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4.2 Congenital Malformations of the Nervous System


Neural tube defects affect the brain and spinal cord, and are among the most common of the congenital anomalies (see Fig. 4.1). Panel A shows a cross section of the rostral end of the embryo at approximately three weeks after conception, showing the neural groove in the process of closing, overlying the notochord. The neural folds are the rising margins of the neural tube, topped by the neural crest, and demarcate the neural groove centrally. Panel B shows a cross section of the middle portion of the embryo after the neural tube has closed. The neural tube, which will ultimately develop into the spinal cord, is now covered by surface ectoderm (later, the skin). The intervening mesoderm will form the bony spine. The notochord is regressing. Panel C shows the developmental and clinical features of the main types of neural tube defects. The diagram in the centre is a dorsal view of a developing embryo, showing a neural tube that is closed in the centre but still open at the cranial and caudal ends. The dotted lines marked A and B refer to the cross sections shown in Panels A and B. Shaded bars point to the region of the neural tube relevant to each defect.

In anencephaly, the absence of the brain and calvaria can be total or partial. Craniorachischisis is characterized by anencephaly accompanied by a contiguous bony defect of the spine and exposure of neural tissue. In open spina bifida, a bony defect of the posterior vertebral arches (in this case, the lower thoracic vertebrae) is accompanied by herniation of neural tissue and meninges and is not covered by skin. In iniencephaly, dysraphia in the occipital region is accompanied by severe retroflexion of the neck and trunk. In encephalocele, the brain and meninges herniate through a defect in the calvaria. In closed spina bifida, unlike open spina bifida, the bony defect of the posterior vertebral arches (in this case, the lumbar vertebrae), the herniated meninges, and neural tissue are covered by skin.

The most prevalent types of neural tube defects are anencephaly, encephalocele and spina bifida.

Fig. 4.1. Neural tube defects [1]

Neural tube defects

Neural tube defects detailed illustration

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Anencephaly (Q00.0)

Anencephaly (see Fig. 4.2) is characterized by either total or partial absence of the brain, together with total or partial absence of the cranial vault and the covering skin. In addition to the term anencephaly, two other terms are used, although rarely, to describe this anomaly. One is holoanencephaly, in which the bone defect extends through the foramen magnum, affecting the entire skull; in the other, meroanencephaly, the bone defect is limited to the anterior part of the skull.

Two additional terms that are occasionally used as synonyms of anencephaly may be sources of confusion, because they also are used to describe other conditions. One is acrania, often used to refer to acalvaria, or absence of the neurocranium (calvarial bones, dura mater, and associated muscles) and believed to be unrelated to neural tube defects. The other is acephaly, which literally means “absence of the head” and is part of a pattern of anomalies observed in acardiac twins. These two terms – acrania and acephaly – are not coded as anencephaly; a diagnosis of acrania can be scrutinized to determine whether the diagnosis of anencephaly is more appropriate.

  Fig. 4.2. Anencephaly
Anencephaly illustration
Anencephaly (Q00.0)
Anencephaly photo
Anencephaly (Q00.0)[2]

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Craniorachischisis (Q00.1)

Craniorachischisis (see Fig. 4.3) refers to the presence of anencephaly with a contiguous spine defect without meninges covering the neural tissue (rachischisis). It may be limited to the cervical region or affect the entire spine. Neonates with craniorachischisis may also have spinal retroflexion resembling the body habitus of neonates with iniencephaly.

  Fig. 4.3. Craniorachischisis
Craniorachischisis illustration
Craniorachischisis (Q00.1)
Craniorachischisis photograph
Craniorachischisis (Q00.1)[2]

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Iniencephaly (Q00.2)

Iniencephaly (see Fig. 4.4) is a rare and complex neural tube defect involving the occiput and inion, resulting in extreme retroflexion of the head, variably combined with occipital encephalocele or rachischisis of the cervical and thoracic spine. In iniencephaly, the cranium is always closed. This fact helps to differentiate iniencephaly from cases of anencephaly with spinal retroflexion.

  Fig. 4.4. Iniencephaly
Iniencephaly illustration
Iniencephaly (Q00.2)
Iniencephaly photograph
Iniencephaly (Q00.2)[2]


Relevant ICD-10 codes and Royal College of Paediatrics and Child Health (RCPCH) uses:
Q00 Anencephaly and similar malformations (avoid using this general code if more specific information is available)
Q00.0 Anencephaly; Holoanencephaly; Meroanencephaly
Q00.00 Acrania
Q00.00 Amyelencephaly
Q00.01 Hemianencephaly
Q00.01 Hemicephaly
Q00.1 Craniorachischisis
Q00.2 Iniencephaly
Note: in cases in which anencephaly and spina bifida are present, but are not continuous, both are to be coded.
Q00.00 Acephaly
Q75.8 Acalvaria (specified absence of skull bone)
Q79.80 Other congenital malformations of musculoskeletal system. Amniotic band / constriction band presencea
aThe ICBDSR uses Q79.80 to identify the presence of an amniotic band. The anomaly is coded as the code for the specific congenital anomaly, as well as the Q79.80 amniotic band code.

