A 12 week old female infant presents to the emergency department with progressive vomiting, lethargy, and difficulty feeding over the past two days. Her mother reports that the infant has been increasingly irritable in the last week, and does not appear to be herself. She has been less interactive, and her cry has become more high-pitched and weak. She has not been breastfeeding well. Additionally, her mother is concerned because she thinks her infant's head has grown, and the "soft spot" on her head appears more tense. She thinks that the infant has felt "warm", but she has not measured the temperature with a thermometer. The infant has had fewer wet diapers and no bowel movements today.
She reports that the infant was born on time and that there were no prenatal or perinatal complications. The infant was released after a 48 hour stay in the regular newborn nursery, and had follow-up initially with her pediatrician about one week after discharge. She has had no further follow-up. From the previous medical records it is confirmed that the infant was born at term. There was poor prenatal care, but the labor and delivery were unremarkable. Mother's prenatal labs were normal. The infant weighed 2900 grams at birth (25th percentile), measured 47.8 cm in length (10th - 25th percentile), and had a head circumference of 34 cm (25th percentile).
Exam: VS: T 36.5 C, P 165, R 45, BP 98/65. Weight 4.20 kg (5th percentile), length 57 cm (10th - 25th percentile), HC 42.6 cm (95th percentile). In general this is a lethargic infant with a weak, high-pitched cry. Her head is oddly shaped and looks like an inverted pear. Her scalp veins are prominent, and the anterior fontanelle is tense and bulging. Eyes show pupils which are equal and round, but are sluggishly reactive to light. Red reflex is present bilaterally. EOMs are clearly dysconjugate. There is mild tachypnea with slight intercostal retractions. Lung fields are clear to auscultation bilaterally. Her heart exam reveals tachycardia with a regular rhythm and a grade 2/6 systolic ejection murmur at the left sternal border. Capillary refill is 2 seconds. Her spine is straight without protrusions or apparent defects. Her upper extremities show good tone and full range of motion with slightly brisk reflexes. Her lower extremities show increased tone with brisk reflexes bilaterally. There is 4+ clonus bilaterally. On neurologic examination, her eyes show rotated downward gaze bilaterally. There is a poor suck. The startle response is minimally present. The grasp and glabellar reflexes are present. No parachute reflex can be elicited. The Moro is present.
Imaging studies demonstrate hydrocephalus and aqueductal stenosis. A ventriculoperitoneal shunt procedure is performed by a neurosurgeon. Following surgery, the patient's anterior fontanelle is concave and the head circumference has decreased.
Hydrocephalus is the pathologic enlargement of the cerebral ventricles secondary to increased intracranial pressure caused by a mismatch between the production of cerebrospinal fluid (CSF) and its absorption (1). CSF is produced mostly by the ependymal cells of the choroid plexus within the ventricular system. CSF flows in a directional, pulsatile movement (2,3) from the lateral ventricles through the Foramina of Monro into the third ventricle. It then drains through the cerebral aqueduct (of Sylvius) into the fourth ventricle. CSF then exits the ventricular system through the Foramina of Luschka and Magendie, where it thereby gains access to the basilar cisterns, which communicate with the subarachnoid spaces over the cerebrum and spinal cord. Finally, CSF is returned to the vascular system by absorption through the subarachnoid villi and granulations.
A blockage of CSF along any point in this pathway may result in increased intracranial pressure and subsequent dilatation of the ventricular system upstream to the obstruction. Classically, hydrocephalus has been divided into two subtypes: communicating and non-communicating. Non-communicating hydrocephalus results from obstruction of CSF within the ventricular system. This includes obstruction at the outlet foramina of the fourth ventricles. Therefore, if CSF flow does not pass into the basilar cisterns, the corresponding hydrocephalus is classified as non-communicating. Some authors will utilize the term obstructive hydrocephalus to indicate non-communicating hydrocephalus.
Communicating hydrocephalus implies that there is free-flowing CSF within the ventricular system (and through its outlet foramina), and usually occurs secondary to impaired absorption at the subarachnoid villi and granulations. It should be noted from a pathophysiologic standpoint, however, that in almost every case, an obstructive process produces hydrocephalus. The single exception to this is the rare case of a congenital choroid plexus papilloma, where the mismatch between CSF production and absorption occurs as a function of excessive CSF production (1,3). Additionally, the term hydrocephaly should be distinguished from the terms macrocephaly and megalencephaly. Macrocephaly is a descriptive term for any head circumference larger than two standard deviations from the mean. Megalencephaly refers to an increase in brain parenchymal volume.
