Surgical management of refractory idiopathic intracranial hypertension

Introduction

Idiopathic intracranial hypertension (IIH) is a disorder characterized by symptoms secondary to increased intracranial pressure (ICP) with no clear identifiable cause. It is therefore a diagnosis of exclusion and necessitates the exclusion of differential diagnoses of increased ICP (i.e., intracranial mass, blockage of the ventricular system, blockage of cerebrospinal fluid [CSF] absorption, communicating hydrocephalus, obstruction of venous outflow, diffuse cerebral edema, and increased CSF secretion). The cause of IHH remains unclear. Hypothetical mechanisms include increased CSF production (secondary to abnormal function of the ependymal cells in the choroid plexus), decreased CSF absorption (secondary to the obstruction of cortical venous sinuses, and increased venous sinus pressure. Contributing factors include obesity, which leads to hormonal changes and increased intraabdominal, pleural, cardiac filling, and central venous pressures, leading to increased ICP.

Idiopathic intracranial hypertension affects 0.5 to 2 people per 100,000 of the general population, but it is significantly more prevalent in obese women of reproductive age. The ongoing worldwide obesity epidemic has increased the incidence of IIH and its impact on patients and society. Visual impairment and headaches are the most common symptoms associated with IIH. Visual deficits are secondary to the progressive atrophy of the optic disc, which is due to papilledema caused by elevated ICP, first leading to transient visual loss and visual field defects and declining color vision and diminished visual acuity. Additional symptoms include pulsatile tinnitus, back and neck pain, olfactory changes, encephaloceles, CSF leaks, and cognitive and mood changes. The clinical presentation is variable, which may lead to delays in diagnosis and management. ^{, ,} In addition to its direct impact on patients and families, IIH is also associated with increased costs. In the United States, the total hospital costs per IIH admission were four times greater than for a population-based per person admission, and the total economic costs of IIH were greater than $444 million per year ( Table 17.2 ).

The main goals of treatment are to preserve visual function and alleviate associated symptoms. Primary treatment consists of weight reduction and medical therapy with acetazolamide, topiramate, or both. Patients who do not respond to medical therapy and present with progressive visual decline should be considered for surgical intervention. Multiple surgical approaches are available for the management of IIH, including CSF shunting, venous stenting, optic nerve sheath fenestration (ONSF), and bariatric surgery. These options have different responses and complication rates and are selected according to the patient’s symptoms, anatomic features, and preferences, combined with the treating team’s experience ( Fig. 17.1 and and Table 17.1 ). ^,

This chapter reviews the surgical interventions currently available for the management of refractory IIH in the context of the current understanding of the pathophysiology of IIH, including the role of venous system obstruction.

Pathophysiology of idiopathic intracranial hypertension: The role of venous system obstruction and sinus stenosis

The pathophysiology of elevated ICP in IHH has not been fully elucidated. Several theories have been suggested, most of them centering on increased CSF production, decreased CSF absorption, increased venous sinus pressure, and other factors such as obesity that may influence these mechanisms.

Approximately 600 mL of CSF is produced daily. This value decreases with increasing age, which may explain why IIH is a disorder of younger people. CSF is produced in the choroid plexus, located in the cerebral ventricles, by a lining of specialized epithelial tissue known as ependyma. This CSF production mechanism involves the action of Na ⁺ /K ⁺ -ATPase channels, which results in other electrolytes (e.g., K ⁺ , Cl ^– , HCO3 ^– ) following the electrochemical gradient. This leads to a large amount of H ₂ O crossing the epithelium via aquaporin-1 channels. Carbonic anhydrase produces protons and bicarbonate used by the transporters and is inhibited by acetazolamide, one of the medical treatments for IIH. CSF flows out of the ventricular system through arachnoid granulations (with microscopic villi) because of pressure difference between CSF space and venous sinuses where it is resorbed.

Obesity is linked to a prothrombotic state via increased plasminogen activator inhibitor activity and therefore an increase the likelihood of obstruction in the cerebral venous sinus and consequently increases in CSF pressure. Additionally, obesity may also lead to decreased venous return and contribute to a decrease in the rate of CSF reabsorption. ^{, ,}

Cerebral venous sinus system

The observation that patients with IIH frequently have compression or stenosis of the transverse and sigmoid sinuses supports a potential role of the cerebral venous system in the primary and secondary mechanisms related to IIH. The proposed physiologic relationship between dural sinus stenosis and IIH is impaired venous drainage. Increased venous pressure gradients have been observed in this population, associated with dural venous system stenosis (DVSS) in the transverse and sigmoid sinuses, leading to decreased CSF absorption in arachnoid granulations. DVSS has been reported in the majority of the IIH patients in recent studies. ^,

Aplastic or hypoplastic venous sinuses, intrinsic and extrinsic compression, and acquired stenosis likely raise CSF pressure. Bilateral transverse sinus stenosis results in more significant changes in CSF pressure than unilateral stenosis. It is important to mention, however, that there is evidence that stenosis of the sinuses may be secondary to intracranial hypertension rather than the cause of IIH itself. ICP reduction after lumbar puncture and, consequently, improvement in CSF dynamics and reduced flattening of the cerebral venous sinuses, may explain the improvement in patients who undergo venous stenting.

Surgical treatment