filter through the membranous roof of the primitive fourth ventricle to dissect open the subarachnoid space. Eventually the roof of the fourth ventricle perforates to form the foraminae.’ Once the foraminae open, CSF escapes freely into the subarachnoid space. The central canal becomes compressed as result of the increased subarachnoid pressure and becomes a vestigial structure’ that has been described as a functionless remnant containing stagnant CSF and cellular debris. l ” CSF Flow Dynamics In the dog, CSF circulates from the ventricular system to the subarachnoid space by way of the lateral apertures of the
Several theories exist to explain the formation of hydromyelia in man. Failure of the fourth ventricle foraminae of Magendie and Luschka to perforate during fetal life, or caudal herniation of the cerebellum with subsequent foraminal compression, forces ventricular CSF through into the central canal. The CSF pulsations are transmitted to the central canal resulting in dilation.6’’ Another theory is that partial obstruction at the foramen magnum area, for example in Chiari malformations, may result in a ball valve effect obstructing CSF flow. Coughing or other maneuvers that result in increased intrathoracic and intra-abdominal pressure, result in spinal epidural venous distension. In the confined space of the spinal canal this results in rapid shunting of CSF from the subarachnoid space to the ventricles. Due to the partial obstruction ball valve effect, the CSF does not drain back immediately, resulting in increased intracranial pressures forcing CSF into the central canaL6 These differing haemodynamic theories are not mutually exclusive and it is likely that a combination of these mechanisms acts at various stages during the development of h y d r ~ m y e l i aIn .~ patients with cerebellar herniation the central canal at the level of the hernia is usually patent but cannot dilate because of the impaction; hydromyelia results below the impaction.’ In experimental hydrocephalic studies in cats, occlusion of the foraminae of Luschka resulted in dilation of the ventricles and central canal within 10 days, with accompanying clinical signs caused by increased intracranial pressure. After 10-14 days communication between the dilated
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central canal and the sacral subarachnoid space became evident. At this stage the cats showed rapid recovery from the initial clinical symptoms. l 4 Even though the pressures decrease with the opening of a sacral shunt, the waterhammer pulsations of CSF may still result in continuous expansion of the ventricles and the central canal.I4 There are often “haustra” and/or synechiae at multiple intervals along the length of the ~ a v i t y This . ~ saccular appearance of the central canal may be due to differences in resistance at the somite level, pulling by dentate ligaments or septa within the cavity.15 In man, congenital malformations play a major role in the development of hydromyelia with Chiari malformations and, to a lesser extent, Dandy-Walker syndrome being the most common (Table 1). Spinal dysraphism and cerebellar cysts may also result in hydromyelia. Chiari I malformation is a congenital disorder resulting in cerebellar dysplasia. Hydromyelia is commonly associated with Chiari I malformation and has been suggested to play an important part in the pathogenesis of intramedullary cavities.’ Chiari I malformation in man is characterized by a malformed craniovertebral junction, caudal elongation of the cerebellar tonsils through the foramen magnum, an extremely small cisterna magna, often with fibrous adhesions which mat the dura, arachnoid and cord together, hydromyelia in 20-8576 of patients, and hydrocephalus in up to 55% of patients.2”6-’8 Other authors state that hydrocephalus does not occur. l 9 The fourth ventricle may be stretched and thinned but is not enlarged.’‘ Chiari malformations are usually accompanied by a closed foramen of Magendie.9 An abnormally large foramen magnum has been found in Chiari I and 11 malformations.’‘ In small-breed dogs, a dorsal notch of the foramen magnum (occipital dysplasia) commonly occurs but is not associated with clinical signs.20 This dorsal notch is usually covered by a membrane and protrusion of the cerebellum has not been observed.2” The foramen magnum was not specifically evaluated in the 3 dogs described but this should be done as part of the routine diagnostic work up of patients with hydromyelia.