The differential diagnosis of hydrocephalus may be considered by age (3). Congenital hydrocephalus has an estimated incidence of about 3 to 4 per 1000 live births (4,5). The most common causes of congenital hydrocephalus are due to structural defects such as Chiari malformations and aqueductal stenosis (3,6). Dandy Walker is also an important cause of congenital hydrocephalus, although it occurs less frequently. Additionally, intrauterine infections, especially toxoplasmosis, rubella, cytomegalovirus, and syphilis, may be associated with congenital hydrocephalus. In the neonatal period, acquired causes of hydrocephalus include perinatal infections and intracranial bleeding secondary to trauma or anoxia. Premature infants are particularly susceptible to intraventricular hemorrhage (IVH), which may subsequently lead to hydrocephalus. AV malformations of the Great Vein of Galen, or the straight sinus may also present in this period and may cause hydrocephalus secondary to blockage or rupture. Much less common causes of hydrocephalus in the neonatal period include arachnoid cysts, congenital choroid plexus papillomas, and tumors (1,3). The common causes of congenital and neonatal hydrocephalus will be discussed below.
Chiari malformations are a very common cause of congenital hydrocephalus, and account for up to 40% of cases (3). Chiari malformations refer to a set of congenital anomalies of the hindbrain where there is a downward displacement of the brainstem and cerebellum through the foramen magnum. There are three types depending on the degree of herniation though the foramen (1,7). Chiari type I is caudal displacement of the cerebellar vermis or tonsils through the foramen. In type II, the fourth ventricle and lower medulla are also herniated. The Chiari type III malformation involves extension of the cerebellum through the foramen magnum and into an associated cervical spina bifida. Chiari type II is the most common of the Chiari lesions and almost always occurs in conjunction with a myelomeningocele and hydrocephalus (1,7).
Aqueductal stenosis is also a very common cause of congenital hydrocephalus. With an occurrence of 0.5 to 1.0 per 1000 live births, it accounts for approximately 20% of hydrocephalus cases (3). Although commonly recognized at birth, the disorder may have an insidious onset, and should be considered in the differential diagnosis of hydrocephalus at any age. Aqueductal stenosis refers to narrowing of the fourth ventricular aqueduct of Sylvius and results in obstructive, non-communicating hydrocephalus. One form of aqueductal stenosis, associated with a syndrome called X-linked hydrocephalus, is caused by a mutation of the X-linked recessive L1 gene, which is responsible for the production of specific neuronal cell adhesion molecules (3,6). Aqueductal stenosis may also be associated with gliosis, which results after destruction of ependymal cells after a hemorrhagic or viral infectious process. Hydrocephalus that occurs as a result of toxoplasmosis or cytomegalovirus is usually the result of aqueductal stenosis (4). There are also multiple case reports of aqueductal stenosis after viral encephalitis caused by mumps (9,10,11).
The Dandy-Walker malformation is a cystic dilatation of the fourth ventricle following partial or complete agenesis of the cerebellar vermis, which leads to obstruction of CSF outflow though the foramina of the fourth ventricle. The syndrome occurs in approximately 1 per 30,000 live births (2,3), and is associated with less than 5 percent of all cases of hydrocephalus (4). Although the defect is present at birth, hydrocephalus does not always present in the neonatal period. Approximately 80% of all Dandy-Walker malformations will be diagnosed by one year of age, although some diagnoses may be delayed until adolescence or adulthood (3,12).
