Michael Reese, DVM, DACVIM (Neurology)
Congenital Central Nervous System Disorders
Congenital central nervous system (CNS) disorders is a general description that incorporates all
abnormalities that are present at the time of birth. These can be due to inherited genetic defects, or
from prenatal infections, intoxications, or nutritional deficiencies. Following birth, the central nervous
system continues to develop. Thus, some abnormalities can be identified early on, but some are not
apparent until months to years later in life. Many congenital CNS disorders are uncommon, but others
occur with greater frequency. The following is an incomplete list for congenital brain and spine
disorders:
BRAIN
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Hydrocephalus: A disorder that arises due to an imbalance between cerebrospinal fluid (CSF)
production and reabsorption. Congenital hydrocephalus is a disorder resulting in impaired CSF
reabsorption. There is speculation that this is due to a prenatal infection leading to this occurrence,
but a primary genetic component cannot be ruled out due to breed specific predilections. The
breeds more commonly affected include the Chihuahua, Yorkshire Terrier, Pug, and Pomeranian.
Common presenting signs would include: domed head, difficulty house training, strabismus (“setting
sun eyes”), dull mentation, circling, and ataxia. MRI is the best tool for evaluation due to its
sensitivity for other pathologies that could lead to hydrocephalus, but CT is also a viable way to
identify hydrocephalus. Persistent fontanelle can often be identified, and do give an opportunity to
image using ultrasonography. The brain is more fragile than normal, so minor head trauma can
create acute problems. Medical management for hydrocephalus requires altering CSF production
and/or reabsorption. Omeprazole (0.5-1 mg/kg/day) has been shown to reduce the amount of CSF
produced. Corticosteroids (0.5-1.0 mg/kg/day of prednisone) can assist with inflammation and help
increase CSF reabsorption, if there are inflammatory changes effecting the routes of CSF
reabsorption. In very acute cases mannitol (0.5-1 gram/kg/day, up to 2 grams/kg/day) can also be
used to aid in CSF reabsorption. Also, depending on the age and whether or not there is a persistent
fontanelle, a CSF tap into the lateral ventricle can quickly alleviate elevated intracranial pressure. In
some cases, medical management can be continued long term with good control of clinical signs.
However, surgical treatment of hydrocephalus offers a better long term control compared to
medical management. Ventriculoperitoneal shunts allow for CSF diversion to control clinical signs
and intracranial pressure, but there is a fairly high failure rate (50%) within the first two year period.
The most common causes for ventriculoperitoneal shunt failure are from shunt obstruction
(proteinaceous debris), infection, or shunt migration.1-7
Caudal Occipital Malformation Syndrome (COMS): A disorder that arises from a caudal fossa (area
within the skull that contains the cerebellum) that is inappropriately sized to contain the
cerebellum. The caudal cerebellar vermis extends into the foramen magnum, leading to disruption
of the normal CSF flow dynamics. This alteration in CSF flow dynamics lead to the development
syringohydromyelia, which is a dilation of the central canal (hydromyelia) with penetration of CSF
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into the spinal cord parenchyma (syringomyelia). Hydrocephalus can also be seen concurrently in
patients with COMS. The most common breed associated with this congenital malformation is the
Cavalier King Charles Spaniel, but other small breed dogs can be effected. Clinical signs associated
with syringohydromyelia include phantom neck scratching (from a parasthesia), neck pain, nerve
root signature/thoracic limb lameness, tetraparesis, and tetra-ataxia. Diagnosis is made using MRI,
looking for flattening of the caudal aspect of the cerebellum, herniation of the cerebellar vermis into
the foramen magnum, and fluid dilation of the central canal. Medical management involves
moderating CSF production, treating inflammation, and treating the paresthesia or the nerverelated pain. Omeprazole (0.5-1 mg/kg/day) can be used to reduce CSF production. Corticosteroids
(0.5-1.0 mg/kg/day of prednisone) can assist with inflammation and help increase CSF reabsorption.
