Summers Ch 2

Question 1

What are the two classifications of CNS malformations based on aetiology?

Answer 1

Primary malformations and secondary malformations

Primary malformations are caused by spontaneous or inherited gene mutations, while secondary malformations arise from exposure to teratogens.

Question 2

Define cerebral aplasia.

Answer 2

Partial absence of the cerebral hemispheres

Often seen in calves, with some ability to stand and walk if the brain stem is intact.

Question 3

What is anencephaly?

Answer 3

Absence of cerebral hemispheres

True anencephaly is very rare.

Question 4

What is a cranium bifidum?

Answer 4

A defect in the calvarium with a small opening on the midline between the frontal bones

Usually associated with cerebral aplasia in calves.

Question 5

What is an encephalocoele?

Answer 5

Protrusion of brain tissue through a defect in the calvarium covered by skin

More accurately referred to as meningoencephalocoele.

Question 6

What is exencephaly?

Answer 6

Protrusion of brain tissue not covered by skin or meninges

Rare and often associated with severe skull and brain malformations.

Question 7

What is holoencephaly?

Answer 7

Development of a single, non-divided cerebrum

Most common in sheep and associated with cyclopia and arhinencephaly.

Question 8

What is cyclopia?

Answer 8

Development of a single, median-positioned eye

Results from interference in the separation of the eye fields during development.

Question 9

What is microphthalmia?

Answer 9

Inadequate development of the eye resulting in a smaller than normal eye

Often associated with other brain malformations.

Question 10

What is anophthalmia?

Answer 10

Complete absence of any evidence of eyeball tissue

Represents failure of optic vesicle-cup development.

Question 11

What is agenesis of the corpus callosum?

Answer 11

Failure of the commissure between the neocortex of each hemisphere to form.

Associated with absence of the septum pellucidum and hippocampal commissure.

Question 12

What characterizes Dandy-Walker syndrome?

Answer 12

Partial or complete absence of the vermis of the cerebellum

Commonly seen in calves and dogs, often leading to cerebellar ataxia.

Question 13

What is hydranencephaly?

Answer 13

Nearly complete destruction and lack of development of the neocortex

Often results from viral infections during gestation.

Question 14

What is porencephaly?

Answer 14

Presence of cystic cavities in the cerebrum due to less severe brain damage

Cavities may communicate with the lateral ventricle or sub-arachnoid space.

Question 15

What does lissencephaly refer to?

Answer 15

Abnormality where the cerebrum has a smooth surface without gyri and sulci

This condition is abnormal in cats and dogs but normal in some other species.

Question 16

What is polymicrogyria?

Answer 16

Excessive production of smaller gyri in the brain

Rare malformation usually associated with more extensive brain malformations.

Question 17

What is microencephaly?

Answer 17

Overall reduction in the size of the brain, especially in the cerebral hemispheres

Typically virus-induced and seen with certain viral infections.

Question 18

Define megalencephaly.

Answer 18

Brain volume that exceeds the mean by more than twice the standard deviation

Subgroups include anatomic, metabolic, and dynamic megalencephaly.

Question 19

What is hydrocephalus?

Answer 19

Increase in volume of cerebrospinal fluid (CSF)

Various classifications exist, including internal, external, obstructive, and communicating hydrocephalus.

Question 20

What is the most common cause of hydrocephalus?

Answer 20

Aqueductal stenosis associated with fused rostral colliculi

Can be caused by pre- or postnatal infections affecting the ependymal surface.

Question 21

What alterations can cause secondary stenosis in hydrocephalus?

Answer 21

Pre- or postnatal inflammations altering the ependymal surface of the aqueduct

Exposure to substances like griseofulvin or infections like feline panleukopenia virus.

Question 22

What is hydromyelia?

Answer 22

Abnormal dilation of the central canal with an intact ependymal layer

It is regarded as the preliminary stage of syringomyelia.

Question 23

What is syringomyelia?

Answer 23

Progressive development of a fluid-filled cavitation or syrinx within spinal cord parenchyma

Most often affects the dorsal column.

Question 24

What are the pathophysiologic mechanisms of acquired hydrocephalus?

Answer 24

Direct or indirect obstruction, loss of brain parenchyma, increased CSF production from a tumor

Hydrocephalus ex-vacuo occurs with destruction or lack of development of cerebral tissue.

Question 25

What causes hydrocephalus ex-vacuo?

Answer 25

Destruction or lack of development of cerebral tissue during critical development periods ## Footnote Examples include hydranencephaly and porencephaly.

Question 26

What is the common cause of syringomyelia in dogs?

Answer 26

Chiari-like malformation ## Footnote Characterized by overcrowding of the craniocervical junction and obstruction of CSF flow.

Question 27

What are the different forms of spina bifida?

Answer 27

Spina bifida occulta, spina bifida aperta, meningocoele, meningomyelocoele ## Footnote Each variant has distinct characteristics and severity.

Question 28

What is meningomyelocoele?

Answer 28

A more severe variant of spina bifida with inclusion of spinal cord in the herniated cyst ## Footnote Often involves sacrocaudal roots and spinal nerves.

Question 29

What is diplomyelia?

Answer 29

Spinal cord duplication occurring within a common covering of leptomeninges and dura ## Footnote It involves variable and often incomplete duplication.

Question 30

What is diastematomyelia?

Answer 30

Development of two separate spinal cords, each in its own meningeal covering ## Footnote Usually associated with paired vertebral canals and a bony partition.

Question 31

What does myelodysplasia refer to?

Answer 31

Malformation of the spinal cord including various morphological abnormalities ## Footnote Often includes segmental hypoplasia and other related issues.

Question 32

What are common abnormalities associated with myelodysplasia?

