Perpendicular Incidence occurs when soundwave hits a surface or tissue boundary that is perpendicular to the media. The variables involved in perpendicular incidence consists of the incident pulse, reflected pulse, and transmitted pulse. The incident pulse is the sound sent out from the transducer and hits the first media at a 90 degree angle at 100%. Once the incident pulse hits the surface, and depending upon the impedance of the first medium, an echo is "reflected" back to the sound source and this is called the reflected pulse. If, however, the impedance on the first medium is less, then a pulse echo is transmitted through the first medium into the second thereby inducing a transmitted pulse. The reflected pulse is the only sound that changes direction opposite to the incident pulse, whereas the transmitted pulse is unchanged and remains in the same direction as that of the incident pulse.
Impedance is the resistance of the medium, and is equal to the density(p) multiplied by the propagation speed. Impedance units are in rayls, and the greater the acoustic impedance, the more dense the material (i.e. bone, air). Therefore, if the medium is more resistant (more incompressible), then density and propagation speed increases. This will cause more echoes to reflect back to the sound source thus an image will be produced (mostly shadowing or attenuation). On the other hand, if resistance or impedance decreases, then more transmitted pulses are generated and no image will be produced (i.e. CW).
Impedance can be broken down into two types: Intensity Reflection Coefficient (IRC) and Intensity Transmission Coefficient (ITC). IRC is the fraction or percentage of the reflected intensity going back through the first medium, and is determined by dividing the relected echo and incident pulse. ITC, on the other hand, is the fraction or percentage of transmitted intensity going through the first medium and into the second, and is determined by dividing the transmitted echo and incident pulse. Both IRC and ITC depend upon the impedances of media one and media two (Z1 & Z2). If there is a large difference between the two media, a stronger echo is produced and IRC increases. If the two media are almost alike, a weaker echo is produced and ITC increases. If however, both mediums are equal, then no echo is produced thus no reflection and ITC is equal to the incident intensity.
If sound hits the media at an oblique angle, then Oblique Incidence occurs. In Oblique Incidence, echoes may reflect back to the sound source or may transmit or "refract" through the medium. Oblique Incidence is defined as the change or shift in the direction of sound due to differences of propagation speeds. As previously mentioned, Perpendicular Incidence depend upon the resistance or impedance of the medium, whereas in Oblique Incidene, the variables invovled depend upon the propagation speeds on either side of the boundary. Oblique Incidence consists of the incident angle, reflected angle, and transmitted angle. The incident angle is "always" proportional to the reflected angle. The transmitted angle plays a role in refraction and depends upon the difference between the propagation speed of medium one and medium two.
Refraction is the change of direction in sound when crossing a boundary. It is caused by Oblique Incidence and a difference in propagation speeds on either side of the boundary. That is, if the propagation speed in medium two is greater, then the transmitted angle increases thus making it greater than the incident angle (and vice-versa). If, however, both of the propagation speeds are equal, then the transmitted angle will be proportional to the incident angle. The relationship between the incident and transmitted angle can be expressed in Snell's Law
Sunday, January 3, 2010
Attenuation
Keywords:
Amplitude
Intensity
Penetration
Incident Pulse
Refraction
Reflection
Scattering
Absorption
Inhomogenous/Heterogenous
Half Power Distance
Noise
Shadowing
Anechoic
Output Power
TGC
Attenuation is the reduction of amplitude and intensity as sound propagates or penetrates farther down into the medium. Amplitude is definded as the maximal cyclical change in a quantiy such as pressure (Mpa), whereas intensity is defined as the rate at which energy passess through a unit area (power over area). Both are indicators of strength of sound and are affected as penetration decrease.
Attenuation is caused by a number of factors: refraction, reflection, scattering, and absorption. Refraction is the change or shift in the direction of sound transmitted from the incident pulse after going through the tissue surface at an angle that may or may not be equal to the incident pulse angle (i.e. Oblique Incidence). Reflection is the reflected echo coming back to the sound source (i.e. transducer)from the tissue surface going in opposite direction to the incident pulse that may be perpendicular or oblique. Scattering is when sound hits an inhomogenous or heterogenous tissue surface and more reflected echoes are sent back to the sound source thus displaying a suboptimal image.
