Anatomy and Physiology

The anatomy associated with cerebrospinal fluid flow within the brain is depicted in Figure 37-1. Cerebrospinal fluid is produced by two major processes. One process involves cerebrospinal fluid production by the choroid plexuses located in the lateral, third, and fourth ventricles. This is an energy-dependent process requiring the enzyme carbonic anhydrase. The remainder of cerebrospinal fluid production occurs via cellular elements; this fluid travels via extracellular pathways. Production of cerebrospinal fluid is constant and is unaffected by intracranial pressure. Most cerebrospinal fluid flows through the ventricular system to reach the subarachnoid space. The pathway starts in the lateral ventricles; it then goes into the third ventricle (via interventricular foramina), through the mesencephalic aqueduct, and into the fourth ventricle and reaches the subarachnoid space by flowing through the lateral apertures of the fourth ventricle. Cerebrospinal fluid is absorbed by the arachnoid villi, as well as by capillaries throughout the brain and spinal cord.

Physical Examination

Common physical characteristics of hydrocephalic patients include a large, dome-shaped head, open fontanelles or larger calvarial defects, and bilateral ventrolateral strabismus (Figure 37-2). The strabismus may be due to orbital skull malformations, rather than to vestibular dysfunction, and has been referred to as the setting sun sign. It is important to realize that small-breed dogs often have persistent fontanelles with no attendant clinical signs of neurologic dysfunction. Clinical signs of neurologic dysfunction associated with hydrocephalus usually reflect a forebrain disorder and include obtunded mentation, behavior abnormalities, circling, pacing, restlessness, and seizure activity.^33,38,50 In comparison with behavioral abnormalities and abnormal mentation, seizures are not commonly associated with congenital hydrocephalus. Some hydrocephalic patients may also exhibit vestibular and/or cerebellar dysfunction. Concurrent congenital abnormalities of the brain (e.g., intracranial arachnoid cyst, Dandy-Walker syndrome, Chiari-like malformation) occasionally occur in hydrocephalic dogs, accounting for cerebellovestibular dysfunction.^8,19,48

Diagnosis

The diagnosis of congenital hydrocephalus is based on a combination of characteristic clinical features, demonstration of ventriculomegaly, and the absence of other causes of encephalopathy. Ultrasonography (through a fontanelles or calvarial defects) and advanced imaging (Figure 37-3) have largely supplanted more invasive methods of documenting ventriculomegaly (e.g., contrast ventriculography). A vast majority of congenital hydrocephalus cases primarily involve dilation of the lateral ventricles (internal hydrocephalus). Occasionally, the bulk of the cerebrospinal fluid accumulation is found within the subarachnoid space overlying the brain (external hydrocephalus), with the brain being displaced axially (Figure 37-4).¹⁴ Electroencephalography has been used historically to assist in the diagnosis of congenital hydrocephalus, with affected patients typically exhibiting slow-frequency, high-voltage activity. However, these electroencephalographic findings are relatively nonspecific and seldom contribute to the diagnosis of congenital hydrocephalus.¹⁹

Medical Treatment

The prognosis for dogs and cats with congenital hydrocephalus is variable but is generally guarded. Medical treatment of congenital hydrocephalus is aimed at reduction of cerebrospinal fluid production. Oral prednisone, at an initial dosage of 0.25 to 0.50 mg/kg q12h, may decrease cerebrospinal fluid production. Prednisone should be reduced over several weeks to the lowest possible dosage required to control clinical signs. Furosemide, a loop diuretic, decreases cerebrospinal fluid production via inhibition of the sodium/potassium/chloride co-transport system. The recommended dosage range is 0.5 to 4.0 mg/kg body weight PO q12-24h. The diuretic acetazolamide is a carbonic anhydrase inhibitor (carbonic anhydrase is a necessary enzyme for cerebrospinal fluid production) and is typically dosed at 10 mg/kg body weight PO q6-8h. Omeprazole, a proton pump inhibitor, has been shown to decrease cerebrospinal fluid production in dogs by 26%. The oral dose for dogs is 10 mg (for dogs weighing less than 20 kg) q24h and 20 mg (for dogs weighing more than 20 kg) q24h. For all of these drugs, it is recommended that the dose be tapered to the lowest dose needed to control clinical signs of disease to avoid serious side effects. Anticonvulsant drugs are administered if the patient is experiencing seizure activity.8,18,19,48

Medical therapy for congenital hydrocephalus may provide some level of disease palliation in mild cases, but often fails in the long term. When treatment decisions are made, the potential side effects of long-term corticosteroid and/or diuretic therapy should be considered, along with the questionable efficacy of medical therapy for congenital hydrocephalus. Electrolyte depletion (especially potassium) and dehydration are concerns when diuretics are used for prolonged periods, particularly when combined with corticosteroids.

Surgical Treatment

Medical therapy may be effective in some patients, whereas others require surgical shunting procedures for long-term control of clinical signs. The goal of surgical treatment for hydrocephalus is to continually divert excessive cerebrospinal fluid from the ventricles of the brain to the peritoneal cavity. The prognosis for sustained clinical improvement in neurologic status after surgical shunting procedures varies in the literature from 50% to 90% for dogs.8,19 In the authors’ experience, the success rate is approximately 75% to 80%. Potential postoperative surgical shunt complications in dogs and cats include shunt obstruction, shunt dislodgment, mechanical damage to the shunt, and shunt infection.

