Fixed Differential Pressure Valves

Fixed differential pressure valves represent the simplest shunt valve design and were the first introduced into wide use practice more than 70 years ago. The mechanism of this type of valve is based on the difference in pressure between the 2 ends of the shunt; it is this pressure differential that is responsible for the shunt flow. These valves are still widely used in hydrocephalus surgery worldwide and the market offers various designs and models alternatives. Their simple mechanism results in significantly cheaper costs compared with adjustable valves; this is one of the main reasons they are more widely used in developing countries. These valves are generally categorized as low, medium, or high differential pressure depending on the level of resistance applied, ranging from the lowest to the highest. A major drawback of this valve type is that clinicians must predict the patient’s CSF diversion needs prior to implantation, which can be challenging due to the significant variability among different conditions, as well as amongst patients with similar diagnoses. Furthermore, a patient need for drainage could change according to clinical status (eg, bed bound patient becoming more mobile) or advancing age. As a result, many patients with fixed differential pressure valves experience symptoms of under-drainage or over-drainage, often necessitating revision surgeries to replace the valve.

Antisiphon Devices

Another important limitation of simple fixed pressure valves is the siphoning effect. This phenomenon occurs when a patient changes from a supine to an upright posture, leading to excessive CSF drainage through the shunt due to gravitational effects. Such over-drainage can result in significant symptoms and increases the risk of developing subdural hematomas. In order to reduce the risk of siphoning, antisiphon devices were developed. There are 2 main categories of antisiphon devices, flow regulated and gravitational. Flow regulated examples include Siphonguard (Integra) and the Delta valve (Medtronic). In gravitational antisiphon devices, resistance is dependent on the patient’s position, and these devices are designed for greater accuracy. Examples include Shuntassistant (Miethke) and the SiphonX (Sophysa). Fig. 1 displays the structure of a Shuntassistant (Miethke), which features a tantalum and sapphire ball system that activates only when the patient assumes an upright posture ( Fig. 1 ). Anti-siphon devices can often be integrated with fixed pressure valves; this is the case for the GAV (Miethke) and Codman-Hakim Micro Precision Valve with Siphonguard (Integra).

Shuntassistant (Miethke) antisiphon device in upright and supine posture. This valve is characterised by a tantalum and sapphire ball system that activates only when the patient assumes an upright posture.

( Image reproduced with permission from Miethke.)

Flow Regulated Valves

The Orbis Sigma valve (Integra) is designed with the aim of maintaining constant flow CSF drainage at specific differential pressure intervals. Previous case-control studies have suggested that it may reduce the risk of over-drainage. However, 1 drawback of flow-regulated valves is the potential for under-drainage when intracranial pressure is significantly elevated.

Adjustable (Programmable) Valves

These valves were developed to allow change of opening pressure of the valve without need for a surgical intervention. The term adjustable is more accurate, as there are no truly programmable valves available yet. The first adjustable differential pressure valve introduced to the market was the Sophy valve (Sophysa). Codman Hakim programmable valve (Integra) and Strata (Medtronic) are still widely use. A major limitation of these valves was the fact that they are not MRI stable meaning that the opening pressure of the valve could change when exposed to strong magnet. Therefore, these valves require assessment and readjustment each time the patients undergo MRI imaging, inevitably increasing the demand for health care providers.

More recent generations of valves include designs that prevent inadvertent valve adjustments, examples are Polaris (Sophysa), ProGAV (Miethke), and Codman Certas plus (Integra) ( Fig. 2 ).

Examples of MRI-stable shunt valves: ( A ) Codman Certas (Integra) valve; ( B ) Polaris (Sophysa) valve.

([A] Permission granted by Integra LifeSciences Corporation, Princeton, New Jersey, USA. [B] With permission from Sophysa.)

Most adjustable valves are available in combination with antisiphon and gravitational mechanisms. Notable examples include the Codman Hakim or Certas plus with Siphonguard (Integra), the Strata II (Medtronic), and the ProGAV with Shuntassisstant (Miethke). These valve systems allow for adjustable differential pressure settings and introduce additional resistance when the patient is in an upright position.

Miethke created the first adjustable antisiphon gravitational valve, initially represented by the proSA valve and more recently updated in the M.blue valve. M.blue plus (Miethke) is a combination of proGAV 2 adjustable differential pressure valve and M.blue. This gravitational unit addresses the limitations of committing to a specific anti-siphon setting prior to implantation.

Adjustable valves are significantly more expensive than fixed pressure ones. There is an ongoing debate whether the use of adjustable valves has real benefit in reducing likelihood of shunt failure or other complications, including overdrainage and headache. There is no high-quality trial evidence to prove superiority of adjustable valves over simpler fixed pressure ones, as found by a Cochrane review on this subject published in 2020. Similarly, a systematic review and meta-analysis comparing fixed and adjustable pressure valves in normal pressure hydrocephalus found insufficient evidence to support the superiority of adjustable valves.

