Any abnormality that offsets the delicate intracranial pressure-volume balance can lead to symptoms. Altered CSF dynamics due to fluid drainage from functioning shunts affects brain function in many ways, sometimes causing symptoms suggestive of intermittent malfunction. Lowered intracranial pressure changes CSF output resistance and in some situations, over-drainage may cause collapse of the ventricles with associated symptoms of shunt malfunction.

Implantation of a shunt device establishes a more physiologic CSF circulation phenomenon, where resistance to CSF outflow is affected, end-plateau pressure and baseline pressures are altered, and compensatory reserve is improved [23]. Shunting often drains large volumes of fluid due to decreased resistance, leading to a temporary over-drainage phenomenon in some cases. Naturally, this impact on pressure-volume relationship is an important factor in symptom causation.

CSF leak leads to intracranial hypotension and diminished brain turgor such as NPH due to decreased spinal fluid volume rather than decreased pressure [6, 26–28]. Major issues associated with shunting in patients with CSF leak include decreased brain turgor or further worsening low ICP symptoms [9, 8, 22, 25].

The dural opening does not completely seal in some cases after shunting, maintaining a potential avenue for organisms or air entry, increasing the risk of subsequent infection or tension pneumocephalus [24]. Studies suggest that persistent CSF leak after shunt surgery has a very high incidence of infection, which is a major concern for any neurosurgeon who must diligently observe for spinal fluid leakage at the surgical site [13, 16]. Often, aggressive measures to revise/replace the shunt mechanism are warranted since a negative pressure situation may allow air to enter the cranial cavity [11]. In cases of traumatic CSF leak, the dual tear often causes a ball valve phenomenon, which becomes an avenue of air entry into the cranium.

Patients undergoing tethered cord release may develop persistent CSF leak, leading to shunt malfunction [32]. Although the exact mechanism is unknown, it is possible that a delicate balance in CSF dynamics is upset, or blood products in the spinal fluid occlude a functioning shunt. Another possibility is collapse or coaptation of the ventricular walls due to CSF loss during surgery, leading to temporary shunt malfunction. It is possible that a vicious cycle of persistent CSF leak leads to shunt malfunction or an improperly opening shunt causes increased pressure causing a persistent leak at the site of tethered cord release.

Most shunts function as like on-off mechanisms with altering flow rates and intracranial pressures during the day. Although evidence is lacking on how CSF dynamics are affected after shunting, it is interesting and perhaps beneficial to identify its role during hydrocephalus management. Typically, the opening and closing at various pressures does not fully replicate physiologic responses, sometimes mimicking symptoms of intermittent malfunction (Czosnyka [5, 23]). Although improvements in spinal fluid dynamics may not be observed in imaging or clinical observations, resistance to CSF outflow (RCSF) decreases with a functioning shunt, which improves the vasogenic components of the ICP waveform (such as the respiratory, pulse, and B waves) [34]. It is unclear if antisiphon devices function as expected to completely correct this phenomenon, although recent advances inflow-regulated valve mechanisms have decreased the overall revision rates.

It is known that abnormal intra-abdominal pressures caused by constipation may lead to transient shunt malfunction. CSF dynamics affecting CSF flow rates are altered in these situations as the rising intra-abdominal pressure offsets ventricle-to-abdomen pressure gradient [18]. Two pediatric cases were reported of temporary ventriculoperitoneal shunt malfunction with increased ICP. These children spontaneously improved after the constipation was resolved, and a similar description was previously offered by others [19].

Treatment of aqueductal stenosis by endoscopic third ventriculostomy versus VP shunt insertion is a frequent dilemma, and identification of certain parameters may facilitate the decision process. Oi and Di Rocco offered novel ideas regarding CSF dynamics explaining why endoscopic third ventriculostomy does not fully correct the symptoms in approximately 1/3 of infants, suggesting improperly developed “minor pathways” that maybe necessary in adulthood [20]. This “evolution theory in CSF dynamics” may suggest newer mechanisms of normal circulation and CSF absorption pathways within the cranial cavity. The authors proposed five stages of CSF dynamics during maturation, with the last two stages developing postnatally. The “minor pathway” of CSF absorption via the extra-arachnoid sites plays a significant role soon after birth until the first year of life, and the “major pathway” begins to take effect at approximately 6 months. This overlap between the two pathways may explain some of the reasons why our initial attempts at third ventriculostomy may not be as successful.

During the later stages of development, increased ICP due to shunt malfunction may require these “minor” (extra-arachnoid) pathways, which in some situations are no longer available in an acute situation. However, certain patients maintain these “minor” CSF absorption pathways that can be recruited as necessary during periods of intermittent shunt malfunction or as a buffer during periods of excess need [20]. Identification of such factors may offer a better understanding to guide attempts at correcting abnormal CSF dynamics.

Patients with CSF fistulae have altered intracranial fluid dynamics and the approaches to correct them require individual consideration. For example, some patients undergo shunt insertion as a primary treatment rather than a procedure to control the CSF leak; however, a direct attack at the site of CSF leak is more appropriate than CSF diversion (which again affects overall pressure-volume balance). Local treatment of CSF fistula may be accomplished by various methods, but symptoms often persist in patients with increased intracranial hypertension, if the fluid leakage occurred as a vent to diminish the pressure.

Persistent CSF fistulae with functioning shunt systems may develop entry of air or organisms into the cranial cavity due to the relative negative pressure. Postoperative pneumocephalus sometimes occurs after shunt insertion in patients with an occult CSF leak, and management of patients with hydrocephalus is more difficult if associated with infection, pneumocephalus, or persistent symptoms of low intracranial pressure [16].