A 26-month-old boy presents for mild altered mental status and balance issues following a fall the day before. There was no loss of consciousness or vomiting but he subsequently complained of left-sided head pain. What's the diagnosis?
A 26-month-old boy presents to the emergency department (ED) for mild altered mental status and balance issues following a fall the day before. The patient fell backward off a step and hit the back of his head on a rubber-matted floor. The fall was witnessed by his father, who reports that the child had no loss of consciousness or vomiting but subsequently complained of left-sided head pain. After an uneventful night, the child was noted to be less active upon waking in the morning. During a telehealth visit with the pediatrician, the patient appeared off-balance and was not acting normally, prompting a referral to the ED.
Examination and imaging studies
Upon arrival to the ED, the patient is acting normally as per his father but is holding his neck abnormally. He seems comfortable and playful during the physical examination, which shows normal vital signs. On first impression, he is noted to have a large head. He keeps his neck flexed and laterally rotated to the right, with his chin pointing left. He turns his entire body during attempts to perform rotation of the neck. There is no cervical spine tenderness. The anterior fontanelle is open about 2 cm. There is no evidence of Battle sign, raccoon eyes, or hemotympanum. The remainder of the physical examination is unremarkable.
The patient is given ibuprofen to see whether that improves his range of motion of the neck. A cervical spine x-ray is ordered to evaluate for subluxation; because patient positioning limits the study, C2 subluxation cannot be ruled out. A noncontrast cervical spine computed tomography shows no cervical spine injury, but the radiologist visualizes a large low-density region in the posterior fossa that appears to follow the density of cerebrospinal fluid. A noncontrast head CT reveals the cause of the patient’s signs and symptoms.
Given the history of the fall in conjunction with the patient’s presentation of holding his head abnormally, the leading diagnosis is cervical spine subluxation, but the differential diagnosis includes torticollis, cervical spine fracture, intracranial hemorrhage, musculoskeletal injury, and intracranial mass. Additionally, his initial altered mental status, noted by the pediatrician and now resolved, was most likely secondary to a mild concussion. A significant intracranial injury is possible but less likely.
It is noted that this child has macrocephaly, defined as a large head, and his anterior fontanelle is open. The differential diagnosis of macrocephaly in infants and children includes megalencephaly, congenital hydrocephalus, acquired causes of increased cerebrospinal fluid (CSF), increased intracranial blood volume, bone thickening, increased intracranial pressure (ICP), and an intracranial mass.1 Common causes of macrocephaly in infants and children vary with age of onset. Benign enlargement of the subarachnoid space, known as benign extra-axial fluid of infancy or benign external hydrocephalus, is a cause of macrocephaly.
Evaluation for the etiology of macrocephaly should begin if the occipitofrontal head circumference (OFC) is greater than 2 standard deviations (SDs) above the age-related mean; serial measurements of the OFC cross at least 1 major percentile line at wellness visits; or, in infants younger than 6 months, OFC increases more than 2 cm per month.1
The evaluation of macrocephaly, including imaging, is directed by history and physical examination with special consideration of signs or symptoms suggesting elevated ICP, central nervous system (CNS) trauma or infection, syndromic features, neurodevelopmental abnormalities, and family history of neurodevelopmental or cutaneous abnormalities.1 If there are no syndromic features and development is normal, the OFC of first-degree relatives (parents, siblings) should be measured to assess for familial macrocephaly.
Macrocephaly and megalencephaly both present with head circumference greater than 2 SDs above the age-related mean but are differentiated in that megalencephaly is defined as increased growth of actual cerebral structures. Pediatricians often see both benign familial macrocephaly and benign familial megalencephaly.
Megalencephaly can be caused by anatomic or metabolic abnormalities or can be benign with no neurological impairment.2 Familial megalencephaly, the most common anatomic type, is confirmed in a child with normal development and neurologic examination, no syndromic features, no family history of developmental or neurologic abnormalities, and an OFC within the normal range, as estimated by Weaver curves. Weaver curves use parental OFC to better define ranges of normalcy for a child’s head circumference.3
The differential diagnosis of persistently open anterior fontanelles includes rickets, congenital hypothyroidism, achondroplasia, trisomy 21, ICP, arachnoid cyst, familial macrocephaly, and normal anatomic variation.4 Median time to anterior fontanelle closure is 13.8 months.5 At ages 3 months, 1 year, and 2 years, the anterior fontanelle is closed in 1%, 40%, and 96% of children, respectively.5 Generally, the anterior fontanelle closes earlier in boys than girls.5 Less common causes of persistently open anterior fontanelles include other skeletal disorders, other chromosomal abnormalities, congenital infections, drugs and toxins, dysmorphogenesis syndromes, and malnutrition.4
A noncontrast head CT shows moderate to marked enlargement of the lateral and third ventricles compatible with hydrocephalus. The fourth ventricle is normal in size. A large cyst (5.8 × 3.9 × 4.4 cm) that is isodense to CSF is visualized in the posterior fossa. The radiologist’s over- all impression is that the findings are compatible with a large posterior fossa arachnoid cyst demonstrating local regional mass effect resulting in moderate to marked supratentorial hydrocephalus.
After these data are obtained, further discussion with the father reveals that his son has always been “clumsy,” falling often, and known to have a large head. The father adds that his son frequently “keeps his head down” and that the pediatrician once stated that the boy’s head circumference was greater than the 100th percentile.