Hydrocephalus practice and patients’ circumstances are complex and variable. Furthermore, different shunt valves have different characteristics that might influence the outcome of intervention. While robust evidence supporting the superiority of adjustable valves is lacking in the literature, the UK shunt registry shows that in adults of all ages, but especially aged 70 and over, there is a significant advantage in using an adjustable valves ( P <.001) in terms of valve revision rate reduction ( Fig. 3 ). There is a notable trend in clinical practice toward increased use of adjustable valves, particularly in centers specializing in hydrocephalus and CSF dynamics disorders. This shift likely stems from the clear advantage of being able to tailor CSF drainage to meet patients’ evolving needs, an option not available with traditional fixed pressure valves.

Cumulative valve revision rates: Fixed Pressure versus Adjustable valves.

( Reproduced with permission from Prof. John Pickard, Dr Alexis Joannides, UK Shunt Registry Draft Report 2017. https://www.sbns.org.uk/index.php/audit/shunt-registry/ . ©2017 John Douglas Pickard, Hugh Richards and Alexis Joannides.)

There are subtle differences between various adjustable shunt valves currently available in the market. The Certas plus valve and the Sophysa polaris valve can both be adjusted with minimal skin contact. Although the opening pressure could be set to relatively low level in both, it cannot be adjusted to zero. The Certas plus allows a virtual block (level of 8), where the shunt is unlikely to be draining, unless the intracranial pressure (ICP) achieves a very high. This is a useful option in certain indications. Certas valve is often used in combination with Siphonguard. As a flow-limiting device, the valve has no orientation restrictions, whereas gravitational valves must be positioned perpendicularly for optimal functioning and to avoid inappropriate activation of the gravitational unit. Another disadvantage of gravitational units arises in patients with reduced consciousness levels when lying supine with neck flexion, such as when using a large pillow. In these cases, patients may develop clinical and radiologic signs of under-drainage syndrome, as neck flexion can activate the gravitational antisiphon component while supine. Ensuring proper head and trunk alignment through attentive nursing care can help mitigate this issue. Alternatively, an adjustable gravitational antisiphon device (such as the M.blue) can be set to a low level initially, with gradual increases as the patient improves and is able to sit upright.

The Miethke ProGAV valve allows change of opening pressure between 0 and 20 cm H2O. This ability to allow drainage at very low pressures (setting of zero) is particularly useful in low pressure hydrocephalus patients, when used with a gravitational antisiphon system like Shuntassistant or M.blue valve. This has been reported to result in more optimal clinical improvement without increasing risk of over drainage and subdural hematomas. ^, Furthermore, in certain patients, the use of adjustable antisiphon gravitational devices may lead to improved clinical outcomes by deliberately allowing a degree of siphoning. This is particularly noticed in some normal pressure hydrocephalus patients who benefit from lower anti-gravity settings or elderly patients with hydrocephalus secondary to brain hemorrhage, during initial hospitalization period. The use of both proGAV and M.blue in M.blue plus combination allows a wide range of valve opening pressure choices, from being completely open to almost completely blocked. A disadvantage of the Miethke valves is the need to press on the skin with a special tool to adjust them, which can cause discomfort in the immediate postoperative period. However, newer generation valves (such as proGAV 2 and M.blue) are much easier to adjust compared to earlier versions (proGAV and proSA).

There is variability of patients need when it comes to appropriate adjustable valve opening pressure. This primarily depends on the diagnosis and age. For idiopathic intracranial hypertension, many hydrocephalus surgeons tend to start with high opening pressure. The rational is that cerebral ventricles are small, and excessive drainage would result in complete ventricles collapse and intermittent catheter blockage, hence resulting in worsening headaches. In addition, the response of ICP to valve settings adjustments in vivo can be difficult to predict and is often paradoxic , with ICP measured following shunt valve opening pressure increase going down and vice versa. The optimal opening pressure for low-pressure hydrocephalus remains a topic of debate, with 2 distinct schools of thought. Some surgeons advocate for a low opening pressure to achieve optimal clinical improvement, while others recommend starting with a higher pressure and only reducing it if there is no clinical progress. This approach aims to minimize the risk of overdrainage and subdural hematoma. For ventriculo-atrial shunts there is some evidence of paradoxic overdrainage in supine rather than upright position, due to a respiration-generated pressure gradient favoring venous outflow through the cerebral sinuses, and CSF outflow through the differential valve, with no added resistance from the additional gravitational valve.

Choosing the Right Valve for the Patient

When selecting a shunt valve for a patient, several critical factors must be considered. Patient-specific characteristics such as age, hydrocephalus type, height, weight, prior surgeries, and prognosis play an essential role in determining the optimal valve, particularly with respect to opening pressure range and anti-siphon mechanism.

Valve-related factors are also crucial, including the valve’s size, availability of adjustment kits, local expertise, MRI compatibility, potential MRI-induced artifacts, and overall cost. Notably, a simpler valve design does not guarantee better outcomes, and a higher cost does not necessarily reduce complication risks. For instance, Warf BC and colleagues examined the 1-year outcomes of hydrocephalus treatment in Ugandan children, comparing the inexpensive Chhabra shunt (priced at $35) commonly used in East Africa, with the more costly Codman-Hakim Micro Precision Valve ($650). The study found no significant difference in outcomes between the 2 shunt types.