35
Chiari I1 has no relationship to Chiari I malformation and results from a congenital disorder of neural tube closure and is often accompanied by meningomyelocele. Patients present with neurologic symptoms at birth.2 The condition in man is characterized by downward displacement of the spinal cord often resulting in a kinking at the cervicomedullary junction; marked elongation of the brain stem so that the medulla extends into the cervical canal, dysplasia and caudal elongation of the cerebellum so that it protrudes through the foramen magnum, and concomitant hydromyelia in 20-70% of patient^.^,''^'^ Dandy-Walker syndrome, a congenital cerebellar dysplasia, is a rare, but well-known condition in and has been described in the dog23 and cat.24 The condition is associated with a classical triad of signs: 1) cerebellar hypoplasia, 2) caudal fossa cysts originating from the fourth ventricle and, 3) communicating hydrocephalus. Associated cerebral anomalies such as dysgenesis of the corpus callosum and dysplasia of the cerebral cortices may also occur. 18.21,22,25 Systemic malformations, which may worsen the prognosis, are also described.2’ In man, elevation of the tentorium cerebelli with resultant posterior fossa enlargement occurs commonly.22 The cystic fourth ventricle may descend into the upper cervical this appears to have occurred in dog 2. Hydromyelia has been described in 3 human patients.9 Several theories exist regarding the etiology of the syndrome but essentially it involves a developmental anomaly in the rostra1 area of the roof of the fourth ventricle. 2’,22 Cerebellar cysts are occasionally associated with hydromyelia and may27 or may not2’ have concomitant hydrocephalus. Distal hydromyelia has been described in children with tethered cords and occult d y ~ r a p h i s m . ~ Acquired causes of hydromyelia include infection, trauma and neoplasia.”‘ Partial occlusion of the third and fourth ventricles by pyogranulomatous reactions resulted in hydrocephalus and hydromyelia in a cat with feline infectious peritonitis .29 This hydromyelia was also diagnosed “accidentally” during a cisterna magna myelogram, most
TABLE1. Anatomic Characteristics of Congenital Malformations Resulting in Hydromyelia in Humans1~2~16-’9~21~22,25 Pathology/Anatomical Region
Chiari I
Hydromyelia Hydrocephalus Fourth ventricle
Common Fairly common Normal
Cerebellum Cerebellar position
Dysplasia Elongation of tonsils through foramen magnum
Posterior fossa cysts Associated cerebral abnormalities Meningomyelocele
None Nonc Rare
Chiari I1 Fairly common Common Displaced into cervical canal and rarely enlarged Dysplasia Caudal displacement of vermis and tonsils resulting in cord kinking None May occur Usually
Dandy-Walker Uncommon Common, mild to severe Cystic and may descend into cervical canal Hypoplasia or aplasia Normal Present Common None
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KIRBERGER ET
likely due to an exudate obliterating the subarachnoid space in the cisterna magna area. In a retrospective epidemiologic study of canine hydrocephalus, the Maltese poodle was the breed most at risk for h y d r o ~ e p h a l u s Considering .~~ that two of our patients were Maltese poodles, one can only ponder on the likelihood of a relationship between hydrocephalus in the above study and hydromyelia. In the authors’ experience, during cervical myelography, small breed dogs often have a relatively wide cervical spinal cord in relation to the vertebral canal. The possibility of hydromyelia playing a role in these patients requires further investigation. In dogs a widened central canal may occur primarily as in hydromyelia or may be secondary to iatrogenic damage during the m y e l ~ g r a m . ~
Clinical Signs Human patients with Chiari I malformation present in adolescence or early adulthood with a variety of symptoms occurring in various combinations.221933‘ Three patterns of neurologic deficit occur: 1) foramen magnum compression with ataxia, sensory deficits, cerebellar signs, lower cranial nerve palsies and headaches, 2) central cord syndrome with dissociated sensory loss, segmental weakness and long tract signs possibly combined with lower cranial nerve palsies and, 3) cerebellar syndrome. The majority of human patients with Dandy-Walker syndrome are seen before the age of 1 year25 although clinical signs may develop later in life as was the case in Dog 2. Clinical signs have been described in a 72-year old man.22 Clinical features include a large or enlarging head and seizures. Neurologic signs of foramen magnum compression, cranial nerve deficits or cerebellar signs are rarely encountered.,’ Clinical signs in man appear to be due primarily to concurrent central nervous system anomalies such as hydrocephalus. 22 Scoliosis has been described to accompany hydromyelia and syringomyelia in 2 dogs32 and in a dog with spinal dysraphism with associated hydromyelia.33 In man a wide range of fluctuating neurologic complaints in the early stages of hydromyelia often result in an incorrect initial diagnosis. This was reflected in our 3 dogs which had a variety of clinical signs that altered over time. This may be due to varying intracranial pressures resulting from partial or intermittent obstruction of the original inciting cause, or of the shunt that develops between the central canal and distal subarachnoid space as described in experimental studies. l4 Cervical pain is not specifically associated with hydromyelia in man. Cervical pain was noted in the 3 dogs described but it is uncertain whether this is related to the hydromyelia.
