Neuro-Ophthalmology & Neuro-Otology
Isolated sixth nerve palsy
Nov. 28, 2023
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The mainstay treatment for hydrocephalus remains the placement of a ventricular shunt with a terminus most commonly in the peritoneum, atrium, or pleural cavity. Although this is an effective treatment, there are numerous complications that can occur in the operative placement and management of neurosurgical shunts. The latest thoughts on the placement and management of shunts are highlighted in this overview. The importance of minimizing radiation exposure in the evaluation of the shunted patient is discussed, including the use of rapid sequence MRI scans for surveillance and emergent evaluation. The potential impact of tablet computers on programmable shunts is also reviewed.
• Shunt malfunction is a life-threatening condition and must be treated urgently.
• It is imperative when evaluating for a potential shunt malfunction, the “baseline” image is known to have been taken at a time of shunt function to avoid the misinterpretation of “no change” as the incorrect conclusion of “no evidence of shunt malfunction.”
• Image gently in shunted patients as their lifetime exposure to unnecessary radiation puts them at increased risk of developing leukemia, brain tumors, and other cancers.
• Attention to detail in the operating room (reduced traffic, preoperative antibiotics, patient preparation, and efficient operating) is key to reducing shunt infections.
Hydrocephalus is one of the most commonly encountered pediatric neurosurgical diseases.
The first implantable shunt was placed in 1952 by Nulsen and Spitz, and since then shunt placement has remained the most common treatment of hydrocephalus. The peritoneum is the most common location for placement of the distal catheter; however, the distal end can also be placed in the right atrium, pleural cavity, or gallbladder. Since the 1960s, the use of modern shunts has become commonplace. As a mechanical device, a shunt can be expected to fail at a rate of approximately 10% a year. Shunt malfunction is a life-threatening condition, and any neurosurgical center serving a large population performs a significant number of ventriculoperitoneal shunt revisions. This procedure, most frequently performed in children by neurosurgeons, remains one of the most failure-prone medical devices, with failure rates of 30% to 40% within the first year after implantation and 50% within 2 years of implantation (14). More importantly, the possibility of malfunction impacts significantly on the quality of life of patients with shunted hydrocephalus. Because shunt failure is often progressive and symptomatic, recognition of early signs of shunt failure is imperative for earlier operative exploration. This article concentrates on the nature of shunt malfunction, its diagnosis, and management.
When assessing shunt malfunctions, it is critical that the comparison images are known to have been taken in the setting of a functioning shunt. If the comparison image is from a time of shunt malfunction, the interpretation may be that there is no change in the ventricular size and, therefore, no shunt malfunction. Shunt failures can occur due to obstruction, disconnection, migration, fracture, ascites, abdominal pseudocyst, infections, and even constipation (24). Occlusion of the ventricular catheter is the cause of approximately half of mechanical obstructions of shunts (18). The most common type of mechanical obstruction is blockage of the ventricular catheter. This is typically thought to occur by the growth of choroid plexus into the ventricular catheter, although more recent evidence suggests that the type of cellular infiltrate is dependent on the timing of shunt obstruction (33). For instances of choroid plexus obstruction, there is an increased risk of hemorrhage during a shunt revision as the removal of the ventricular catheter can cause bleeding from the highly vascular choroid plexus, resulting in intraventricular and parenchymal hemorrhage. This hemorrhage increases the risk of subsequent shunt malfunction and can also result in more severe neurologic injury. The shunt system can occlude anywhere along its course, including the proximal catheter, reservoirs, the valve, and the distal tubing.
Disconnection of a shunt rarely occurs shortly after placement of a new shunt unless there was a technical error, such as not attaching the catheter effectively. In some shunt systems, the ventricular catheter snaps on to a reservoir, and if this is not done adequately the ventricular catheter can disconnect from the reservoir, resulting in malfunction.
Late fractures often result from chronic mechanical stress associated with patient growth and movement. Using spectroscopy, conventional histology, and scanning electron microscope, Yamamoto and colleagues found that mineral deposits in degraded shunt tubing consisted of hydroxyapatite (39). The most extensive calcification was seen in the neck, presumably due to the increased mechanical stress in this region.
Symptomatic complaints of shunt calcification may include pain in the neck or chest wall region, limitation of neck movement due to tethering of the shunt, and cutaneous manifestations such as irritation of the skin overlying the shunt (32). Most surgeons will revise a shunt when a fracture or disconnection is demonstrated because the fibrous tract along a fractured shunt can drain CSF for an indeterminate length of time but will ultimately fail. Deposition of sodium chloride and other crystals involving the valve has also been demonstrated as a contributing factor in shunt malfunction (34).
Distal obstruction is far less common than proximal obstruction, and the occurrence is more likely to be delayed (14). In particular, premature infants with a high protein count in their CSF may present with a distal obstruction without infection. In premature infants with certain types of shunts, the obstruction can often be cleared by irrigating the shunt, obviating the need for an operation. Distal obstructions can be associated with an infection. When distal obstruction is suspected, an abdominal ultrasound study can be useful to look for a pseudocyst, ascites, or constipation. CSF ascites can be due to infection, shunt disseminated metastases, or poor absorption of CSF into the peritoneum. Infection must be ruled out in cases of ascites. It is more common for a small pseudocyst to be infected than a large one. Treatment of infected pseudocysts involves administration of antibiotics and externalization of the shunt for a brief period of time (11).
The evaluation of a shunt malfunction is first and foremost a complete history and physical examination including a funduscopic examination. The Image Gently campaign has emphasized the importance of reconsidering the need for radiation as well as altering the type of radiation performed, particularly in children (13). Children with ventriculoperitoneal shunts are at risk of developing malignancies at a later date if the exposure to diagnostic radiation is not carefully considered with every evaluation (26). There are studies to support not performing a routine shunt series in every child with a possible shunt malfunction unless there is clinical evidence to suggest a break. In a retrospective longitudinal cohort analysis performed by Antonucci and colleagues, most CT scans obtained in the cohort were not followed by surgical intervention (03). If available, rapid sequence MRI can be used to assess the ventricular size. The advantage of rsMRI is that there is no radiation and it does not require sedation as it can be performed in less than 1 minute. If that modality is not available, consideration of a low-dose CT would be another option for minimizing radiation exposure. In the child with an open fontanelle, ultrasound is effective. In any event, careful consideration should be given to ordering diagnostic radiation in these patients. The routine of ordering a shunt series and a CT scan on all cases, often even prior to notifying the neurosurgeon, must be stopped.
One publication describes the use of cerebral regional oxygen saturation monitoring in the assessment of pediatric shunt patients. This technology has been used most commonly for the intraoperative monitoring of children undergoing repair of congenital heart disease. The advantage is that it is a noninvasive, easy-to-apply technology that gives instantaneous readings. The researchers found that in the setting of malfunctioning shunts, there was an asymmetrical hemispheric cerebral regional oxygen saturation, and greater changes in regional oxygen saturation occur for distal versus proximal shunt malfunction (01). Although this methodology requires more study, it potentially provides another noninvasive and safe technique for the evaluation of shunt malfunction.
In shunted adult patients with a diagnosis of normal pressure hydrocephalus, a technique of using noninvasive auditory tests (as currently used for universal neonatal hearing screenings) for the diagnosis of cerebrospinal fluid shunt malfunction was described by Sakka and colleagues (31). Additionally, ultrasound-based measurements to characterize the movement of flow through the shunt-valve interface in response to CSF flow has been shown to be another potential mode of noninvasive testing for shunt malfunction (04).
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