Sound scatter occurs with increasing frequences thus attenuation increases because penetration (depth of images) decreases. Frequency is inversely proportional to penetration, and as penetration decrease, amplitude and intensity become reduced. Therefore, lower frequencies are used for deeper penetration in which attenuation is decreased, whereas higher frequencies are used for structures located superficially requiring no deeper penetration. If, for example, a 5MHz frequency transducer is used on an overweight patient for an abdominal scan, sound scatter would increase thus the image displayed would be highly attenuated. The only way to fix this problem is to select a low frequency transducer, adjust output power, or TGC in order to decrease attenation and improve image resolution.
Absorption is the dominant factor contributing to attenuation of ultrasound in soft tissues, and is defined as the conversion of sound to heat. That is, absorption occurs when the tissue absorbs the sound and converts the echo to heat. As a result, the transferred energy (or heat) becomes lost and does not come back to the sound source thus a suboptimal image is displayed. The rest of the other factors contribute little to attenuation in most cases. The units for attenuation are in decibels (db), and is proportional to half of the frequency multiplied by the path length or wavelength. Therefore, as frequency and the path length or wavelength increases, attenuation increases.
Attenuation is higher in air (i.e. lung) and in bone than in other soft tissues (half power distance = 1-5cm) with a half power distance of 0.08 and 0.7cm, respectively. Attenuation occurs least in liquid or water due to a half power distance of 380cm. The half power distance refers to the distance that sound will travel to a particular tissue before its energy or amplitude is attenuated to half of its original value. Therefore, the presence of air and bone can interfere the ultrasound image by displaying increased noise or shadowing. In the presence of liquid or water, the image displayed in the monitor is anechoic.
In order to avoid attenuation, it is important for the tech to know the factors contributing to attenuation. The half power distances should also be noted and when scanning, the tech should avoid the presence of air or bone, which could interfere ultrasound image. Attenuation could also be avoided by adjusting the frequencies, output power, or TGC, which can be found on or near the machines to improve image resolution.
Amplitude
Intensity
Penetration
Incident Pulse
Refraction
Reflection
Scattering
Absorption
Inhomogenous/Heterogenous
Half Power Distance
Noise
Shadowing
Anechoic
Output Power
TGC
Attenuation is the reduction of amplitude and intensity as sound propagates or penetrates farther down into the medium. Amplitude is definded as the maximal cyclical change in a quantiy such as pressure (Mpa), whereas intensity is defined as the rate at which energy passess through a unit area (power over area). Both are indicators of strength of sound and are affected as penetration decrease.
Attenuation is caused by a number of factors: refraction, reflection, scattering, and absorption. Refraction is the change or shift in the direction of sound transmitted from the incident pulse after going through the tissue surface at an angle that may or may not be equal to the incident pulse angle (i.e. Oblique Incidence). Reflection is the reflected echo coming back to the sound source (i.e. transducer)from the tissue surface going in opposite direction to the incident pulse that may be perpendicular or oblique. Scattering is when sound hits an inhomogenous or heterogenous tissue surface and more reflected echoes are sent back to the sound source thus displaying a suboptimal image.
Sound scatter occurs with increasing frequences thus attenuation increases because penetration (depth of images) decreases. Frequency is inversely proportional to penetration, and as penetration decrease, amplitude and intensity become reduced. Therefore, lower frequencies are used for deeper penetration in which attenuation is decreased, whereas higher frequencies are used for structures located superficially requiring no deeper penetration. If, for example, a 5MHz frequency transducer is used on an overweight patient for an abdominal scan, sound scatter would increase thus the image displayed would be highly attenuated. The only way to fix this problem is to select a low frequency transducer, adjust output power, or TGC in order to decrease attenation and improve image resolution.
Absorption is the dominant factor contributing to attenuation of ultrasound in soft tissues, and is defined as the conversion of sound to heat. That is, absorption occurs when the tissue absorbs the sound and converts the echo to heat. As a result, the transferred energy (or heat) becomes lost and does not come back to the sound source thus a suboptimal image is displayed. The rest of the other factors contribute little to attenuation in most cases. The units for attenuation are in decibels (db), and is proportional to half of the frequency multiplied by the path length or wavelength. Therefore, as frequency and the path length or wavelength increases, attenuation increases.