Intraventricular hemorrhage (IVH) is becoming an increasingly important cause of hydrocephalus secondary to the increased survival of very low birth weight infants (<1,500 grams) (13). Intraventricular hemorrhage is the result of vascular instability of cerebral vessels in the germinal matrix at the level of the head of the caudate in the premature infant. Bleeding of these vessels has been classified into 4 grades. Grade I is hemorrhage within the germinal matrix only. Grade II is hemorrhage from the matrix into the ventricles, but without ventricular dilatation. Grade III is IVH with resultant ventricular dilatation. Grade IV is IVH with ventricular dilatation and extension of the bleeding into the surrounding brain parenchyma. When present, IVH usually occurs in low birth weight infants within 72 hours of delivery, and 50% of these occur without immediate clinical symptomology (13). Post-hemorrhagic hydrocephalus may occur immediately as a result of clotting of blood in the ventricular system, which leads to obstruction. More commonly, however, communicating hydrocephalus develops within weeks to months after the hemorrhagic event as the breakdown products of blood lead to diffuse fibrosis of the leptomeninges (13). In order to detect asymptomatic intraventricular hemorrhage, it is recommended that all premature neonates of gestational age <30 weeks of life undergo screening cranial ultrasound at 7 to 14 days of life. If none is detected, a follow-up cranial ultrasound is recommended at 36 to 40 weeks postconception (14).
After the newborn period, common causes of hydrocephalus are hemorrhage, and post-viral or post-bacterial meningitis. The mechanism of formation of hydrocephalus secondary to hemorrhage in other age groups is the same as for premature infants described above, except that the origin of bleeding is different and may be due to rupture of an AV malformation, subarachnoid bleed, or as a result of a traumatic injury. In bacterial meningitis, clumping of increased cellular and infectious matter within the CSF may produce non-communicating hydrocephalus. However, communicating hydrocephalus that occurs as a result of permanent scarring of the meninges is the most common outcome, and occurs in up to 1% of survivors of bacterial meningitis (5). Tumors, cysts, and other space occupying lesions should be considered in the differential diagnosis of hydrocephalus at any age, although they become more important causes in this age group.
The signs and symptoms of hydrocephalus may also be considered as a function of age. The most visually dramatic cases of hydrocephalus occur in infants prior to the close of the anterior fontanelles at 18 months, where the increase in head size may become very large secondary to decreased containment of swelling by open sutures. Subsequently, limited expansion of the more developed sutures leads to earlier neurologic signs, which promote detection of lesions prior to the onset of massive hydrocephalus. Before 2 years of age, hydrocephalus will invariably present with some enlargement of the head. There may be cephalofacial disproportion, possibly with frontal bossing. Sutures may be splayed, and scalp veins may be very prominent. The "setting sun" sign is an ocular abnormality where the eyes appear to look downward such that the whites of the sclera form an arc above the irises. Papilledema is a rare finding; however, long-standing hydrocephalus may be associated with optic atrophy. In infants, primitive reflexes may persist and there may be delayed development of the more mature reflexes. Motor spasticity with hyperreflexia and clonus may be present and will be more prominent in the lower extremities before the upper extremities. This is secondary to increased stretching of the motor fibers of the lower extremities as they traverse longer pathways (1,3). It should be noted that if the onset of hydrocephalus is acute with rapid progression, then vomiting, lethargy, seizures, and cardiorespiratory compromise may occur in infants despite open sutures (3). In older infants, pressure on the brainstem bilaterally may lead to a condition known as pseudobulbar palsy where poor oral-motor control is manifest by difficulty with swallowing and changes in speech.
In older children with hydrocephalus, more focal neurologic signs will be apparent and suggestive of the lesion. Older children present with more classic signs of increased intracranial pressure such as headache and vomiting that is worse in the morning especially upon wakening. Papilledema and strabismus will likely be present. The "bobble head doll syndrome" is a manifestation of obstructive lesions around the third ventricle or aqueduct and is characterized by 2 to 4 oscillations of the head per second along with psychomotor retardation (2,3). Spasticity is particularly prominent in the lower extremities, and there may be a positive Babinski sign. Endocrine abnormalities may be apparent due to long-standing perturbation of the hypothalamic-pituitary axis and may result in growth derangements, delay or acceleration of sexual maturity, fluid and electrolyte disturbances, and thyroid dysfunction (2,3). Cognitive deficits may be suggestive of the lesion, and emotional lability may be a presenting sign.
The diagnosis of hydrocephalus is made more readily apparent with the increasing availability of imaging techniques. Ultrasonography may be used to detect hydrocephalus in the fetal and neonatal periods. In older infants and children, computed tomography (CT) may be utilized. However, magnetic resonance imaging (MRI) is the preferred diagnostic tool in this age group as it provides superior resolution of the brain, spinal cord, and subarachnoid spaces such that specific lesions are more easily detected (3,15). The characteristic lesion of non-communicating hydrocephalus will show dilatation of the ventricles proximal to the site of obstruction with periventricular edema of the adjacent white matter caused by disruption of the ependymal lining in the affected area. In communicating hydrocephalus, the entire ventricular system will be dilated with distinct enlargement of the subarachnoid space over the cerebrum.
