Motor developmental delay in a healthy 11-month-old infant: answer

MRI showed a large, well-defined non-enhancing thin-walled cyst with a T2 hypointense circumferential rim, arising above the sella turcica. Occlusion of the bilateral foramina of Monro by the thin-walled cyst resulted in severe obstructive hydrocephalus. Radiographic characteristics of the cyst content were consistent on all image sequences with cerebrospinal fluid (CSF), suggesting the diagnosis of a suprasellar arachnoid cyst (SAC). The child was taken to the operating room for cyst fenestration, restoration of CSF ventricular drainage, and relief of hydrocephalus. A right transfrontal endoscopic approach was undertaken (Fig. 1A, as viewed along the arrow trajectory seen in Fig. 2A of Images in Neuroscience: Question), confirming the presence of an occlusive cyst. Fenestration of this wall revealed the cyst interior (Fig. 1B). Chronic thinning of the third ventricle floor allowed striking visualization of the posterior circle of Willis (Fig. 1B, arrowheads), pituitary stalk and gland (black arrow), and bilateral third cranial nerves (white arrows). Arachnoid cysts are CSF-filled structures formed by arachnoid membranes, commonly found arising in the subarachnoid cisterns or major fissures of the brain. These cysts account for 1% of all intracranial space-occupying lesions, and are typically single, left-sided, and have a male to female predominance of 2:3 [1]. SAC represent only 9% of all arachnoid cysts in adults and 15% of all arachnoid cysts in the pediatric population [2]. While often an incidental finding on cranial imaging, surgical intervention can become necessary when compression of adjacent structures results in hydrocephalus, neurological symptoms, endocrine abnormalities, or developmental delay. As seen with this classic presentation of a SAC, approximately 90% of symptomatic SAC present with hydrocephalus due to obstructed third or lateral ventricular CSF outflow [3]. The exact etiology of SAC is unknown, though it may be linked with weakening of an arachnoid wall that later forms an inversion of the membrane, favoring cyst development [4]. Early identification of symptomatic SAC can facilitate prompt surgical interventions to relieve symptomatic compressions [1–3,5]. The morphology and periventricular location of SAC often permit endoscopic fenestration. These procedures are often effective, minimally disruptive to collateral structures, and associated with low morbidity and mortality rates, particularly in children [5]. Here, fenestration of the cyst floor allowed decompression into the adjacent subarachnoid cisterns. Despite initial successful cyst decompression, our patient later developed hydrocephalus requiring ventriculoperitoneal shunting with improvement in psychomotor development. SAC should be considered in patients demonstrating evidence of progressive hydrocephalus and a suprasellar lesion, as surgical intervention may dramatically improve cognitive trajectories and relieve potentially life-threatening hydrocephalus. Further elucidation of the natural history and evolution of SAC, including associated CSF hydrodynamic abnormalities, may help direct the optimum treatment strategy.