Attenuation is higher in air (i.e. lung) and in bone than in other soft tissues (half power distance = 1-5cm) with a half power distance of 0.08 and 0.7cm, respectively. Attenuation occurs least in liquid or water due to a half power distance of 380cm. The half power distance refers to the distance that sound will travel to a particular tissue before its energy or amplitude is attenuated to half of its original value. Therefore, the presence of air and bone can interfere the ultrasound image by displaying increased noise or shadowing. In the presence of liquid or water, the image displayed in the monitor is anechoic.
In order to avoid attenuation, it is important for the tech to know the factors contributing to attenuation. The half power distances should also be noted and when scanning, the tech should avoid the presence of air or bone, which could interfere ultrasound image. Attenuation could also be avoided by adjusting the frequencies, output power, or TGC, which can be found on or near the machines to improve image resolution.
Sunday, December 20, 2009
Physics 101: The Basics II
Keywords:
Continous Wave
Pulsed Ultrasound
Pulse Repitition Frequency
Pulse Repitition Period
Duty Factor
Spatial Pulse Length
Bandwidth
Fractional Bandwidth
Quality factor
Continuous wave (CW) is described where cycles are being repeated indefinitely thereby defined as a single frequency. On the other hand, pulsed wave or pulsed ultrasound (PW) is described as an ultrasound pulse of a few cycles seperated by gaps in time. The Pulse Repitition Frequency (PRF) is the number of pulses occuring in one second, and the units are in kilohertz (kHz). The Pulse Repition Period is the time measured from the beginning of one pulse to the beginning of the next, and is reciprocal to PRF. The Pulse Duration (PD) is the time it takes for one pulse to occur, and is proportional or equal to the period times the number of pulses or cycles in the pulse (n). The Pulse Duration (PD) is also reciprocal to the frequency, that is, if the frequency increases, then PD decreases thus a shorter pulse is produced. Shorter pulses are necessary to improve the quality of sonographic images.
In CW, the ultrasound is on 100% all the time, whereas in PW, the ultrasound is not on all the time. The time it takes for ultrasound to be on depends upon the Duty Factor (DF), which is the "fraction" of time that pulsed ultrasound is on. In other words, DF represents the amount of time that the sound is on. The Duty Factor (DF) is proportional to PRF and PD and reciprocal to PRP; that is, as PRF increases then DF increases and PRP decreases. In relation to frequency, the acutal number of cycles in PW depends upon DF as well. For example, a 5-MHz frequency CW ultrasound has 5-million cycles occuring per second, whereas in a 5-MHz frequency PW ultrasound has 50,000 cycles occuring per second or 50kHz. This is due to DF being only 1.0%, which makes the ultrasound being on only one hundreth of the time.
A pulse can be measured by acquiring the Spatial Pulse Length, which is the length of the pulse from front to back. The Spatial Pulse Length is proportional to the number of cycles in a pulse and the wavelength of the pulse. Units for SPL are in millimeters. Since wavelength increases with decreasing frequency, SPL increases with decreasing frequency. Therefore, if frequency increases then SPL and wavelength decreases and a shorter pulse is produced, which improves sonographic image resolution.
As frequency increases, shorter pulses are generated, which will increase the bandwith (more broader) and decrease the wavelength as well as SPL. In other words, the shorter the pulse (the fewer number of cycles), the more frequencies present in it (broader bandwidth). Bandwidth is defined as the range of frequencies contained in a pulse. Therefore, shorter pulses have broader bandwidths and fractional bandwidths, which is the bandwidth divided by the operating frequency and is reciprocal to the Quality factor (Q). In this case, Q would be low and thus signify a shorter pulse with a broad bandwidth at higher frequencies.