Once the underlying etiologic condition has been addressed, the mainstay of therapy for progressive hydrocephalus is a shunt procedure, which allows for diversion of CSF with the overall goal of ventricular decompression. In general, CSF fluid proximal to the site of obstruction is shunted through a catheter and drained into a body space that allows for absorption of the ventricular fluid. The most common and preferred site of drainage is into the peritoneum (VP-ventriculoperitoneal shunt), although many other sites have been utilized (16,17). The most common secondary sites are the pleural space and the venous system or right atrium. The shunt catheter contains a valve to assure one-way CSF flow and is concealed within a subcutaneous tract. Extra tubing is usually curled into position at the distal catheter end to allow for growth of the infant or child. Other treatments for hydrocephalus include the endoscopic third ventriculostomy, which involves fenestration of the third ventricle in obstructive hydrocephalus to provide a direct communication with the subarachnoid space. Additionally, lumboperitoneal shunts may be utilized in cases of communicating hydrocephalus (16,17).
Medications that reduce intracranial pressure such as mannitol may be utilized for cases of rapidly progressive hydrocephalus as a palliative measure while awaiting surgery. Additionally, specific medications that decrease the production of CSF may be useful such as acetazolamide and furosemide. These latter medications may also be utilized temporarily for slowly progressive hydrocephalus or hydrocephalus that is transient, e.g., while awaiting shunt revision (1,16).
Shunt malfunction is a fairly common occurrence with a one-year failure rate of 30-40% (18,19). Higher rates of failure have been described in younger patient populations with the most significant risk occurring in patients younger than 6 months of age at the time of implantation (18,20). The most common time for shunt failure to occur is within six months of surgery (18,21), and causes of shunt malfunction include obstruction, infection, and over-drainage (16,18,21). Obstruction occurs generally because of collection of organic matter in the catheter tubing. Symptoms of shunt malfunction include headache and vomiting. Sunsetting of the eyes, vision changes, diplopia, and distended veins may also be noted. CT scanning is fairly reliable in identifying shunt malfunction. Enlargement of the ventricles is diagnostic of shunt malfunction. Unfortunately, a baseline CT (i.e., when the shunt is working) is not always available for comparison. Frequently, the patient's previous CT scans were obtained when the shunt was malfunctioning. This must be taken into consideration when comparing the size of the ventricles.
Infections usually come to attention about two months after shunt insertion, suggesting that infection may be occurring at the time of surgery, although subsequent infection through contaminated skin surfaces also occurs (2,18). Infection rates vary from 1-10% (3,18,20,21). The most common causative agent of infection is coagulase-negative staphylococci, especially Staphylococcus epidermidis, although Staphylococcus aureus has also been implicated. Treatment usually mandates removal of the shunt, and intraventricular as well as intravenous antibiotics may be required. If shunt revision is necessary, sterility of the CSF is first documented by culture prior to surgery. In cases of community-acquired meningitis, however, treatment may be given as usual with the shunt left in place, as the usual causative agents are unable to colonize the shunt and the catheter may actually lessen the severity of symptoms (16,20).
The overall outcome and prognosis of hydrocephalus is highly dependent on multiple factors including the age of onset, etiology, ventricular expansion, and extent of neurologic damage prior to correction of the intracranial insult. Mortality rates have been reduced to less than 5% in ten years after shunt placement (22). In one study of 129 children followed ten years post-operatively, who had had shunts placed prior to the age of two, 60% were found to have motor deficits, 25% had visual or auditory deficits, and 30% had epilepsy (22). About 1/3 of children had IQs above 90, 28% had IQs between 70 and 90, and another 30% had IQs below 70. Twenty-one percent had IQs less than 50. The presence of behavioral disorders was frequent in this study. Other researchers have also found a relationship between hydrocephalus and behavior problems (23,24). Among higher functioning children, a discrepancy has been noted between verbal and nonverbal (performance) IQs, with nonverbal skills being reduced in hydrocephalic children (23,24,25,26). It has been postulated that disruption of cerebral white matter tracts leads to this decrease in nonverbal skills, which may promote behavioral maladjustment in these children. These studies indicate that despite the decreased mortality associated with hydrocephalus, there is still much long-term morbidity associated with the disorder. Multidisciplinary planning and close follow-up is needed to ensure the maximal developmental potential of these children.
