Arachnoid cysts are common in the middle cranial fossa and are usually asymptomatic; this infant had an arachnoid cyst in the posterior fossa and was symptomatic.

Born to non-consanguineous parents, the antenatal and birth history were normal. There was no family history of seizure activity. Developmental milestones were normal and the immunisation status was appropriate for the age.

The infant's weight, height and head circumference were 3.88 kg, 56 cm and 37 cm, all of which were below the third percentile. This infant had a cleft palate and low-set ears, but no other dysmorphism (figure 1). Physical examination was unremarkable except for a palpable urinary bladder.

Ultrasonography revealed bilateral hydronephrosis, hydroureters and thickened bladder wall. Both the kidneys were of normal size and echotexture. The micturating cystourethrogram showed grade V VUR on right side and dilated posterior urethra (figures 2 and ​and33).

MRI of the brain showed large lobulated cystic lesion in both the occipital lobes, mainly on the left side (the dimensions 6.3×4.8×4.6 cm), crossing the midline, leading to splaying of the occipital horns and compressing the posterior aspect of third ventricle causing moderate ventricular dilatation (figures 4 and ​and55).

PUVs are the most common form of urethral obstruction in boys.¹ It is part of the spectrum of obstructive congenital anomalies of the urinary tract. The urethral valves are tissue leaflets fanning distally from the prostatic urethra to the external urinary sphincter. A slit-like opening usually separates the leaflets. Valves are of unclear embryological origin and cause varying degrees of obstruction. Approximately 30% of patients experience end-stage renal disease or chronic renal insufficiency. The prostatic urethra dilates and the bladder muscle undergoes hypertrophy. VUR occurs in 50% of patients.²

There are a wide range of renal changes from mild hydronephrosis to severe renal dysplasia; their severity probably decided by the severity of the obstruction and its time of onset. As in other cases of obstruction or renal dysplasia, there may be oligohydramnios and pulmonary hypoplasia, these two being very important poor prognostic factors.

Affected boys with PUVs are discovered prenatally when maternal ultrasonography reveals bilateral hydronephrosis, a distended bladder and, if the obstruction is severe, oligohydramnios.

Prenatally diagnosed PUVs, particularly when discovered in the second trimester, carry a poorer prognosis than those detected after birth. In the male neonate, PUVs are suspected when there is a palpably distended bladder and the urinary stream is weak. If the obstruction is severe and goes unrecognised during the neonatal period, infants may present later in life with failure to thrive due to uraemia or sepsis caused by infection in the obstructed urinary tract. With lesser degrees of obstruction, children present later in life with difficulty in achieving diurnal urinary continence or with urinary tract infection.

Cystoscopy is considered to be the reference standard for determining the presence of a urethral obstruction. It is supposed to be more reliable than voiding cystourethrography and even more reliable than urodynamic studies.³

The relationship between valves, reflux and dysplasia has been identified. In comparing refluxing and non-refluxing kidneys, it was recognised that infants with unilateral reflux had markedly different renal function. Reflux was consistently associated with dysplastic and damaged kidneys. It was postulated that the reflux allowed the high bladder pressures to focus on the refluxing kidney, sparing and protecting the non-refluxing kidney. The vesicoureteral reflux and dysplasia relationship is found in 13% of patients with valve issues, and interestingly, the refluxing kidney is on the left in 92%.⁴

The proposed mechanism of this protection is that the refluxing collecting system acts as a pressure pop-off valve. It is thought that the protected kidney would ultimately lead to a better long-term prognosis than for boys without unilateral reflux.⁵

Arachnoid cysts are commonly encountered fluid collections in the central nervous system. These are more common in men, as was the case in our patient too. Reporting a series of 20 cases, the authors describe a case series that illustrates the diverse forms of presentation and the treatment modalities commonly used for arachnoid cysts. A total of 70% of the cysts were supratentorial, 5% infratentorial and 25% spinal. Symptoms at presentation included headache (41%), weakness (23%), seizure (14%), hydrocephalus (9%), scoliosis (4%), cognitive decline (4%) and visual loss (4%).⁶

Primary arachnoid cysts are present at birth and are the result of developmental abnormalities in the brain and spinal cord that arise during the early weeks of gestation. Secondary arachnoid cysts are not as common as primary cysts and develop as a result of head injury, meningitis, or tumours, or as a complication of brain surgery. The majority of arachnoid cysts form in the middle crania fossa. Arachnoid cysts involving the spinal cord are more rare. The location and size of the cyst determine the symptoms and when those symptoms begin. Most individuals with arachnoid cysts develop symptoms before the age of 20, and especially during the first year of life, but some people with arachnoid cysts never have symptoms. Males are four times more likely to have arachnoid cysts than females.

Symptomatic arachnoid cyst may be treated by fenestration or cystoperitoneal shunt. Asymptomatic arachnoid cyst may be watched, or drained if expansion or haemorrhage would threaten vital structures.