What does enlargement of the subarachnoid space mean?

Enlargement of the Subarachnoid Space: Clinical Significance and Interpretation

Enlargement of the subarachnoid space refers to an abnormal increase in the fluid-filled area between the arachnoid and pia mater membranes that surround the brain and spinal cord, which can indicate either benign developmental variants (particularly in infants) or pathological conditions requiring urgent intervention (such as increased intracranial pressure, leptomeningeal disease, or CSF leakage). 1, 2

Normal Anatomy and Physiology

The subarachnoid space is the compartment between the arachnoid and pia mater layers of the meninges, containing cerebrospinal fluid (CSF). 1

• In adults, this space normally contains approximately 140 mL of CSF, with the entire volume replaced more than 5 times daily as approximately 800 mL is produced over 24 hours. 1
• CSF flows from the lateral ventricles through the foramina of Monro, into the third ventricle, through the aqueduct of Sylvius to the fourth ventricle, then through the foramina of Magendie and Luschka to circulate around the brain and spinal cord. 1

Clinical Contexts and Differential Diagnosis

Benign Enlargement in Infants

In infants, benign enlargement of the subarachnoid space (BESS) is the most common cause of macrocephaly and represents a self-limiting condition that does not require active treatment. 3, 4, 5, 6

Key features include:

• Rapid head circumference increase (often from 50th percentile to >95th percentile within weeks to months) 6
• Normal or only mildly delayed neurodevelopment 3, 4, 5
• Family history of macrocephaly is common 3
• Imaging shows enlarged anterior subarachnoid spaces with normal or slightly enlarged ventricles 4, 6
• Spontaneous resolution typically occurs by 18-24 months of age, though head circumference may remain at or above the 95th percentile 3, 6

Critical caveat: While BESS is generally benign, enlargement of subarachnoid spaces can be an early sign of specific genetic disorders requiring recognition and appropriate workup. 7

Increased Intracranial Pressure (Pseudotumor Cerebri/Idiopathic Intracranial Hypertension)

In children and adults with increased intracranial pressure, particularly pseudotumor cerebri syndrome, subarachnoid space enlargement manifests as specific imaging findings. 2

Key diagnostic features on MRI include: 2, 8

• Distention of the perioptic subarachnoid space (dilated optic nerve sheaths) 2, 8
• Empty sella 2
• Flattening of the posterior aspects of the globes 2
• Tortuous or enhancing optic nerves 2

The pathophysiology involves increased intracranial CSF volume that accumulates in the subarachnoid space due to impaired CSF homeostasis and altered venous hemodynamics. 2

Leptomeningeal Metastases

Tumor cells within the subarachnoid space can cause focal or diffuse enlargement through direct infiltration or obstruction of CSF flow. 1

• Most common in breast cancer (particularly infiltrating lobular subtype), lung cancer, melanoma, and gastrointestinal malignancies 1
• Tumor cells accumulate at sites of slow CSF flow, particularly the basilar cisterns, posterior fossa, and cauda equina 1
• CSF cytology is the diagnostic gold standard, though false-negative rates are significant 1

CSF Leakage and Intracranial Hypotension

In spontaneous intracranial hypotension, decreased CSF volume paradoxically can be associated with compensatory changes including subdural fluid collections and altered subarachnoid space appearance. 9

• CSF leakage through dural defects, leaking meningeal diverticula, or CSF-venous fistulas causes volume depletion 9
• Imaging findings include epidural fluid collections, dilated epidural venous plexus, and subdural hygromas 9
• The spine represents the anatomical source of most symptomatic CSF leaks 9

CSF Leak (Sinonasal)

Enlargement or communication of the subarachnoid space with the sinonasal cavity occurs through osteodural defects. 10

• May be traumatic, surgical, spontaneous, or associated with idiopathic intracranial hypertension 10
• Requires accurate localization for successful surgical repair due to meningitis risk 10

Diagnostic Approach

For infants with macrocephaly: Obtain detailed developmental history, family history of macrocephaly, and neurological examination. MRI brain is the imaging modality of choice to distinguish BESS from hydrocephalus or other pathology. 3, 5, 6

For children/adults with signs of increased intracranial pressure: MRI of the brain and orbits with and without contrast is the most sensitive modality for detecting secondary signs of increased intracranial pressure, including perioptic subarachnoid space distention. 2, 8

For suspected leptomeningeal disease: CSF cytology from large-volume sampling (10-15 mL) with immediate processing; repeat sampling if initial cytology is negative. 1