Questions
1. Define hydrocephalus and distinguish this term from macrocephaly and megalencephaly.
2. What are the two classic classifications of hydrocephalus and give examples of each?
3. What are the most common causes of congenital hydrocephalus?
4. What is X-linked hydrocephalus?
5. True/False: The Dandy-Walker syndrome is usually diagnosed at birth.
6. What is the purpose of routine cranial ultrasound screening in the very low birth weight infant?
7. True/False: CT is the best imaging method for the diagnosis of hydrocephalus after the neonatal period?
8. What is the frequency of shunt failure after initial surgical treatment of hydrocephalus?
9. What is the rate of infection after shunt insertion, and what is the most likely etiologic agent?
10. True/False: Most children with hydrocephalus go on to have IQs consistent with mental retardation.
References
1. Fishman MA. Chapter 396 - Developmental Defects. In: McMillan JA, DeAngelis CD, Feigin RD, Warshaw JB (eds). Oski's Pediatrics: Principles and Practice, 3rd edition. 1999, Philadelphia: Lippincott Williams and Wilkins, pp. 1906-1909.
2. Ashwal S. Chapter 17 - Congenital Structural Defects. In: Swaiman KF, Ashwal S (eds). Pediatric Neurology: Principles and Practice, third edition. 1999, St. Louis: Mosby, pp. 266-273.
3. Menkes JH, Sarnat HB. Chapter 4 - Neuroembryology, Genetic Programming, and Malformations. In: Menkes JH, Sarnat HB (eds). Child Neurology, sixth edition. 2000, Philadelphia: Lippincott Williams and Wilkins, pp. 354 - 377.
4. James HE. Hydrocephalus in Infancy and Childhood. Am Fam Phys 1992;45(2):733-742.
5. Pattisapu JV. Etiology and Clinical Course of Hydrocephalus. Neurosurg Clin North Am 2001;12(4):651-659.
6. Partington MD. Congenital Hydrocephalus. Neurosurg Clin North Am 2001;12(4):737-742.
7. Fishman MA. Chapter 25.3 - Disturbances in Neural Tube Closure and Spine and Cerebrospinal Fluid Dynamics. In: Rudolph AM, Rudolph CD (eds). Rudolph's Pediatrics, 21st edition. 2003, New York: McGraw-Hill Medical Publishing Division, pp. 2179-89.
8. Thilo EH, Rosenberg AA. Chapter 1 - The Newborn Infant. In: Hay Jr. WW, Hayward AR, Levin MJ, Sondheimer JM (eds). Current Pediatric Diagnosis, 16th edition. 2003, New York: The McGraw-Hill Companies, Inc., pp. 35-36.
9. Viola L, Chiaretti A, Castorina M, et al. Acute hydrocephalus as a consequence of mumps meningoencephalitis. Pediatr Emerg Care 1998;14(3):212-214.
10. Ogata H, Oka K, Mitsudome A. Hydrocephalus due to acute aqueductal stenosis following mumps infection: of a case and review of the literature. Brain Dev 1992;14(6):417-419.
11. Rotilio A, Salar G, Dollo C, et al. Aqueductal stenosis following mumps virus infection (Case Report). Ital J Neurol Sci 1985;6(2):237-239.
12. Pascual-Castroviejo I, Velez A, Pascual-Pascual SI, et al. Dandy-Walker malformation: analysis of 38 cases. Childs Nerv Syst 1991;7(2):88-97.
13. Hudgins RJ. Posthemorrhagic hydrocephalus of infancy. Neurosurg Clin North Am 2001;12(4):743-751.
14. Ment LR, Bada HS, Barnes P, et al. Practice parameter: Neuroimaging of the neonate. Neurology 2002;58(12):1726-1738.
15. Bradley Jr WG. Diagnostic tools in hydrocephalus. Neurosurg Clin North Am 2001;12(4):661-684.
16. Kanev PM, Park TS. Treatment of hydrocephalus. Neurosurg Clin North Am 1993;4(4):611-619.
17. Li V. Methods and complications in surgical cerebrospinal fluid shunting. Neurosurg Clin North Am 2001;12(4):685-693.
