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Genetic Welfare Problems of Companion Animals
An information resource for prospective pet owners
Siamese
Hydrocephalus
Related terms: Hydrocephaly, primary hydrocephalus, "Water on the brain"
Outline: Siamese cats are reported to be at risk of an inherited congenital hydrocephalus in which cerebrospinal fluid (fluid normally present in the brain and spinal cord) builds up causing pressure on the brain. The skull may become abnormally large and the pressure can cause affected animals chronic pain and loss of brain function. The disease usually begins early in life and typically progresses. Treatment is difficult. It is not possible to determine which animals are likely to have affected offspring, but affected animals should not be bred from and those that have produced affected offspring should not be bred again
Summary of Information
(for more information click on the links below)
1. Brief description
Hydrocephalus is characterised by the presence of an abnormally large volume of cerebrospinal fluid (CSF) present within the brain or between the brain and skull. In both forms, the pressure exerted by the CSF on the bones of the skull may result in the cranium becoming excessively domed in shape and to thinning of the skull bones. The disease can cause brain damage and dysfunction. Signs are often not present at birth but become apparent over the first few weeks or months of life. Affected kittens are often slower to learn than littermates, have gait abnormalities eg being ataxic (wobbly) and can exhibit bizarre behaviours such as compulsive circling, and may have seizures (fits). The progress of the disease is variable and unpredictable. Signs usually progress from mild to severe and then to stupor and death, but in some animals the disease may stabilise before this, meaning that they exhibit only minor signs that persist throughout life.
Treatment is difficult but can involve medication aimed at reducing fluid production and brain swelling, or surgery to implant a shunt to assist drainage of the fluid from the skull (eg into the abdominal cavity).
2. Intensity of welfare impact
Affected cats are likely to suffer from pain, and probably also from nausea (feeling sick) and other forms of discomfort (mild to severe depending on the circumstances) as they are not able to function normally. The disease frequently leads to death.
3. Duration of welfare impact
This tends to be a relatively slowly developing condition and the duration of welfare problems is likely to be prolonged. Unless successfully treated, the problems are likely to persist and become progressively worse until the animal dies or is euthanased.
4. Number of animals affected
The proportion of Siamese cats affected by this condition is unknown.
5. Diagnosis
The diagnosis of hydrocephalus may be suspected in any young Siamese cat with signs of brain disease. Signs include a marked dome-shape to the cranium (the skull bones that encase the brain), unfused skull bones and ventrolateral strabismus (eye squint). Confirmation of the diagnosis requires ultrasound examination, magnetic resonance imaging (MRI) or computer tomography (CT) scans of the brain.
6. Genetics
It has been suggested that hydrocephalus in Siamese cats is an autosomal recessive condition (Silson & Robinson 1969) but this has yet to be confirmed. If this is the case, then kittens with two mutated genes (one from each parent) will have the condition whilst animals with only one mutated gene may carry the condition and pass it on to the next generation without being affected themselves.
7. How do you know if an animal is a carrier or likely to become affected?
Currently it is not possible to identify carrier animals as the genetics are yet to be confirmed and no test for carrier status has been developed.
All kittens should be examined prior to purchase. Many affected individuals may be identified by this age.
8. Methods and prospects for elimination of the problem
It is not possible, at present, to determine which individuals may produce affected offspring. The best advice to avoid perpetuating conditions in which there is strong evidence for a genetic basis but where no specific tests are available, is not to breed from affected individuals and, generally, to avoid breeding from animals that have affected relatives or which have produced affected offspring.
Progress in eliminating this condition may be facilitated by the elucidation of the details of its genetic basis and the development of a genetic test.
For further details about this condition, please click on the following:
(these link to items down this page)
- Clinical and pathological effects
- Intensity of welfare impact
- Duration of welfare impact
- Number of animals affected
- Diagnosis
- Genetics
- How do you know if an animal is a carrier or likely to become affected?
- Methods and prospects for elimination of the problem
- Acknowledgements
- References
1. Clinical and pathological effects
Siamese cats appear to be predisposed to a hereditary congenital hydrocephalus in which cerebrospinal fluid builds up in the skull causing pressure on the brain. Cerebrospinal fluid (CSF) circulates in cavities within and around the brain and spinal cord and provides a protective, cushioning effect to these organs as well as supplying them with nutrients.
There are four cavities (ventricles) within the brain where most of the CSF is produced. It flows from the larger, paired lateral ventricles into the smaller, third and fourth ventricles and then to the spaces between the tissue of the brain and spinal cord and the bones that surround them (the skull and vertebrae). The connecting passages between the ventricles, and between these internal brain cavities and the outside of the brain and spinal cord, are all relatively narrow. Any excess of CSF is absorbed into the blood circulation at special blood vessel sites called arachnoidal villi, although absorption into capillaries throughout the brain and spinal cord also occurs (Dewey et al 2006).