Continous Wave
Pulsed Ultrasound
Pulse Repitition Frequency
Pulse Repitition Period
Duty Factor
Spatial Pulse Length
Bandwidth
Fractional Bandwidth
Quality factor
Continuous wave (CW) is described where cycles are being repeated indefinitely thereby defined as a single frequency. On the other hand, pulsed wave or pulsed ultrasound (PW) is described as an ultrasound pulse of a few cycles seperated by gaps in time. The Pulse Repitition Frequency (PRF) is the number of pulses occuring in one second, and the units are in kilohertz (kHz). The Pulse Repition Period is the time measured from the beginning of one pulse to the beginning of the next, and is reciprocal to PRF. The Pulse Duration (PD) is the time it takes for one pulse to occur, and is proportional or equal to the period times the number of pulses or cycles in the pulse (n). The Pulse Duration (PD) is also reciprocal to the frequency, that is, if the frequency increases, then PD decreases thus a shorter pulse is produced. Shorter pulses are necessary to improve the quality of sonographic images.
In CW, the ultrasound is on 100% all the time, whereas in PW, the ultrasound is not on all the time. The time it takes for ultrasound to be on depends upon the Duty Factor (DF), which is the "fraction" of time that pulsed ultrasound is on. In other words, DF represents the amount of time that the sound is on. The Duty Factor (DF) is proportional to PRF and PD and reciprocal to PRP; that is, as PRF increases then DF increases and PRP decreases. In relation to frequency, the acutal number of cycles in PW depends upon DF as well. For example, a 5-MHz frequency CW ultrasound has 5-million cycles occuring per second, whereas in a 5-MHz frequency PW ultrasound has 50,000 cycles occuring per second or 50kHz. This is due to DF being only 1.0%, which makes the ultrasound being on only one hundreth of the time.
A pulse can be measured by acquiring the Spatial Pulse Length, which is the length of the pulse from front to back. The Spatial Pulse Length is proportional to the number of cycles in a pulse and the wavelength of the pulse. Units for SPL are in millimeters. Since wavelength increases with decreasing frequency, SPL increases with decreasing frequency. Therefore, if frequency increases then SPL and wavelength decreases and a shorter pulse is produced, which improves sonographic image resolution.
As frequency increases, shorter pulses are generated, which will increase the bandwith (more broader) and decrease the wavelength as well as SPL. In other words, the shorter the pulse (the fewer number of cycles), the more frequencies present in it (broader bandwidth). Bandwidth is defined as the range of frequencies contained in a pulse. Therefore, shorter pulses have broader bandwidths and fractional bandwidths, which is the bandwidth divided by the operating frequency and is reciprocal to the Quality factor (Q). In this case, Q would be low and thus signify a shorter pulse with a broad bandwidth at higher frequencies.
Friday, December 18, 2009
Physics 101: The Basics
Keywords:
Medium
Acoustic Variables
Pressure
Density
Compression
Rarefaction
Propagation Speed
Stiffness
Longitudinal
Mechanical
Frequency
Infrasound
Ultrasound
Wavelength (Lambda)
Period
Sound is described as a wave that propagates through a medium (i.e air) associated with acoustic (hearing) variables. These acoustic variables are the following: Pressure, Density, Compression, Rarefaction, Temperature, and Particle motion. Without a medium, sound cannot travel (as in a vaccum). Gas, liquids, solids, and soft tissues are the types of mediums that sound can travel with differences of propagation speeds. Essentially, the medium determines the Propagation speed, and the Propagation speed, denoted as c (mm/microsecond), depends upon the stiffness of the material. In other words, the stiffer the material (more incompressible), the higher the propagation speed. Therefore, bone has the highest propagation speed, whereas gas or air has the lowest. In soft tissue, the propagation speed is "constant" and is calculated to be at 1.54mm/microsecond (1540 m/s). With that in mind, whenever the frequency changes (thus wavelength changes), the speed remains unchanged.
As sound travels through a medium, pressure and density go through cycles (frequency) of increase and decrease, and particles of the medium oscillate back and forth. As a result, sound waves can be broken down into two types: longitidunal and mechanical (or transverse). In longitudinal, the sound oscillate back and forth in the same direction of the linear plane or wave travel. In mechanical, sound inceases and decreases becoming perpendicular to the linear plane (90 degree angle). Therefore,sound is a wave of pressure and density variations with particle vibrations and is broken down into longitidinal and mechanical that describe the frequency or wave cycle.