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Diagnosis In man, Chiari and other congenital brain malformations as well as hydromyelia are being diagnosed incidentally, or during investigations of patients with minimal neurologic disease, with greater frequency due to the increased use of computed tomography and MRI.4,1”34Computed tomography will only allow detection of large intramedullary cavities but it typically is not possible to determine whether the cavities communicate with the ventricular system. Smaller cavities are only seen if they fill with contrast medium. I Magnetic resonance imaging is the ideal non-invasive imaging technique to screen for suspected syringomyelia and hydromyelia and allows for assessment of concomitant pathology and a possible aetiology . I Exact midsagittal images most effectively display possible abnormalities. Cerebrospinal fluid behaves like water and the ventricular system appears black (hypointense) under T I weighting compared with brain parenchyme and white (hyperintense) under T, weighting.35 Protein content of the hydromyelic cavity is normal and the fluid T1 and T2 relaxation times similar to that of CSF.3 Substantial flow within the hydromyelic cavity is p~ssible.~.‘ Since cortical bone has no signal on MRI and appears similar to subarachnoid space on short Spin-echo techniques the posterior margin of the foramen magnum is seldom identified, making identification of small cerebellar prolapses difficult. Although it is possible to increase the signal of CSF compared to cortical bone by increasing the TE and TR in Spin-echo techniques, the resulting loss of resolution usually makes it impossible to image the edge of the foramen.34 The degree of subarachnoid space narrowing caused by cerebellar ectopia is difficult to assess.34 Cavitation due to cord neoplasia can be distinguished from hydromyelia by means of MRI. The cystic and solid parts of the neoplasm have different signal intensities and can be distinguished from hydromyelia on that basis. In humans with hydromyelia, CSF analysis was normal and fluid aspirated percutaneously from cavitatory lesions was within normal limits and similar to each other in 74 patients.”I5 In a study involving 60 human patients with hydromyelia, enlargement of the spinal cervical canal was frequently noticed on survey radiography, and some degree of spinal cord enlargement was documented in all patients on myelography. It is uncertain what proportion of such cavities are missed by conventional radiographic studies.34 The remodelling of the vertebral canal roof of C2 in Dog 1 is indicative of a widened vertebral canal and should alert the clinician to the possibility of hydromyelia. In the 3 dogs described, hydromyelia was diagnosed accidentally during myelography . The dilated central canal results in a widened spinal cord which may be accompanied by arachnoiditis of the cisterna magna area, resulting in an
’’
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absent or very small dorsal subarachnoid space. On passing the spinal needle during a cisterna magna myelogram, the first CSF encountered is that from the dilated central canal.7 It is essential to evaluate carefully the occipital region in such patients for the conditions described. If necessary, the X-ray table can be tilted to allow cranial gravitation of contrast medium to improve delineation of anatomical structures. Hydromyelia should also be a differential diagnosis in patients with diffuse cord swelling seen on myelography . Diagnostic ultrasonography of the occipital region may prove to be of benefit to visualize cerebellar or ventricular abnormalities, particularly in the presence of occipital dysplasia. The malformations described, and their various subtypes, may be difficult to accurately classify by means of survey radiographs, contrast medium studies, ultrasonography and even MRI, particularly considering the wide range of canine skull types. Greater accessibility for animals to MRI may result in increasing numbers of patients with hydromyelia being diagnosed.
Treutment Treatment depends upon the obstructive site and pathophysiology of the ~ o n d i t i o n .In ~ man, posterior fossa decompression by means of suboccipital craniectoniy and
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laminectoniy of C 1-3 is performed to relieve the compression in Chiari malformations.4331Ultrasonography may be used intra-operatively to further evaluate the anatomy of any malformation and assists in surgical decision making.4 Results of treatment are variable and are often complicated by respiratory depression within 5 days after surgery. ‘7,31 Ventricular drainage may be implanting tubes into the ventricles and allowing drainage into other body cavities as was done in Dog l . 3 ’ 9 3The 6 shunt was poorly positioned in Dog 1, probably due to the mild hydrocephalus and the use of equipment designed for human use. The retrospective diagnosis of a Chiari I malformation suggested that intraventricular shunting may have been an inappropriate treatment in this dog. However, the classification of the structural defect was not possible until the postoperative MRI had been performed. Dandy-Walker syndrome is treated by ventricular shunting, particularly from the cystic fourth ventricle to other body cavities .*’ Hydromyelia without hindbrain malformation or hydrocephalus is treated by means of placing a shunt between the cavity and the p l e ~ r a If . ~the hydromyelia extends to the terminal cord a terminal ventriculostomy is p e r f ~ r m e dIt. ~is concluded that surgical intervention should only be considered if clinical signs persist or become progressive.