Gabapentin (about 10 mg/kg TID) is commonly used to treat the nerve-related pain or parasthesia.
Surgical management can be pursued in severe, non-responsive, or recurrent cases. This involves
removal of the occipital bone to alleviate the compression of the cerebellum, with or without a
cranioplasty. Some advocate the use of a bone cement cap to reform the occipital bone, but there is
no proven benefit to one over the other. 8-11
Cerebellar abiotrophy: A disorder resulting from postnatal degeneration of the cerebellum. This
often occurs in younger animals from a few months of age to about a year old. In two breeds,
Brittany Spaniels and Staffordshire Terriers, this degeneration begins when they are middle aged.
The progression of clinical signs related to cerebellar dysfunction varies depending on breed, and
can be from several weeks to years. Some of the other breeds described with this condition include
the Kelly Blue Terrier, Old English Sheepdog, Scottish Terrier, Gordon Setter, and domestic shorthair
cats. Diagnosis is made using MRI, to evaluate the subarachnoid space around the cerebellum and
the ratio of the brainstem to cerebellum. There is no treatment to stop the progression of the
disease, and treatment centers around supportive care. Meclizine (about 4 mg/kg PO QD) or Cerenia
(2-8 mg/kg PO QD) can be used to help with the motion sickness related to vestibular signs, if
present. Not all cases progress to a point where they are incapacitated.12-16
Cerebellar Hypoplasia: A disorder resulting from a malformation of the cerebellum. This is most
commonly seen in kittens due to panleukopenia virus infection in utero. This can also be a result of
vaccination during pregnancy. Cerebellar hypoplasia has also been described in some terrier breeds
and Chow Chows. Clinical signs associated with cerebellar dysfunction are apparent as soon as the
animals begin to try to ambulate. This is a nonprogressive disease, and the degree of clinical signs
depends on the severity of malformation. Some animals will adapt as they mature, thus can appear
to improve a little. Others will remain static with their clinical signs. Depending on the severity of the
clinical signs, these animals often can enjoy a good quality of life. There is no treatment available.
The environment they live will need to be set up for their protection, so they do not get injured
because of their incoordination.17-19
SPINE
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Vertebral malformations: There are multiple malformations that fall under this category, such as
hemivertebrae, butterfly vertebrae, block vertebrae, spina bifida, and transitional vertebrae. These
often do not cause clinical signs themselves, but they can change the dynamics of the spinal column
and lead to other problems such as disc disease and subarachnoid diverticulum (cyst) formation. The
common breeds associated with vertebral malformations would include the French Bulldog,
Bulldogs, and Pugs.
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Hemivertebrae and butterfly vertebrae both occur due to an abnormal formation of the
vertebral body. Hemivertebrae are a triangular shaped vertebra due to failure of one half of
the vertebral body to form.
o Butterfly vertebrae are an anomaly that result in a sagittal cleft, due to failure of the central
and ventral aspects of the vertebral body to form.
o Block vertebrae result from failure of separation of two vertebral bodies during embryonic
development, leading to one large vertebra.
o Spina bifida is an anomaly due to failed fusion of the dorsal lamina, leading to a defect
dorsally with two spinous processes. This can be associated with no protrusion of nervous
tissue into the defect, protrusion of the meninges into the defect (meningocele), or
protrusion of meninges and spinal cord into the defect (meningomyelocele).
o Transitional vertebrae are fairly common. Lumbarization of the last thoracic vertebra and
sacralization of the last lumbar vertebra are the most common. This malformation creates a
vertebra that shares characteristics of a thoracic and a lumbar vertebra (lumbarization),
with a transverse process on one side and rib on the other; or a lumbar spine and sacral
vertebra (sacralization), with a transverse process on one side and fusion to the ilium on the
other. These malformations in themselves often do not lead to neurologic deficits. However,
there does seem to be some increased incidence of disc disease adjacent to these abnormal
vertebrae. Sacralization of the last lumbar vertebra is associated with the development of
lumbosacral stenosis in German Shepherd Dogs (GSD).