18. Lo P, Drake JM. Shunt malfunctions. Neurosurg Clin North Am 2001;12(4):695-701.
19. Drake JM, Kestle J. Determining the best cerebrospinal fluid shunt valve design: the pediatric valve design trial. Neurosurgery 1996;38(3):604-607.
20. Bayston R. Epidemiology, diagnosis, treatment, and prevention of cerebrospinal fluid shunt infections. Neurosurg Clin North Am 2001;12(4):703-708.
21. Drake JM, Kestle JRW, Milner R, et al. Randomized trial of cerebrospinal fluid shunt valve design in pediatric hydrocephalus. Neurosurgery 1998;43(2);294-303.
22. Hoppe-Hirsch E, Laroussinie F, Brunet L, et al. Late outcome of the surgical treatment of hydrocephalus. Childs Nerv Syst 1998;14(3):97-99.
23. Riva D, Milani N, Giorgi C. Intelligence outcome in children with shunted hydrocephalus of different etiology. Childs Nerv Syst 1994;10(1):70-73.
24. Fletcher JM, Brookshire BL, Landry SH, et al. Behavioral adjustment of children with hydrocephalus: relationships with etiology, neurological, and family status. J Pediatr Psychol 1995;20(1):109-125.
25. Hirsch JF. Consensus: long-term outcome in hydrocephalus. Childs Nerv Syst 1994;10(1):64-69.
26. Brookshire BL, Fletcher JM, Bohan TP, et al. Verbal and nonverbal skill discrepancies in children with hydrocephalus: a five-year longitudinal follow-up. J Pediatr Psychol 1995;20(6):785-800.
Answers to questions
1. Hydrocephalus refers to pathological enlargement of the cerebral ventricles secondary to a mismatch between the amount of production of CSF and its drainage. Macrocephaly is a general term for any head circumference greater than two standard deviations from the mean. Megalencephaly refers to increased volume of the brain parenchyma.
2. Hydrocephalus is divided into two types: communicating and non-communicating. Communicating hydrocephalus is used if CSF flows freely throughout the ventricular system. Non-communicating hydrocephalus indicates that obstruction of CSF occurs somewhere within the ventricular system, including the outlet foramina of Luschka and Magendie. Communicating hydrocephalus may occur from scarring of the leptomeninges after viral or bacterial meningitis, or after a hemorrhagic brain event where the breakdown products of blood lead to diffuse fibrosis of the meninges. Non-communicating hydrocephalus occurs in cases of discreet obstruction within the ventricular system, such as occurs with aqueductal stenosis, the Chiari Malformations, the Dandy-Walker malformation, or mass effect from brain tumors or other mass lesions.
3. The most common causes of congenital hydrocephalus are Chiari malformations and aqueductal stenosis.
4. X-linked hydrocephalus is a form of aqueductal stenosis in which there is a mutation on the X-linked recessive L1 gene, which produces a family of abnormal neuronal cell adhesion molecules that leads to narrowing and obstruction at the level of the cerebral aqueduct.
5. False. The Dandy-Walker malformation, although present at birth, is responsible for less than 5% of cases of congenital hydrocephalus. Approximately 80% of cases will eventually be detected by one year of age.
6. When present, intraventricular bleeding in the very low birth weight infant usually occurs within the first 72 hours of life. Because up to 50% of these events will occur without immediate clinical symptomology, it is recommended that routine screening be performed between days 4 to 7 of life.
7. False. MRI is the preferred imaging method for the diagnosis of hydrocephalus after the neonatal period as it will also elucidate more precisely than CT the specific etiology of the hydrocephalus. However, in an emergency situation, CT is preferred because it can be done rapidly.
8. Shunt malfunction is a fairly common occurrence with a one-year failure rate of 30 to 40%.
9. The rate of infection after shunt insertion varies among different institutions, and has been reported from 1 to 10%. The most likely etiologic agent is Staphylococcus epidermidis.
10. False. The overall outcome and prognosis of hydrocephalus is highly dependent on multiple factors, including age of onset, etiology, the rate of ventricular expansion, and the extent of neurologic damage prior to shunt placement or other corrective intervention. In one study that looked at 129 children 10 years after shunt placement (shunts were placed prior to age two), approximately 60% had IQs over 70, which is generally considered the cutoff for one of the diagnostic criteria for mental retardation.