Cerebrospinal Fluid

The cerebrospinal fluid (CSF) is ultrafiltrate plasma that protects, supports, and nourishes the central nervous system (CNS). It originates from the choroid plexus of the lateral, third, and fourth ventricles directly from the brain by way of the ependymal lining of the ventricular system and pial-glial membrane covering the external brain surface as well as from the blood vessels of the pia and arachnoid.

It is the most representative extra cellular fluid of the CNS and flows over the lateral ventricles through Monro's foramen until the third ventricle. From there, it flows through the midbrain aqueduct into the fourth ventricle and reaches the caudal fossa. In the caudal fossa, it follows directly to the subarachnoid layers of the skull and spinal cord where it is passively absorbed in the arachnoid villi.

The skull is a rigid structure with a fixed volume which contains three components: brain tissue, vascular intracranial volume and intracranial CSF. Thus, in a normal situation, the CSF and vascular volume vary inversely to keep the intracranial pressure (ICP) within the normal limits.

Another function of the CSF is the excretion of toxic products originated from the brain metabolism and transport of biologically active substances, such as releasing hormones that are produced in the hypothalamus and discharged into the third ventricle and then spread to the whole CNS. The CSF canal so serves as a carrier of opioids and other neuroactive substances derived from the systemic circulation.

In cats, the production of CSF occurs at a rate of 0.017 ml/min, about 25 ml/day and in dogs at a rate of 0,047 ml/minute, approximately 68 ml/day. In both species the ICP average is 8.0 mm Hg (108.7 mm/H₂O).

The CSF production is independent of hydrostatic pressure within the ventricular system but is influenced by the osmotic pressure of the blood. Because of this, the use of osmotic agents as mannitol and corticosteroids can reduce drastically the production rate of CSF.

Normally the production and flow of CSF is relatively slow and the brain's visco-elastic properties ensure that there is no measurable pressure difference within the ventricular system. The balance between the rate of formation and the rate of absorption determines the volume of CSF within the skull. The CSF absorption depends on the pressure difference across the arachnoid villi which act as a valve system to keep intracranial pressure in the normal range. When intracranial pressure decreases below 5 mm Hg, there is no CSF absorption. When this pressure increases, absorption increases in proportion to the pressure within the ventricles.

Hydrocephalus

The term hydrocephalus implies the presence of an excessive accumulation of CSF within the cranial cavity with dilation of the ventricular system. Hydrocephalus occurs when there is resistance in the CSF passage that causes a higher pressure gradient between CSF proximal and distal to the obstruction and, moreover, alternate pathways of CSF absorption are unable to reduce the increased CSF volume within the ventricles and return it to normal range. It has been reported that the disruption of CSF absorption is connected with the duration of hydrocephalus.

Depending on the location of the accumulated CSF, hydrocephalus is classified further as internal, in which the ventricle's enlargements are apparent, or external, with an enlarged subarachnoid space. The increased volume of CSF in hydrocephalus is rather caused by a decreased resorption (secondary to under development of arachnoid villi or inflammatory processes) than increased production of CSF (apparent in choroid plexus tumors).

According to aetiology, hydrocephalus can be classified as congenital or acquired. In veterinary patients, congenital hydrocephalus is more common than acquired. Causes of congenital hydrocephalus are reported as follows: fusion of rostral colliculi with secondary mesencephalic aqueduct stenosis; prenatal inflammation with lesions of the ependymal surface; and malformations of the cerebellum, as in caudal occipital malformation syndrome. In the latter two causes, secondary cerebellar herniation and syringohydromyelia may occur and obstruct CSF drainage.

In acquired hydrocephalus, the increased volume of CSF is rather caused by a decreased resorption (secondary to inflammatory processes of arachnoid villi) than increased production of CSF (apparent in tumors of choroid plexus). Hydrocephalus ex vacuum is an acquired alteration featured by a decrease in parenchyma that occurs following trauma, infarction, necrosis or degenerative process.

In obstructive no-communicating hydrocephalus occurs the blockage of CSF flow within the ventricular system or at the outflow through the lateral apertures that prevents the communication between ventricular system and the subarachnoid space. This situation can occur in congenital (Chiari-like malformation, Dandy-Walker syndrome) or acquired (encephalitis, intracranial tumors) diseases that compress the mesencephalic aqueduct and the caudal fossa.

Clinical Signs

Clinical signs of hydrocephalus reflect the anatomic level of disease involvement. Forebrain, vestibular, or cerebellar signs are most common. A ventral or lateral strabismus or both, alterations in cognition, dementia, circling gait, paresis, and seizures are common in dogs with hydrocephalus. Congenital hydrocephalus is typically recognized in patients 2–3 months of age. Animals with congenital hydrocephalus are often smaller than their litter mates and show calvarium distortion according to the rate of fluid accumulation and stage of ossification of cranial sutures.

It has been reported that the intensity of clinical signs is depended on the increased intracranial pressure. In obstructive hypertensive hydrocephalus clinical signs are far more apparent than in normotensive hydrocephalus. Neurological deficits may progress over time, remain static or improve after a while.

Diagnosis

The diagnosis of hydrocephalus is aided by information obtained from a variety of imaging. Ultrasound examination can be used to diagnose hydrocephalus when a fontanel is present providing an 'acoustic window' as ultrasound waves do not adequately penetrate the skull. In small dogs, the ventricular enlargement (ventricle to brain (VB) ratio) can be correlated to severity of clinical signs using a transcranial Doppler ultrasonography. Dogs with VB ratio >60% presents more chances to develop neurological signs.

Usually, these dogs show greater resistance in the basilar artery that can be correlated to elevation in ICP.

Computed tomography (CT), as a non-invasive intracranial imaging modality, is often useful in defining ventricular size. As CSF is less attenuating than brain parenchyma, the ventricular system is usually readily identifiable on images due to its relative blackness in comparison to parenchyma. CT evaluation also affords the ability to examine the majority of the ventricular system as well as additional intracranial structures.

Magnetic resonance imaging (MRI) has revolutionized the diagnosis of intracranial malformations in small animal practice. Low-field MRI predominates in investigating hydrocephalus because it has the best diagnostic volume, is a sensitive method and produces high quality images with minimum artefacts.

MRI also affords evaluation of the ventricular system. This modality provides superior neural parenchyma resolution and is especially useful for evaluation of the infratentorial structures. It is now recognized that many toy breed dogs predisposed to hydrocephalus also have infratentorial anomalies, such as Chiari-like malformations that potentially complicate their management and are detected only on MRI.

References

1.  Terlizzi R, Platt S. The function, composition and analysis of cerebrospinal fluid in companion animals: part I - function and composition. Vet J. 2006;172:422–431.

2.  Coates JR, Axlund TW, Dewey CW. Hydrocephalus in dogs and cats. Compend Contin Educ. 2006;28:136–147.

3.  Przyborowska P et al. Hydrocephalus in dogs: a review. Veterinarni Medicina. 2013;58(2):73–80.

4.  Saito M et al. The relationship between basilar artery resistive index, degree of ventriculomegaly and clinical signs in hydrocephalic dogs. Vet Radiol Ultras. 2003;44:687–694.

5.  Carvalho CF et al. Transcranial duplex Doppler ultrasonography in dogs with hydrocephalus. Arq Bras Med Vet Zootec. 2010;62(1):54–63.