The child likely experienced a mild concussion from the fall, resulting in mild behavioral changes and exacerbating his baseline balance disorder. His clinically important large arachnoid cyst was an incidental finding during a CT scan evaluating his torticollis, which is likely the result of his posterior fossa cyst. Case reports have described posterior fossa arachnoid cysts causing torticollis, though the mechanism is unclear.6 Hypotheses include mass effect compressing structures that control head movement via stretching and irritation of the dura of the ascending meningeal branches of nerves C1 through C3 and the accessory nerve.6 It is also possible that the neck positioning may be acute on chronic presentation, possibly from musculoskeletal pain, considering the father’s reported history of the patient keeping his head down—but, of note, no signs of muscular or ligamentous trauma appeared on CT or the magnetic resonance imaging obtained later.
Arachnoid cysts are collections found within the arachnoid membranes, arachnoid cells lining the cyst secrete CSF.7 Arachnoid cysts are nonneoplastic, with 2 classifications: primary developmental, which are more common, and secondary. Primary cysts result when the arachnoid membrane splits in utero, whereas secondary cysts occur after trauma, surgery, infection, or intracranial hemorrhage.8 Arachnoid cysts account for approximately 1% of all intracranial masses and occur in 2.6% of children, more commonly affecting boys; 75% of symptomatic arachnoid cysts occur in children.8 Intracranial arachnoid cysts are more common in certain genetic syndromes, including trisomy 21, mucopolysaccharidosis, schizencephaly, neurofibromatosis, autosomal-dominant polycystic kidney disease, acrocallosal syndrome, and Aicardi syndrome.9
The most common location for arachnoid cysts is the sylvian fissure. They may also occur in the cerebral convexity, interhemispheric fissure, suprasellar cistern, quadrigeminal cistern, cerebellopontine angle, mid-line of the posterior fossa, spine, or any part of the nervous system where arachnoid mater is present.10
Most arachnoid cysts are asymptomatic, are found incidentally, and do not require surgery.8,9 When symptomatic, the most common presentation is headache. Additional symptoms include dizziness, nausea, vomiting, mood or mental status changes, ataxia, vision changes, and hearing loss.8 In patients with significant symptoms, including hydrocephalus, focal neurological deficits, localized seizures, or remodeling of the skull bones, surgery is indicated.8,9 The diagnosis of arachnoid cysts is made by CT or MR imaging. Surgical treatment, when indicated, includes cyst wall fenestration or, less commonly, a cyst-peritoneal shunt.7-9 At this time, there is no class I evidence regarding optimal treatment of arachnoid cysts.9
Following the head CT results, neurosurgery is consulted; a brain MRI confirms the CT findings, adding no significant data. Ophthalmologic examination reveals bilateral hyperemic optic discs, without disc edema. The patient undergoes left craniotomy and cyst fenestration. The remainder of his hospital course is unremarkable, and he is discharged 5 days following initial ED presentation.
Lessons for the clinician
1. Boom JA. Macrocephaly in infants and children: etiology and evaluation. UpToDate. 2019. Accessed January 15, 2021. Updated June 21, 2022. https://www.uptodate.com/contents/macrocephaly-in-infants-and-children-etiology-and-evaluation
2. Pavone P, Praticò AD, Rizzo R, et al. A clinical review on megalencephaly: a large brain is a possible sign of cerebral impairment. Medicine (Baltimore). 2017;96(26):e6814. doi:10.1097/MD.0000000000006814
3. Weaver DD, Christian JC. Familial variation of head size and adjustment for parental head circumference. J Pediatr. 1980;96(6):990-994. doi:10.1016/s0022-3476(80)80623-8
4. Kiesler J, Ricer R. The abnormal fontanel. Am Fam Physician. 2003;67(12):2547-2552.
5. D’Antoni AV, Donaldson OI, Schmidt C, et al. A comprehensive review of the anterior fontanelle: embryology, anatomy, and clinical considerations. Childs Nerv Syst. 2017;33(6):909-914. doi:10.1007/s00381-017-3406-1
6. Hanrahan J, Frantzias J, Lavrador JP, Bodi I, Zebian B. Posterior fossa arachnoid cyst causing torticollis and gastro-oesophageal reflux in an infant. Childs Nerv Syst. 2018;34(12):2519-2523. doi:10.1007/s00381-018-3917-4
7. Huang KT, Smith TR. Pediatric neurosurgery. In: Doherty GM, eds. Current Diagnosis & Treatment: Surgery. 15th ed. McGraw Hill; 2020. Accessed January 15, 2021. https://accessmedicine.mhmedical.com/content.aspxbookid=2859§ionid=242161973
8. Mustansir F, Bashir S, Darbar A. Management of arachnoid cysts: a comprehensive review. Cureus. 2018;10(4):e2458. doi:10.7759/cureus.2458
9. Jafrani R, Raskin JS, Kaufman A, Lam S. Intracranial arachnoid cysts: pediatric neurosurgery update. Surg Neurol Int. 2019;10:15. doi:10.4103/sni.sni_320_18
10. Chheda MG, Wen PY. Uncommon brain tumors. UpToDate. 2020. Accessed January 15, 2021. Updated November 2, 2021. https://www.uptodate.com/contents/uncommon-brain-tumors