Figure 1. This diagram of the human brain illustrates the location of the four ventricles/ chambers and is similar to that seen in the cat. Cerebrospinal fluid (CSF) is produced in, and circulates through, these chambers and in the space between the brain and skull (the subarachnoid space) and between the spinal cord and spinal canal.
(This figure, for which we are grateful, is from the Canadian Institutes for Health Research website at http://thebrain.mcgill.ca).
Hydrocephalus is characterised by the presence of an abnormally large volume of CSF within the skull. There are several forms of hydrocephalus reported in the cat and these are classified as internal or external depending on whether the fluid builds up within the ventricles or in the subarachnoid spaces. Communicating hydrocephalus is caused by an outflow obstruction of the venous system or increased CSF production which increases fluid in both spaces and non-communicating only increases the fluid within the ventricular space (Coates & Sullivan 2001). Congenital hydrocephalus in cats can be associated with fusion of part of the brain, the rostral colliculi, causing a narrowing between the third and fourth ventricles (the Aquaduct of Sylvius) (Coates & Sullivan 2001).
Figure 2. A CT scan of a human patient showing ventricles excessively dilated with CSF (darkest areas on the scan). This occurs in both communicating and non- communicating forms of hydrocephalus (see text).
In a healthy brain, the ventricles are much smaller.
(Figure available from Wikipedia: http://en.wikipedia.org/wiki/Hydrocephalus )
The result of these abnormalities is a build-up of CSF within the brain which causes it to swell. When this occurs in young individuals, whose skull bones are still growing and have yet to fuse together, this swelling causes the skull bones to be pushed further apart than usual so that the head enlarges. Further, this build up of fluid causes the tissue of the brain to become compressed between the fluid trapped inside it and the inner surface of the skull. In the communicating form of hydrocephalus the tissue of the brain is squeezed between excessive fluid both within the brain and without (due to a build-up of CSF between the brain and skull). In both forms, this increase in pressure can result in the cranium becoming excessively domed in shape and thinning of the skull bones.
Figure 3. A Siamese cat showing signs of hydrocephalus. The domed skull shape is the result of excess CSF within the brain causing distension of the skull.
(Image property of Dr A Sparkes and the Feline Advisory Bureau (www.fabcats.org), to whom we are grateful for the permission to reproduce it here).
In cats secondary or acquired hydrocephalus can occur following infectious inflammations of the brain caused by feline pathogens, such as feline infectious peritonitis (FIP or coronavirus) or feline infectious enteritis (feline parvovirus),or as a result of toxic side effects of griseofulvin medication to the mother during pregnancy. It can also be seen secondary to other developmental abnormalities or acquired disorders such as neoplasia (tumours) (Coates & Sullivan 2001). Hydrocephalus due to these secondary causes is seen more commonly in cats than the primary congenital form but this document concerns the primary congenital form.
Hydrocephalus can cause brain damage and dysfunction. Signs are often not present at birth but become apparent over the first few weeks or months of life. This is partly due to progressive fluid accumulation and compression of the brain, and partly due to loss of brain tissue and function. Detection of brain impairment becomes easier as the kittens grow. The feline brain normally takes months to mature and abnormal behavioural development may be the first sign to owners that significant disease due to hydrocephalus is present.
Typically, affected kittens are slower to learn and, for example, may not house train (Shell 2009). More severe signs are also often linked to damage to the cerebral hemispheres (the forebrain area that is involved with higher brain functions). Affected kittens can exhibit bizarre behaviours such as circling, dementia, repeated pressing of their heads against a wall, and seizures (fits); they may also walk abnormally with a spastic gait (Shell 2009). It may also lead to visual impairment (due to brain damage). The progress of the disease is variable and unpredictable. Signs usually progress from mild to severe and then to death but in some animals the disease may stabilise before this, meaning that they exhibit only minor signs that persist throughout life.
Treatment is difficult but can involve medication aimed at reducing CSF production and brain swelling or surgery to implant a shunt to permanently aid draining away of excess fluid from the skull (eg into the abdominal cavity).
In brain diseases generally, there is not a close correlation between the volume of brain loss and degree of dysfunction observed and this is also true in hydrocephalus. The degree of hydrocephalus seen (for example on a MRI or CT brain scan) does not predict the extent of disability or the prognosis.
2. Intensity of welfare impact
Hydrocephalus is known to be painful in varying degrees in humans and it may be expected that affected cats experience headaches. In humans, hydrocephalus can also cause nausea (feeling sick/wanting to vomit) and distress as a result of not being able to function normally. It is therefore considered to be a serious welfare issue.