Several shunts are available, all of which have the same basic design, including a ventricular catheter for placement into the lateral ventricle, a one-way valve, and a peritoneal catheter for placement into the peritoneal cavity (Figure 37-5). The shunts used in veterinary medicine are passive shunts that flow at a predetermined pressure level based on the valve. After the patient has been anesthetized, the head, lateral thorax, and entire lateral abdominal region are shaved and prepared aseptically for surgery with the patient in lateral recumbency (Figure 37-6). For very small patients (e.g., Chihuahuas, Yorkshire Terriers), the authors make two incisions—one curvilinear incision over the caudodorsal region of the parietal bone, and one vertical incision caudal to the last rib. For larger patients (e.g., English Bulldogs), an additional incision approximately midway between these two incisions on the lateral thorax is often required. A high-speed drill is used to drill two holes in the parietal bone: one for the ventricular catheter and one for an anchoring suture. The hole for the ventricular catheter is created to be slightly wider than the diameter of the catheter. Once the dura is exposed, it is incised with a #11 scalpel blade. The thin rim of brain parenchyma beneath the dura/arachnoid is also incised; the authors remove this small circular area of parenchyma with suction so that the ventricular catheter can be placed reliably in the lateral ventricle. The ventricular catheter is guided rostrally into the dorsal aspect of the lateral ventricle. After insertion of the ventricular catheter into the lateral ventricle, it is secured with a Chinese fingertrap suture via the anchor hole using 3-0 or 4-0 polypropylene (Figure 37-7). A small piece of Gelfoam is placed over the entry site. A grid approach to the peritoneal cavity is used, and the peritoneal catheter is tunneled from the incision over the parietal bone caudally to the incision over the lateral abdomen using a long Carmalt or Doyen forceps. After placement of the peritoneal catheter into the peritoneal cavity, it is secured to both the last rib and the abdominal musculature.^8,48 Postoperative radiographs (Figure 37-8) or computed tomography (CT) scans (Figure 37-9) can be used to assess the accuracy of shunt placement postoperatively.

Physical Examination

A wide age range at clinical presentation has been noted for dogs with intracranial arachnoid cyst (2 months to 10 years), with an approximate average age of 4 years. The most common clinical signs seen with intracranial arachnoid cyst are forebrain (including seizure activity) and/or central vestibular (cerebellovestibular) dysfunction. Dogs may also present with a primary clinical sign of neck pain.17,19

Diagnosis

The diagnosis of intracranial arachnoid cyst is typically made via CT or (preferably) magnetic resonance imaging (MRI). Intracranial arachnoid cysts can also be visualized with the use of ultrasound imaging (via foramen magnum, temporal window, and/or persistent bregmatic fontanelle), especially in younger dogs. The characteristic appearance of intracranial arachnoid cyst is a large, well-demarcated, fluid-filled structure, isointense with cerebrospinal fluid spaces and located between the caudal cerebrum and the rostral cerebellum (Figure 37-10). Because intracranial arachnoid cyst may be an incidental finding, it is important to rule out concurrent inflammatory disease (i.e., via cerebrospinal fluid examination). In the authors’ opinion, it is often difficult or impossible to discern whether intracranial arachnoid cyst in the presence of another brain disorder is purely an incidental finding. Because the presence of a large, fluid-filled structure within the cranial vault likely decreases intracranial compliance, some intracranial arachnoid cysts may be contributory, rather than an incidental finding. In one study, it was found that the degree of brain compression caused by the intracranial arachnoid cyst as measured on MRI was predictive of whether or not the patient would display clinical signs of dysfunction.²⁶ Because this disorder is believed to represent a developmental abnormality of the intracranial ventricular cerebrospinal fluid system, it may occur concurrently with other fluid abnormalities (e.g., congenital hydrocephalus). The cyst may or may not communicate with the remainder of the ventricular system. When evidence of intracranial arachnoid cyst and another disease (e.g., granulomatous meningoencephalomyelitis [GME]) is found in the same patient, optimal response to treatment may entail treating both conditions.

Treatment

Medical treatment for intracranial arachnoid cyst is identical to that described for congenital hydrocephalus (e.g., corticosteroids, diuretics, anticonvulsants if indicated). Dogs with intracranial arachnoid cyst tend to respond initially to medical therapy, but the response is often temporary. Surgical management of intracranial arachnoid cyst in people is typically via cyst fenestration or cystoperitoneal shunt placement. Both procedures have been reported in dogs with intracranial arachnoid cyst.15,20,33,49,50 Five cases of fenestration have been reported in which intracranial arachnoid cyst was considered the primary disease. The extent of the fenestration performed varies among surgeons. The amount of cyst wall removed is somewhat dependent on the extent of the craniotomy performed. Three fenestration cases were reimaged post surgery; two of the three dogs had evidence of cyst persistence on MRI; however, only one of these two dogs required reoperation.^33,49,50

The authors have reported successful cystoperitoneal shunting of dogs with intracranial arachnoid cyst.¹⁵ Cystoperitoneal shunting has also been successfully used in the treatment of a feline case of intracranial arachnoid cyst.²⁵ With the exception of placement of the end of the shunt into the intracranial arachnoid cyst, the procedure is identical to that described for congenital hydrocephalus. For placement of the catheter in intracranial arachnoid cyst cases, the authors perform a wide caudolateral craniotomy, creating both rostrotentorial and suboccipital skull defects. The transverse sinus between these two defects is sacrificed and occluded with bone wax. As with congenital hydrocephalus, postoperative imaging is performed to assess the accuracy of shunt placement (Figure 37-11). The success rate for surgical management of intracranial arachnoid cyst appears to be high in human beings⁷ and dogs, and whether fenestration or cystoperitoneal shunting is the preferred procedure for both species remains controversial.