Answer 32

Hydromyelia, syringomyelia, absence or duplicated central canal, abnormal grey matter distribution ## Footnote Vertebral abnormalities often accompany these spinal cord malformations.

Question 33

Fill in the blank: The presence of a fluid-filled sac protruding from the spinal canal is known as a _______.

Answer 33

Meningocoele

Question 34

True or False: Hydrocephalus can occur without any observable abnormalities other than dilated ventricles.

Answer 34

True ## Footnote Such cases are assumed to have structural and/or functional abnormalities of the arachnoid villi.

Question 35

What is the role of the choroid plexuses in cerebrospinal fluid (CSF) production?

Answer 35

They produce the majority of CSF in each ventricle of the brain ## Footnote CSF is drained partly into the blood and partly along lymphatic pathways.

Question 36

What are the two theories for how cerebral aplasia develops?

Answer 36

1. If the neural tube was prevented from fusing rostrally, at the level of the rostral neuropore, the prosencephalon would remain attached to the skin ectoderm at that point. This would expose the neural canal to the skin surface and prevent meninges and bone from developing over the brain at that point. This would account for the opening in the calvarium (the cranium bifidum) and the leakage of CSF. 2. The defect represents a rupture of the neural tube after initial closure.

Question 37

| How does a meningoencephalocoele develop embryologically?

Answer 37

Failure of the neural plate to separate from the skin ectoderm and fuse into a neural tube.

Question 38

Describe the embryological development fo holoprosencephaly and cyclopia

Answer 38

Normally, a single ventral region of prosencephalic neuroectoderm is responsible for initiating the development of the eyes. Interaction between this originally single "eye field" and the underlying head mesenchyme is responsible for its separation into two symmetrical eyefields that bulge out as the optic pits and vesicles. The same interaction is responsible for the development of two separate dorsolateral telencephalic vesicles. Hence, interference with this interaction results in holoencephaly and cyclopia.

Question 39

How does microphthalmia develop

Answer 39

Inadequate development of the optic cup

Question 40

How does agenesis of the corpus callosum develop

Answer 40

Both telencephalic vesicles develop into the two cerebral hemispheres, but the commissure that normally develops in the lamina terminalis between the neocortex of each hemisphere fails to form

Question 41

How does Dandy Walker syndrome develop

Answer 41

Primary parenchymal midline developmental field defect of unknown origin

Question 42

How do hydranencephaly and porencephaly develop?

Answer 42

Destruction of the ventricular (germinal) zone of the telencephalon as well as the neocortex that is already formed. The malformation therefore represents both hypoplasia and secondary atrophy.

Question 43

How does lissencephaly develop?

Answer 43

The normal laminar pattern of neuronal cell body organization is disrupted

Question 44

Describe the three subgroups of megalencephaly

Answer 44

Dynamic megalencephaly is the brain enlargement that occurs secondary to obstructive hydrocephalus. Metabolic megalencephaly is seen in some of the storage diseases in children. Anatomic megalencephaly results from a neurodevelopmental disorder with excessive neuronal development or decreased normal programmed neuronal death.

Question 45

Define internal and external hydrocephalus

Answer 45

- Internal hydrocephalus: Increased volume of CSF within the ventricles - External hydrocephalus: increased volume of CSF within the subarachnoid spaces

Question 46

Define obstructive, communicating, and ex vacuo hydrocephalus

Answer 46

- Obstructive (i.e., noncommunicating) hydrocephalus is characterized by occlusion of CSF flow within the ventricular system rostral to the site of obstruction. - Communicating hydrocephalus occurs when normal CSF circulation and absorption is obstructed after it exits the ventricles. Communicating hydrocephalus may also be a result of excessive CSF production, but this is rare. - Compensatory hydrocephalus (i.e., hydrocephalus exvacuo) occurs when CSF occupies space in the cranial cavity that normally would be occupied by brain parenchyma.

Question 47

Where does syrinx fluid come from?

Answer 47

There is no consensus as to whether syrinx fluid is derived from extracellular or cerebrospinal fluid (CSF).

Question 48

Where is CSF produced and drained?

Answer 48

The majority of CSF is produced by the four choroid plexuses (one in each ventricle of the brain), and drainage of CSF is partly into the blood through arachnoid granulations and villi and partly along lymphatic drainage pathways. It has also been suggested that the spinal cord central canal may play a part in drainage of CSF.

Question 49

What is the major factor in the formation of a syrinx?

Answer 49

Disturbance of the normal free flow of CSF through the foramen magnum appears to be a major factor

Question 50

Describe the theory of pathogenesis of syringomyelia

Answer 50

Obstruction to CSF flow in the subarachnoid space results in a mismatch in timing between the arterial pulse peak pressure and CSF pulse peak pressure. Earlier arrival of peak CSF pressure compared to peak spinal arterial pressure encourages flow of CSF into the perivascular space. The perivascular space changes in size during the cardiac cycle and is widest when spinal arteriole pressure is low. If at that time peak CSF pressure is high then the perivascular space could act as a ‘leaky’ one-way valve.

Question 51

Describe how diplomyelia and diastematomyelia develop embryologically

Answer 51

The cervical and thoracic portions of the neural tube form by the classical folding of the neural plate. Abnormal dorsal folding could result in the formation of two adjacent neural tubes. Normally the lateral edges of the neural plate progress dorsomedially to meet and fuse on the median plane, closing the neural tube. If the two approaching edges of the folds do not fuse but turn ventrally and continue to grow ventrally adjacent to each other, they will meet and fuse with the neuroectoderm of the plate ventrally. This will produce two adjacent neural tubes enclosed in a single covering of meninges.