Frequency is defined as the number of cycles in one second. A single cycle consists of high and low pressure associated with density. Pressure is proportional to density, that is, as pressure increases, density increases as well (and vice versa). Density is defined as mass/volume, and should not be confused being proportional to "intensity". Regions of high pressure and density are called compressions, whereas regions of low pressure and density are called rarefactions. Therefore, frequency can also be defined as the number of compressions and rarefactions in one second.
The different types of frequencies are the following: Hz, kHz (1000), and MHz(1,000,000). When comparing infrasound to ultrasound, the range goes from 20Hz to 20,000Hz (20kHz). Infrasound is below 20Hz, whereas ultrasound is greater than 20,000MHz (20kHz). The frequency depends upon the transducer and is reciprocal to the wavelength and period or time. In other words, as the frequency increases, the wavelength and period decreases. Wavelength is defined as the length of space over which one cycle occurs (front to back) and is proportional to the period or time. The period is the time it takes for one cycle to occur. Therefore, as the wavelength or lambda increases, then the period or time increases as well.
Medium
Acoustic Variables
Pressure
Density
Compression
Rarefaction
Propagation Speed
Stiffness
Longitudinal
Mechanical
Frequency
Infrasound
Ultrasound
Wavelength (Lambda)
Period
Sound is described as a wave that propagates through a medium (i.e air) associated with acoustic (hearing) variables. These acoustic variables are the following: Pressure, Density, Compression, Rarefaction, Temperature, and Particle motion. Without a medium, sound cannot travel (as in a vaccum). Gas, liquids, solids, and soft tissues are the types of mediums that sound can travel with differences of propagation speeds. Essentially, the medium determines the Propagation speed, and the Propagation speed, denoted as c (mm/microsecond), depends upon the stiffness of the material. In other words, the stiffer the material (more incompressible), the higher the propagation speed. Therefore, bone has the highest propagation speed, whereas gas or air has the lowest. In soft tissue, the propagation speed is "constant" and is calculated to be at 1.54mm/microsecond (1540 m/s). With that in mind, whenever the frequency changes (thus wavelength changes), the speed remains unchanged.
As sound travels through a medium, pressure and density go through cycles (frequency) of increase and decrease, and particles of the medium oscillate back and forth. As a result, sound waves can be broken down into two types: longitidunal and mechanical (or transverse). In longitudinal, the sound oscillate back and forth in the same direction of the linear plane or wave travel. In mechanical, sound inceases and decreases becoming perpendicular to the linear plane (90 degree angle). Therefore,sound is a wave of pressure and density variations with particle vibrations and is broken down into longitidinal and mechanical that describe the frequency or wave cycle.
Frequency is defined as the number of cycles in one second. A single cycle consists of high and low pressure associated with density. Pressure is proportional to density, that is, as pressure increases, density increases as well (and vice versa). Density is defined as mass/volume, and should not be confused being proportional to "intensity". Regions of high pressure and density are called compressions, whereas regions of low pressure and density are called rarefactions. Therefore, frequency can also be defined as the number of compressions and rarefactions in one second.
The different types of frequencies are the following: Hz, kHz (1000), and MHz(1,000,000). When comparing infrasound to ultrasound, the range goes from 20Hz to 20,000Hz (20kHz). Infrasound is below 20Hz, whereas ultrasound is greater than 20,000MHz (20kHz). The frequency depends upon the transducer and is reciprocal to the wavelength and period or time. In other words, as the frequency increases, the wavelength and period decreases. Wavelength is defined as the length of space over which one cycle occurs (front to back) and is proportional to the period or time. The period is the time it takes for one cycle to occur. Therefore, as the wavelength or lambda increases, then the period or time increases as well.
Monday, December 14, 2009
Holoprosencephaly (HPE)
What is Holoprosencephaly?
Holoprosencephaly (HPE) is a birth defect that occurs during the first few weeks of intrauterine life. HPE is a disorder in which the fetal brain does not grow forward and divide as it is supposed to during early pregnancy (incomplete cleavage of the embryonic forebrain/failure of the prosencephalon to cleave into the cerebral and lateral hemispheres).
This brain malformation can range from mild to severe and is classified into four types:
(1) Alobar (severe)--where the brain is not divided and there are severe abnormalities (there is an absence of the interhemispheric fissure, a single primitive ventricle, fused thalami, and absent third ventricle, olfactory bulbs and tracts and optic tracts).
