The brain is the supporting actor of the drama, accounting for the occurrence of the clinical manifestations in symptomatic cases. Actually, when this actor is not able to perform, namely, in case of atrophy, ACCD is an exceptional event. The intrinsic role of the brain in the genesis of the complication here considered is further demonstrated by the rare occurrence of ACCD in patients shunted in adult age, although in such patients the gradient of the hydrostatic pressure, that is the main factor favoring overdrainage, is higher than in children. In other terms, a necessary factor for the establishment of an ACCD is a brain able to expand in order to compensate for an abnormal volumetric reduction of the ventricles, cisternal, and peripheral subarachnoid spaces. Such an expansion can be a real cerebral growth, as it occurs in the first years of life. More commonly, in subsequent ages, the main cause for the brain to occupy the space, made available by the reduced volume of the ventricles and subarachnoid spaces, is the turgor of the brain determined by the dilation of the cerebral veins associated to the chronically low intracranial pressure related to the continuous depletion of CSF by the extra-thecal CSF shunt device. In the long run, the phenomenon will progressively account for further volumetric reduction of the CSF space that is the disappearance of the peripheral subarachnoid spaces and slit-like ventricles, easily detected on the neuroimaging studies. The increased elastance of the brain, the dilation of the cerebral venous structures, and the volumetric reduction of the CSF space will then account for the reduced ability of the patients to compensate for transient increases in intracranial pressure and for the reduced possibility of the ventricular system to enlarge. Actually, the attention of scientists has been longtime focused on the last aspect, that is on the excessively small ventricular cavities apparently unable to enlarge even when the clinical manifestations of the patients seemed to indicate a CSF shunt malfunctioning. The most shared interpretation was that the chronic drainage of CSF would have led to rigid ventricular walls, thereby preventing the ventricles to dampen any increase in intraventricular volume. Based upon animal studies, it was also suggested that the long-standing presence of a ventricular catheter could have promoted subependymal gliosis, which also contributed to the rigidity of the ventricular walls [11]. However, ex vivo studies have documented that the degree of subependymal gliosis is no different in subjects with dilated or nondilated ventricles at shunt malfunction [12].

Thus, the noncompliant ventricles should be considered the effect of the dynamic unbalance created by the long-standing action of the shunt device rather than the cause of the clinical manifestations presumed to depend on an intermittent and transitory malfunctioning of the CSF shunt apparatus.

The role of a possibly impaired venous outflow in ACCD is still matter of discussion. Albeit in some cases overt stenosis of the main venous sinuses may be identified [13], a venous outflow impairment has been hypothesized and recently attributed to a condition defined as “capillary absorption laziness” [14]. At the moment, however, it is not possible to draw any definitive conclusion on the subject, as a venous outflow impairment in ACCD could be either the cause or the effect of the increased cerebral pressure, resulting in both cases in a modification of the viscoelastic properties of the brain and in its subsequent decreased ability to compensate for increases in intracranial volume [15].

Finally, it is interesting to note how the size of the ventricles has received so much attention, while so scant attention has been paid to the volume of the subarachnoid spaces. Although in SVS there is dissociation between the low intraventricular pressure and the high subarachnoid pressure, the volume of the subarachnoid spaces still allows some degree of brain compliance. On the other side, in case of ACCD the nearly complete obliteration of the cisternal and peripheral CSF space would prevent any physical possibility for the brain to increase its volume so resulting in a more severe pathological condition.

The third actor involved in the drama is the skull. As Hoffman observed in 1976, the placement of the CSF shunt may dramatically decrease the pressure exerted by the growing brain, thus arresting the growth of the skull. As a consequence, “when brain volume has increased to occupy the space provided by diminished ventricular size, the neurocranium must enlarge, but this may not readily occur after a period of arrested growth. The result is a state of cephalocranial disproportion as brain volume exceeds the available [intracranial] space” [16]. The phenomenon described by the author is what actually leads to a secondary “craniosynostosis.” It occurs mainly in infants and in the first months after the placement of a CSF shunting device. A more subtle “overdrainage” which persists for years and even the mere presence of the shunt also diminishes the pressure forces of the intracranial content which act on the calvarium and assure the physiological equilibrium between bone absorption and formation. Dampened CSF pulses and chronically low mean intracranial pressure result in excessive deposition of lamellar bone at the inner surface of the calvarium. The phenomenon represents a reactive change of the skull aimed at compensating the reduced volume of the intracranial content. In some patients, however, it results in the excessive reduction of the volume of the subarachnoid spaces which in normal condition assure to the brain the possibility to expand or, more in general, to accommodate the physiological volumetric changes related to the cerebrovascular dynamics. Thickened skull vault, prematurely fused cranial sutures, hyperpneumatosis of the air sinus cavities, and impacted posterior fossa constitute the radiological evidences of the effect of the long-standing intracranial fluid subtraction determined by the extra-thecal CSF shunt device.