REFERENCES I . Lee BCP, Zimnicrman RD, Manning JJ, Dcck MDF. MR imaging of syringomyclia and hydromyelia. AJNR 1985;6:221-228. 2. McClarty B, Suthcrland S. Bertram EG. The Spine. In: Kucharczyk W, cd. MRI: Central Nervous System. Philadelphia: Raven Press 1990:s. 19-5.20. 3. Norman D. MR of the spinc. In: Brant-Zawadzki M, Norman D, eds. Magnetic restinancc imaging of the central nervous system. New York: Raven Press 198731 1-314. 4. Wisoff JH, Epstein F. Management of hydromyelia. Neurosurgery 1989;25:562-571 5 . Lecouteur RA, Child G. Diseases of the spinal cord. In: Ettinger SJ, cd. Textbook of Vctcrinary Internal Medicine. Philadelphia: WB Saunders Co. 1989:692-693. 6 . Sherman JL, Barkovich AJ, Citrin CM. The MR appearance of syringomyelia. AJNR 1986;7:985-995. 7. Kirberger RM, Wrigley RH. Myelography in the dog: Review of cases with contrast medium in the central canal. Vet Radio1 & Ultrasound 1993;34:253-258. 8. De Lahunta A. Veterinary neuroanatomy and clinical neurology. 2nd ed. Philadelphia: WB Saunders Company, 1983. 9. Gardncr WJ. Hydrodynamic mechanism of syringomyelia: its relationship to myelocele. J Neurol Neurosurg Psychiat 1965;28:247-259. 10. Bradbury MWB, Lathem W. A flow of cerebrospinal fluid along the central canal of the spinal cord of the rabbit and communications between this canal and the sacral subarachnoid space. J Physiol 1965:181: 785-800. 11. Blake JA. The roof and lateral recesses of the fourth ventricle considered morphologically and embryologically. J Comp Neurol 1900; 10:79- 108. 12. Faulhauer K, Donauer E. Experimental hydrocephalus and hydrosyringomyelia in the cat. Radiological findings. Acta Neurochirurgica 1985;74:72-80. 13. Williams B, Bentley J. Experimental communicating syringomyclia in dogs after cisternal kaolin injection. J Neur Sc 1980;48:93-107. 14. Donauer E, Wussow W, Rascher K, Piepgras U. Radiologic studies
of cerebrospinal fluid pathways in experimental hydrocephalus-hydrnsyringomyelia. Acta Radiologica 1986;369:25 1-253. 15. Schlesinger EB, Antunes JL, Michelson J, Louis KM. Hydromyeha: Clinical presentation and comparison of modalities of treatment. Neurosurgery 1981;9:35&365. 16. Aboulezz AO, Sartor K , Geyer CA, Gado MH. Position of ccrebellar tonsils with Chiari malformation: A quantitative approach with MR imaging. J Comput Assist Tomogr 1985;9:1033-1036. 17. Menezes AH. Chiari I malformations and hydromyelia - complications. Pediatr Neurosurg 1991-92;17: 146154. 18. Naidich TP, Zimmerman RA. Common congenital malformations of the brain. In: Brant-Zawadski M, Norman D, eds. Magnetic resonance imaging of the central nervous system. New York: Raven Press 1987:131148. 19. Kucharczyk W, Kelly WM, Chuang S. The Brain. In: Kucharczyk W, ed. MRI: Central Nervous System. Philadelphia: Raven Press 1990:l. 17-1.20. 20. Watson AG, de Lahunta A, Evans HE. Dorsal notch of foramen magnum due to incomplete ossification of supraoccipital bone in dogs. J Small Anim Pract 1989;30:666-673. 21. Hart MN, Malamud N, Ellis WC. The Dandy-Walker syndrome. Neurology 1972;22:771-780. 22. Gardner E, O’Rahilly R, Prolo D. The Dandy-Walker and ArnoldChiari malformations. Clinical, developmental and teratological considerations. Acta Neurol 1975;32:393-407. 23. Schmidt V, Lang J , Wolf M. Dandy-Walker-Like syndrome in four dogs: Cisternography as a diagnostic aid. J Am Anim Hosp Assoc 1992; 28:355-360. 24. Regnier AM, Ducos.de Lahitte MJ, Delisle MB, Dubois GG. Dandy-Walker syndrome in a kitten. J Am Anim Hosp Assoc 1993;29: 5 14-5 18. 25. Sawaya R , McLaurin RL. Dandy-Walker syndrome. Clinical analysis of 23 cases. J Neurosurg 1981;55:89-98. 26. Diebler C, Dulac 0. Pediatric neurology and neuroradiology. Springer Verlag 1987:42.
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