(2) Semi-Lobar (moderate)--where the brain is partially divided and there are some moderate abnormalities; where there are two hemispheres in the rear but not the front of the brain (there are partially separated cerebral hemispheres and a single ventricular cavity).
(3) Lobar (mild)--where the brain is divided and there are some mild abnormalities (there is a well developed interhemispheric fissure however there is some fusion of structures).
(4) Middle Interhemispheric Variant (MIHV) -- where the middle of the brain (posterior frontal and parietal lobes) are not well separated.
Children diagnosed with HPE may have a small head (microcephaly), excessive fluid in the brain (hydrocephalus), variable degrees of mental retardation, epilepsy, endocrine abnormalities, or abnormalities of other organ systems such as cardiac, skeletal, genitourinary, and gastrointestinal. Mildly affected children may exhibit few symptoms and may live a normal life.
Facial deformities are often present in many children diagnosed with HPE. Mild forms of facial abnormalities may include a flat single-nostril nose (cebocephaly), close set eyes (hypotelorism), cleft lip and/or palate, or just one upper middle tooth (single maxillary central incisor). More severe facial deformities may include a single central eye (cyclopia), a nose located on the forehead (proboscis), or missing facial features.
What causes holoprosencephaly?
The cause of HPE is currently unknown. Often, no specific cause can be identified. Suggested risk factors include maternal diabetes, infections during pregnancy (syphilis, toxoplasmosis, rubella, herpes, cytomegalovirus), and various drugs taken during pregnancy (alcohol, aspirin, lithium, thorazine, anticonvulsants, hormones, retinoic acid). Women with previous pregnancy loss and first trimester bleeding are also more likely to have a child diagnosed with HPE.
Although many children with HPE have normal chromosomes, specific chromosomal abnormalities have been identified in some patients. There is evidence that in some families, HPE is inherited (autosomal dominant as well as autosomal or X-linked recessive inheritance).
Several genes have been identified that play a role in holoprosencephaly.
How common is this defect?
It is estimated that HPE affects between 1 in 5,000-10,000 live births. Since many pregnancies with a fetus diagnosed with HPE end in miscarriage, the frequency of HPE among all pregnancies may be as high as 1 in 200-250. Current studies indicate that only 3% of all fetuses with HPE survive to delivery and the vast majority of these infants do not survive past the first six months of life. The prognosis for a child diagnosed with HPE depends on the type of HPE and the presence of associated anomalies. The most severely affected children may live several months or years and the least affected may live a normal life span. Almost two-thirds of affected patients have alobar HPE and approximately one quarter are diagnosed with semilobar HPE.
Summary
HPE is characterized by a failure of transformation of the prosencephalon into cerebral hemispheres with separate lateral ventricles. HPE has many associated anomalies, both of the nervous system and face. HPE is also associated with malformations in other body systems, particularly when it has a chromosomal etiology. The true spectrum of HPE, its clinical manifestations, and underlying etiologies require further elucidation. Applying this knowledge to individuals and their families is of utmost importance.
References
Cohen (1989) Teratology 40:211-235
Cohen (1989) Am J of Med Genet 34:271-288
Cohen & Sulik (1992) J of Craniofacial Genet Dev Bio 12:196-244
DeMyer & Zeman (1963) Confin Neurol 23:1-36
DeMyer, Zeman, & Palmer (1964) Pediatrics 34: 256-263
Ming & Muenke (l998) Clin Genet 53:155-163
Plawner LL, et al. (2002) Neuroanatomy of Holoprosencephaly as Predictor of Function: Beyond the Face Predicting the Brain. Neurology, 59:1058-1066.
Lewis AJ, et al. (2002) Middle interhemispheric variant of holoprosencephaly: a distinct cliniconeuroradiologic subtype. Neurology (in press).
spotted at The Carter Centers for Brain Research
Holoprosencephaly (HPE) is a birth defect that occurs during the first few weeks of intrauterine life. HPE is a disorder in which the fetal brain does not grow forward and divide as it is supposed to during early pregnancy (incomplete cleavage of the embryonic forebrain/failure of the prosencephalon to cleave into the cerebral and lateral hemispheres).
