What is communicating hydrocephalus?

Communicating Hydrocephalus Definition

Communicating hydrocephalus is a condition where cerebrospinal fluid (CSF) flows freely from the ventricles to the subarachnoid space, but accumulates due to impaired CSF absorption at the arachnoid granulations or extracranial lymphatic drainage sites, resulting in ventricular enlargement without obstruction of CSF pathways. 1, 2

Core Pathophysiologic Features

The defining characteristic that distinguishes communicating from non-communicating hydrocephalus is the absence of obstruction to CSF flow between the ventricular system and subarachnoid space. 3, 2 The CSF can move freely through the foramina of Monro, aqueduct of Sylvius, and foramina of Luschka and Magendie, but the problem lies downstream at the absorption sites. 4, 2

Mechanism of CSF Accumulation

• Impaired absorption occurs primarily at the arachnoid granulations due to fibrosis, meningeal scarring, and subependymal gliosis. 1
• Extracranial lymphatic vessels in the ethmoid bone region also contribute to CSF drainage, and dysfunction of these pathways plays a role in communicating hydrocephalus. 4
• Molecular mediators including TGF-β1 and TGF-β2 stimulate extracellular matrix protein deposition, directly impairing CSF resorption at absorption sites. 1
• Elevated aminoterminal propeptide of Type I collagen (PC1NP) and vascular endothelial growth factor levels in CSF reflect the fibrotic process at arachnoid granulations. 1

Major Etiologic Categories

Post-Infectious Causes

• Meningitis is a leading cause, particularly in lower-middle-income countries where it accounts for 19.2% of pediatric hydrocephalus cases. 1
• Inflammation and subsequent fibrosis of the subarachnoid space from bacterial, viral, or fungal meningitis impairs CSF flow and absorption. 1
• Coccidioidal meningitis causes hydrocephalus in approximately 40% of affected individuals, either at presentation or during disease progression. 1

Post-Hemorrhagic Causes

• Subarachnoid hemorrhage (SAH) causes communicating hydrocephalus when blood products obstruct CSF flow in the subarachnoid space and damage absorption sites. 1
• Intraventricular hemorrhage (IVH) in premature infants leads to post-hemorrhagic hydrocephalus through impaired CSF absorption mechanisms. 5, 1
• Ventricular puncture should be reserved for infants in extremis as it increases risk of CSF infection and loculated hydrocephalus. 5

Neoplastic Causes

• Leptomeningeal metastases obstruct CSF flow in the subarachnoid space without blocking ventricular pathways. 1
• Brain tumors near the fourth ventricle can cause communicating hydrocephalus when they partially obstruct CSF flow while maintaining some communication. 1
• Neurocysticercosis presents with subarachnoid involvement that can cause communicating hydrocephalus, meningitis, stroke, or focal neurological findings. 5

Normal Pressure Hydrocephalus (NPH)

• NPH is a specific form of communicating hydrocephalus characterized by the clinical triad of gait disturbance, urinary incontinence, and cognitive impairment. 1, 6
• Prevalence is 3.7% in patients over 65 years of age, making it a significant cause of reversible dementia in the elderly. 1
• Imaging findings include ventriculomegaly, narrowed posterior callosal angle, effaced sulci along high convexities, and widened sylvian fissures. 1
• Defect in CSF resorption is the commonly accepted pathophysiologic mechanism, with recent evidence implicating the glymphatic system. 6

Clinical Distinction from Non-Communicating Hydrocephalus

The critical distinction is that in communicating hydrocephalus, CSF can flow freely from ventricles to subarachnoid space, whereas non-communicating (obstructive) hydrocephalus has a definite anatomic obstruction preventing this flow. 3, 2 This distinction is essential because:

• Serial lumbar punctures can be therapeutic in communicating hydrocephalus if the lumbar subarachnoid space communicates with the ventricular system. 5
• Early intervention with serial LPs reduced surgical intervention needs to 29% in one study, with only 16% eventually requiring permanent shunt insertion. 5
• Non-communicating hydrocephalus may be amenable to endoscopic third ventriculostomy, whereas communicating hydrocephalus typically requires shunt placement. 7

Common Pitfall in Terminology

The older classification of "obstructive versus non-obstructive" or the terms "communicating versus non-communicating" have caused confusion because communicating hydrocephalus can still involve obstruction—just not at the ventricular level. 2 The obstruction in communicating hydrocephalus occurs at the absorption sites (arachnoid granulations or lymphatic drainage), not in the CSF flow pathways themselves. 1, 2