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Encephalocele (Q01, Q01.0, Q01.1, Q01.2, Q01.8, Q01.9, Q61.90)

An encephalocele (see Fig. 4.5) is a pedunculated or sessile cystic lesion protruding through a defect in the skull. Encephaloceles can contain herniated meninges and brain tissue (encephalocele or meningoencephalocele) or only meninges (cranial meningocele). Most frequently, they are located in the occipital area, but in South-East Asia, the anterior location (frontal or nasofrontal) is most common (see Fig. 4.5 a,b) (28). Encephaloceles also are observed in the amniotic band sequence with entrapment of the head.

  Fig. 4.5 Encephalocele
Frontal encephalocele illustration
a. Frontal encephalocele (Q01.0)
Frontal encephalocele photograph
a. Frontal encephalocele (Q01.0)[2]
Nasofrontal encephalocele illustration
b. Nasofrontal encephalocele (Q01.1)
Nasofrontal encephalocele photograph
b. Nasofrontal encephalocele (Q01.1)[3]
Occipital encephalocele illustration
c. Occipital encephalocele (Q01.2)
Occipital encephalocele photograph
c. Occipital encephalocele (Q01.2)[2]


Relevant ICD-10 codes and RCPCH uses
Q01 Encephalocele (avoid using this general code if more specific information is available); Encephalomyelocele; Hydroencephalocele; Hydromeningocele, cranial; Meningocele, cerebral; Meningoencephalocele
Q01.0 Frontal encephalocele
Q01.1 Nasofrontal encephalocele
Q01.2 Occipital encephalocele
Q01.8 Encephalocele of other sites; if more specific codes are available, use the specific code below
Q01.80 Parietal encephalocele
Q01.81 Orbital encephalocele
Q01.82 Nasal encephalocele
Q01.83 Nasopharyngeal encephalocele
Q01.9 Encephalocele, unspecified

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Spina bifida (Q05, Q05.0, Q05.1, Q05.2, Q05.3, Q05.4, Q05.5, Q05.6, Q05.7, Q05.8, Q05.9)

Spina bifida is a general term used to describe a neural tube defect of the spine in which part of the meninges or spinal cord, or both, protrudes through an opening in the vertebral column (see Fig. 4.6). Hydrocephalus is a common complication, especially among children with open (membrane-covered) meningomyeloceles. Specific types of spina bifida include:

  • Meningocele: this type of spina bifida is characterized by herniation of the meninges through a spine defect, forming a cyst filled with cerebrospinal fluid. It does not contain spinal cord, but might have some nerve elements.
  • Meningomyelocele: this type of spina bifida consists of a protrusion of the meninges and the spinal cord through an opening in the vertebral column. This is the most common type of spina bifida, constituting about 90% of all cases.
  • Myelocele: in this type of spina bifida, the open spinal cord, covered by a thin membrane, protrudes through the defect in the vertebral column.


  Fig. 4.6. Spina bifida
Meningocele illustration
a. Meningocele (Q05)
Myelomeningocele illustration
b. Myelomeningocele (Q05)
Myelocele or myeloschisis illustration
c. Myelocele or myeloschisis (Q05)[2]
Lumbar spina bifida photograph
d. Lumbar spina bifida[4]
Cervical spina bifida without hydrocephalus photograph
e. Cervical spina bifida without hydrocephalus (Q05.5)[2]
Lumbosacral spina bifida without hydrocephalus photograph
f. Lumbosacral spina bifida without hydrocephalus (Q05.7)[2]
Lumbar spina bifida without hydrocephalus
g. Lumbar spina bifida without hydrocephalus (Q05.7)


Relevant ICD-10 codes and RCPCH uses:
Q05 Spina bifida (avoid using this general code if more specific information is available) Hydromeningocele (spinal); Meningocele (spinal); Meningomyelocele Myelocele; Myelomeningocele; Spinal rachischisis; Spina bifida (aperta) (cystica); Syringomyelocele
Q05.0 Cervical spina bifida with hydrocephalus
Q05.1 Thoracic spina bifida with hydrocephalus
Q05.2 Lumbar spina bifida with hydrocephalus
Lumbosacral spina bifida with hydrocephalus
Q05.3 Sacral spina bifida with hydrocephalus
Q05.4 Unspecified spina bifida with hydrocephalus (site unspecified)
Q05.5 Cervical spina bifida without hydrocephalus
Q05.6 Thoracic spina bifida without hydrocephalus
Q05.7 Lumbar spina bifida without hydrocephalus
Lumbosacral spina bifida without hydrocephalus
Q05.8 Sacral spina bifida without hydrocephalus
Q05.9 Spina bifida, unspecified
Note: in cases in which anencephaly and spina bifida are present, but are not continuous, both are to be coded; however, when the malformations are counted, only anencephaly is counted.


Q07.0 Arnold–Chiari syndrome
Q76.0 Spina bifida occulta


  1. Source: adapted, with permission  from the publisher, from Botto et al. N. Engl. J. Med. 1999;341:1509–19.
  2. Photograph source: courtesy of CDC-Beijing Medical University collaborative project.
  3. Photograph source: courtesy of Jaime Frías, MD, USA.
  4. Photograph source: courtesy of Idalina Montes, MD and Rafael Longo, MD, FACS, Puerto Rico.

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