This brain malformation can range from mild to severe and is classified into four types:
(1) Alobar (severe)--where the brain is not divided and there are severe abnormalities (there is an absence of the interhemispheric fissure, a single primitive ventricle, fused thalami, and absent third ventricle, olfactory bulbs and tracts and optic tracts).
(2) Semi-Lobar (moderate)--where the brain is partially divided and there are some moderate abnormalities; where there are two hemispheres in the rear but not the front of the brain (there are partially separated cerebral hemispheres and a single ventricular cavity).
(3) Lobar (mild)--where the brain is divided and there are some mild abnormalities (there is a well developed interhemispheric fissure however there is some fusion of structures).
(4) Middle Interhemispheric Variant (MIHV) -- where the middle of the brain (posterior frontal and parietal lobes) are not well separated.
Children diagnosed with HPE may have a small head (microcephaly), excessive fluid in the brain (hydrocephalus), variable degrees of mental retardation, epilepsy, endocrine abnormalities, or abnormalities of other organ systems such as cardiac, skeletal, genitourinary, and gastrointestinal. Mildly affected children may exhibit few symptoms and may live a normal life.
Facial deformities are often present in many children diagnosed with HPE. Mild forms of facial abnormalities may include a flat single-nostril nose (cebocephaly), close set eyes (hypotelorism), cleft lip and/or palate, or just one upper middle tooth (single maxillary central incisor). More severe facial deformities may include a single central eye (cyclopia), a nose located on the forehead (proboscis), or missing facial features.
What causes holoprosencephaly?
The cause of HPE is currently unknown. Often, no specific cause can be identified. Suggested risk factors include maternal diabetes, infections during pregnancy (syphilis, toxoplasmosis, rubella, herpes, cytomegalovirus), and various drugs taken during pregnancy (alcohol, aspirin, lithium, thorazine, anticonvulsants, hormones, retinoic acid). Women with previous pregnancy loss and first trimester bleeding are also more likely to have a child diagnosed with HPE.
Although many children with HPE have normal chromosomes, specific chromosomal abnormalities have been identified in some patients. There is evidence that in some families, HPE is inherited (autosomal dominant as well as autosomal or X-linked recessive inheritance).
Several genes have been identified that play a role in holoprosencephaly.
How common is this defect?
It is estimated that HPE affects between 1 in 5,000-10,000 live births. Since many pregnancies with a fetus diagnosed with HPE end in miscarriage, the frequency of HPE among all pregnancies may be as high as 1 in 200-250. Current studies indicate that only 3% of all fetuses with HPE survive to delivery and the vast majority of these infants do not survive past the first six months of life. The prognosis for a child diagnosed with HPE depends on the type of HPE and the presence of associated anomalies. The most severely affected children may live several months or years and the least affected may live a normal life span. Almost two-thirds of affected patients have alobar HPE and approximately one quarter are diagnosed with semilobar HPE.
Summary
HPE is characterized by a failure of transformation of the prosencephalon into cerebral hemispheres with separate lateral ventricles. HPE has many associated anomalies, both of the nervous system and face. HPE is also associated with malformations in other body systems, particularly when it has a chromosomal etiology. The true spectrum of HPE, its clinical manifestations, and underlying etiologies require further elucidation. Applying this knowledge to individuals and their families is of utmost importance.
References
Cohen (1989) Teratology 40:211-235
Cohen (1989) Am J of Med Genet 34:271-288
Cohen & Sulik (1992) J of Craniofacial Genet Dev Bio 12:196-244
DeMyer & Zeman (1963) Confin Neurol 23:1-36
DeMyer, Zeman, & Palmer (1964) Pediatrics 34: 256-263
Ming & Muenke (l998) Clin Genet 53:155-163
Plawner LL, et al. (2002) Neuroanatomy of Holoprosencephaly as Predictor of Function: Beyond the Face Predicting the Brain. Neurology, 59:1058-1066.
Lewis AJ, et al. (2002) Middle interhemispheric variant of holoprosencephaly: a distinct cliniconeuroradiologic subtype. Neurology (in press).
spotted at The Carter Centers for Brain Research
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