Hydrocephalus

From Dog
Hydrocephalus in a young dog[1]

Hydrocephalus is a rare congenital anomaly which results in neurological disease in dogs[2].

Hydrocephalus is the result of cerebrospinal fluid (CSF) fluid disturbances[3][4] characterized by increased CSF volume and dilatation of the cerebral ventricles.

This condition may be congenital or acquired postnatally and may occur passively due to increased volume of CSF filling a void left by loss of brain parenchyma[5] (compensatory hydrocephalus, hydrocephalus ex vacuo), often seen with chiari malformations, hydranencephaly, cerebellar hypoplasia, cerebellar abiotrophy or following cranial trauma.

The most common form of hydrocephalus in animals is obstructive (noncommunicating) hydrocephalus[6] typically caused by obstruction to CSF flow in the intraventricular pathway, in which ventricular dilatation occurs proximal to the obstruction site, with preservation of normal ventricular size distal to the block. Obstruction may also occur at the point of CSF resorption by the arachnoid villi in the subarachnoid spaces (e.g., meningitis, malformation of arachnoid villi, or tumors compressing the venous sinuses[7]).

CSF pressure tends to be increased (hypertensive) in cases of obstructive hydrocephalus. While obstruction may occur anywhere along the ventricular pathway in congenital hydrocephalus, it occurs most commonly in the mesencephalic aqueduct of Sylvius. Obstructive hydrocephalus in dogs may be associated with various developmental defects, e.g., myelodysplasia, Dandy-Walker syndrome, spina bifida, syringomyelia and hydromyelia, optic nerve hypoplasia, occipital dysplasia, polymicrogyria in Poodles[8], triploidy (a fatal condition characterized by the presence of three haploid sets of chromosomes, instead of two, in all cells) in a stillborn puppy[9], aphakia (absence of the lens) and multiple ocular defects in Saint Bernard puppies[10].

Postnatal, acquired obstructive hydrocephalus, again often involving the mesencephalic aqueduct, has been associated with canine parainfluenza virus [11], necrotizing periventricular encephalitis[12], cryptococcal granulomatous ependymitis[13], methylmercury poisoning[14], parasitic migration[15] and occasionally it may be caused by mass lesions that block CSF flow at the interventricular foramen, third ventricle, mesencephalic aqueduct, or lateral apertures [16].

Hydrocephalus has been seen in several miscellaneous disorders, including galactosialidosis in a 5 year old Schipperke dog[17], cerebellar abiotrophy in Bull Mastiff puppies[18], neuroaxonal dystrophy in a 9-week-old Jack Russell Terrier [19], primary ciliary dyskinesia in dogs[20], hypertrichosis (growth of hair in excess of normal) in Golden Retrievers[21], hereditary cerebellar abiotrophy in Australian Kelpie dogs[22], arachnoid cysts within the quadrigeminal cistern[23], and in a dog presented with continuous tail chasing[24]. Hydrocephalus is thought to play a role in the development of hydrosyringomyelia[25].

'External' hydrocephalus rmay be observed in soem dogs where excessive collection of CSF forms within the subarachnoid space rather than in the ventricular system and is seen in cases of generalized brain atrophy. This form has been reported in a 12 week old Fox Terrier presented with hydrocephalus[26].

Bleeding is a potential complication of hydrocephalus. Chronic subdural hematomas have been reported in a hydrocephalic 2 month old Newfoundland puppy[27].

Small, toy, and brachycephalic breeds (Maltese, Yorkshire Terrier, English Bulldog, Chihuahua, Lhasa Apso, Pomeranian, Toy Poodle, etc.) are at high risk for hydrocephalus. In one study, 53% of 564 hydrocephalic dogs manifested clinical signs by 1 year of age (Selby et al, 1979). A distinction between congenital and acquired forms of hydrocephalus may be very difficult from a clinical viewpoint especially since infectious agents may cause hydrocephalus postnatally in young puppies[28]. Furthermore, the confusion in terminology is reflected in the results of one epizootiologic study from 14 veterinary schools in the United States in which 30% of 564 dogs classified as having "hydrocephalus due to congenital origin" were over 2 years of age. Genetic disease has been reported.

Clinical signs in affected animals (which are usually young) include an enlarged, dome shaped cranium, and open sutures and/or fontanelles that may be bulging in an animal that continuously cries out, has visual and auditory impairment, and altered mental status (ranging from hyper-excitability to severe depression). This should be distinguished from craniosynostosis, where similar features are present , including intracranial hypertension, but without cerebral deficits.

Other signs may include sporadic seizures, a gait that is uncoordinated, spastic, and clumsy, head tilt, circling, dilated and fixed pupils, and head pressing[29]. Ventrolateral strabismus may occur as a result of encroachment on the orbit from expanding frontal bones, in which case eye movements are normal. Less common signs are positional and spontaneous nystagmus, vomiting, and cervical pain.

Some adult Golden Retrievers with hydrocephalus have concurrent hypertrichosis and may exhibit behavioral abnormalities (e.g., aggression, hyperactivity, slow learning and other temperament changes) that make them unacceptable pets[21].

Diagnosis is usually based on CT or MRI imaging. Cranial sutures and/or open fontanelles may be evident after the normal age for closure and skull ossification[30][31].

Measuring ventricular volume using quantitative MRI also appears to be a useful tool and may help in understanding the relationship between ventricular volume and neurological disease[32]. In one study, a high incidence of asymptomatic (ventricular enlargement or ventriculomegaly) was noted in clinically normal dogs[33], a finding sometimes termed 'occult hydrocephalus'.

Electroencephalographic traces usually have a characteristic pattern of high amplitude (25 - 200 mV), slow wave (1 - 5 Hz) activity, often with a superimposed fast frequency of 10 to 12 Hz. Fundic examination may reveal papilledema. Collection of CSF for analysis is usually not performed since it may precipitate brain herniation due to the presence of increased intracranial pressure.

Prognosis is sometimes related to an underlying disease (such as cerebral neoplasm), but tends to be guarded to poor.

The efficacy of corticosteroids and surgical shunt procedures in animals with acquired hydrocephalus remains uncertain but dexamethasone has been used empirically[34]. This dose is gradually reduced over a 2 to 3 week course of therapy. Some animals may be maintained on alternate day dosage schedules.

References

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