These malformations are readily identifiable on radiographs. Determining whether they are causing
impingement of the spinal cord requires advanced diagnostics, such as a CT (with or without
myelography) or an MRI. There is no specific treatment for these malformations if they are not
leading to neurologic weakness. When they do lead to neurologic weakness, the treatment often
requires decompression of the spinal cord along with fusion of the adjacent region of the spine.20-23
Atlantoaxial (AA) subluxation: A disorder leading to instability of the AA joint, which is associated
with a congenital malformation or trauma. The AA joint is stabilized by the apical, paired alar, and
transverse ligaments that connect the dens of C2 to the vertebral body of C1. There is also
ligamentous attachment dorsally between C1 and C2 by the dorsal atlantoaxial ligament. Congenital
malformations weaken the attachment of C1 to C2, which can be from aplasia or hypoplasia of the
dens, ligamentous malformation, or less commonly due to a block vertebrae. Malformation of the
dens occurs most frequently in toy breed dogs. Clinical signs associated with atlantoaxial instability
can present acutely after minor trauma, but also can present as a chronic progressive disease.
Clinical signs can include neck pain, tetra-ataxia, tetraparesis, and vestibular signs. AA subluxation
can be diagnosed with radiographs, with instability that can be appreciated with dynamic views.
Extreme caution should be taken when flexing the neck of any dog that may have AA instability. MRI
and CT can be useful to evaluate for other potential comorbidities (e.g. hydrocephalus, COMS, etc)
and obtain better bony anatomy for implant placement. In patients with just pain and with mild
deficits, AA instability/subluxation can be managed with splinting, which can be a dorsal or ventral
splint. Initially the splints should be changed once to twice weekly, depending on how well the dog
is adapting to the splint and how well the owners are keeping it clean. The splint should be kept on
for a total of 6-8 weeks, along with strict exercise restriction. This is more often successful in
younger patients with acute onset of clinical signs, and less successful in the older patients that have
a more progressive onset. Depending on how the response is to medical management and in
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patients with significant neurologic deficits, surgical stabilization of C1-C2 is the definitive treatment
option.24-27
Sacrocaudal dysgenesis: A disorder that leads to vertebral malformations in the sacrum and tail,
which is associated with the formation of the "screw tail". This is seen primarily in Manx cats, but
can be seen in other animals (e.g. bulldogs). Clinical signs can vary, and if present will often be
apparent as they begin to move around. Clinical signs can static or progressive, and can include
weakness/ataxia, and urinary and/or fecal incontinence. There are no effective treatment options
available, and therapy involves maximizing hygiene to prevent urine scald and contact dermatitis.
Prognosis depends on the degree of neurologic deficits and the commitment of the owner to the
care of a nonambulatory and/or incontinent animal.28-29
Dermoid sinus: A disorder that creates a defect that communicates the spinal column to the skin
due to a failure in the normal separation of the ectoderm from the neural tube during embryonic
development. A sinus tract is created that extends to varying depths into the subcutaneous tissues,
classified from I to V. This is primarily seen in Rhodesian Ridgebacks, but this anomaly has been
identified in other breeds such as Cocker Spaniels, Rottweilers, Boxers, other dogs breeds, and cats.
The onset of clinical signs can occur at any age, but is more common in young dogs. Clinical signs
can include a swelling on the neck or back, bacterial infection of the sinus, or signs associated with a
meningitis or meningomyelitis if it communicates with the central nervous system. Diagnosis can be
made via contrast radiography (fistulogram), but the appropriate contrast agent should be used
(labeled safe for intrathecal use). An MRI or CT can also be used to determine the extent of the sinus
tract with more precision. If there is no clinical disease related to the dermoid sinus, then no
treatment is necessary. If there are neurologic deficits or skin infections related to the dermoid
since, then treatment involves the complete surgical excision of the tract along with antibiotic
therapy for the dermatitis. Prognosis ultimately depends on the neurologic status prior to
intervention.29-33
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