This bone “compensation” phase can last several years without clinical symptoms. However, the brain compliance and the possibility of the cerebral ventricle to enlarge diminish progressively during such a phase until the reduced cranial volume and the concomitant changes in cerebrovascular compliance result in a symptomatic ACCD.

In conclusion, the interaction of three actors is essential for the drama to take place. However, the role of a single actor may be prevalent in a given subject so reminding what can happen when a single actor plays alone that is the skull in cases of osteopetrosis or craniosynostosis, the brain in case of pseudotumor cerebri, and the shunt in case of CSF overdrainage. All these conditions are characterized by CCD, the etiology of which is, however, easy to be recognized. Only the ACCD is multifactorial and difficult to be diagnosed in its early stages, as it requires years to fully establish.

When the role of one actor is prevalent different complications may occur before or during the establishment of a condition of CCD. For example, the mismatch between the volume of the brain and that of skull caused by an overdraining CSF shunt may be complicated by the occurrence of subdural hematomas. Two actors playing together may result in brain turgor and SVS (with working shunt) without any role of the skull, while the last actor may be particularly involved when an atrophic brain is not able to expand sufficiently. In the last occurrence, the action of the extra-thecal CSF shunt may induce over-riding and premature fusion of the cranial sutures with the development of microcrania, as described by Faulhauer and Schmitz [17]. Finally, the partial failure of the first actor, that is the CSF shunt device, while the two remaining actors still play, is clinically translated by the intermittent clinical manifestations of a partially obstructed ventricular catheter, the surgical revision of which usually results only in a relief of the symptoms (Fig. 17.1).

The clinical history is usually that of a subject with a hydrocephalus shunted before the age of 2–3 years, albeit anecdotal cases of ACCD following shunting of an endocranial arachnoid cyst have been described [18–20].

As headache is often the first complaint, the diagnostic algorithms aim at differentiating between headache independent from the shunt from headache due to shunt malfunction, SVS with working shunt, and ACCD [5]. In case of migraine, an appropriate medical treatment often solves the diagnostic dilemma. Similarly, headache disappears after CSF shunt surgical revision in case of shunt malfunction. This result is persistent in time, at least until the next shunt malfunctioning, so confirming the direct relation between the headache and the shunt failure. On the other hand, the differential diagnosis between SVS and ACCD requires the integration of clinical and instrumental data because of the characteristic intermittence of the symptoms and the frequent illusory improvement which follows the revision of the shunt. Even the analysis of the clinical manifestations may be difficult as patients complain of chronic, debilitating headaches that in some cases are prevalent in the nuchal region, so heralding the possible descent of the cerebellar tonsils into the upper cervical canal (Fig. 17.2). In general, the clinical manifestations are progressively worsening and may impact considerably on the social life and on the well-being of the patient. In some cases the diagnosis is unnecessarily delayed by the hypothesis of psychological or merely “functional” disturbances. The clinical picture may be further complicated by additional signs and symptoms of intermittent abnormal elevations in ICP, such as diplopia, ataxia, dizziness, and lethargy. It is worth to note in order to emphasize the need of a prompt diagnosis that acute neurologic deterioration secondary to ACCD evolving rapidly to sudden death has also been described [1].

Collapsed ventricles and absent or small volume subarachnoid spaces, together with a thickened vault of the skull are the usual neuroimaging findings of ACCD (Fig. 17.3). In particular, skull X-ray films or bone window images on the CT scan may document the progressive thickening of the cranial vault, due to the deposition of laminated bone at the inner surface of the skull, the typical appearance of the “copper beaten skull,” the presence of “thumbprintings,” and hyperpneumatosis of the air sinus cavities. Parenchymal window images on the CT scan or MRI studies show slit-like ventricles, lack of CSF signal over the convexities following the obliteration of the cortical subarachnoid spaces, effacement of the cisterns, and, often, a crowded posterior fossa (with or without cerebellar tonsillar herniation) [1]. Cranial volumetric studies may confirm the global reduction in size of the inner cranial volume in a high percentage of the patients and, in several cases, a markedly reduced volume of the posterior cranial fossa which accounts for the upward herniation of the upper cerebellar vermis into the cistern of the great vein of Galen and the downward displacement of the inferior cerebellar vermis and tonsils, usually referred to as Chiari type I malformation [20, 21]. The impact of the cerebrovascular structures contained in a small posterior cranial fossa may be exacerbated in the younger subjects by the robust growth of the cerebellum in postnatal life [22].