3. Duration of welfare impact
The disease tends to develop relatively slowly and the duration of welfare problems is likely to be prolonged. Unless successfully treated, the problems are likely to persist and become progressively worse until the animal dies or is euthanased. Some affected animals will have intermittent suffering that responds to treatment and some may spontaneously recover. Major surgery is required to treat the disease through implantation of a shunt to drain away excess fluid and this carries a risk of painful infections. Such treatment can also fail in the medium term and surgery either has to be repeated or ends with the cat being euthanased (Axland 2004, Dewey et al 2006).
4. Number of animals affected
This is reported to be a hereditary condition in Siamese cats (Silson & Robinson 1969); however, as far as we are aware, there are no published data on the prevalence of the disease in the breed. Congenital hydrocephalus is generally considered to be less common in cats than dogs (Hoskins 2001, Dewey 2008).
5. Diagnosis
The diagnosis of congenital, non-communicating hydrocephalus may be suspected in any young Siamese with signs of brain disease. Signs may include a marked dome-shape to the cranium, unfused skull bones, and open fontanelles (fontanelles are ‘soft spots’ on a kitten’s head that lack bone and which, during birth, enable the bony plates of the skull to flex to allow the kitten’s head to pass through the birth canal) (Shell 2009). Another common sign of hydrocephalus is the so-called “setting-sun sign” or ventrolateral strabismus (in which the eyes look outwards and downwards rather than straight ahead; ie the opposite to being cross-eyed) (Shell 2009). Confirmation of the diagnosis requires ultrasound examination of the brain either through a fontanelle or through the closed skull bones, if they are thin enough to permit this (Hudson et al 1990, Coates & Sullivan 2001). MRI and CT scans can also help to confirm the diagnosis (Coates & Sullivan 2001). Radiography may reveal the thin skull bones typical of the disease, but this is a less useful technique.
6. Genetics
It has been suggested that hydrocephalus in Siamese cats is an autosomal recessive condition (Silson & Robinson 1969) but this has yet to be confirmed. If this is the case, then kittens with two mutated genes (one from each parent) will have the condition whilst those animals with only one mutated gene may carry the condition and pass it on to the next generation without developing the disease themselves.
7. How do you know if an animal is a carrier or likely to become affected?
All Siamese kittens should be examined by a veterinary surgeon prior to purchase and any with possible signs of hydrocephalus should not be purchased. Many affected individuals will have started to show signs by three months of age. Those that develop signs of the condition later should be returned to the breeder. Currently it is not possible to identify carrier animals as the genes involved have not been determined and a test for carriers has not been developed.
8. Methods and prospects for elimination of the problem
It is not possible at present to determine which individuals may produce affected offspring. The best advice to avoid perpetuating conditions in which there is strong evidence for a genetic basis but for which there are no tests, is not to breed from affected individuals and, generally, to avoid breeding from animals that have affected relatives. Healthy Siamese which have produced an affected kitten should be assumed to be carriers of the condition and should not be used for breeding again. Progress in eliminating this condition may be facilitated by the elucidation of the details of its genetic basis and the development of a genetic test.
9. Acknowledgements
UFAW is grateful to Rosie Godfrey BVetMed MRCVS and David Godfrey BVetMed FRCVS for their work in compiling this section.
10. References
Coates JR and Sullivan SA (2001) Congenital Cranial and Intracranial Malformations in August JR’s Consultations in Feline Internal Medicine 4. WB Saunders: Philadelphia, USA
Dewey C (2008) A practical guide to canine and feline neurology. 2nd Ed. Blackwell Publishing: Arnes, USA
Dewey C, Coates J, Axlund T, Smith J and Hopper S (2006) Hydrocephalus in Dogs and Cats. Compendium 28: 136-144
Hudson JA, Simpson ST, Buxton DF, Cartee RE and Steiss JE (1990) Ultrasonographic diagnosis of canine hydrocephalus. Veterinary Radiology and Ultrasound 31: 50-58
Hoskins JD (2001) 1990 The Nervous System. In Hoskin JD’s Veterinary Pediatrics: dogs and cats from birth to six months 2nd ed p 431 WB Saunders: Philadelphia, USA
Shell L (2009) Hydrocephalus, congenital and acquired. VIN Associate. http://www.vin.com/Members/Associate/Associate.plx?DiseaseId=994. Accessed 25.4.11.
Silson M and Robinson R (1969) Hereditary hydrocephalus in the cat. Veterinary Record 84: 477
© UFAW 2011
Credit for main photo above:
By Karin Langner-Bahmann, upload von Martin Bahmann (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons
