neonatal brachial plexus palsy: current knowledge · 2018-12-13 · susceptibility to brachial...
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Neonatal Brachial
Plexus Palsy:
Current Knowledge
Michele J. Grimm, Ph.D.
Department of Biomedical Engineering
© Michele Grimm, 2015
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NBPP and Litigation
• It is permanent NBPP that may result in
litigation
• Cases are not filed based on a shoulder
dystocia alone
• Our understanding of the mechanisms of
injury comes from temporary and
permanent injuries
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NBPP and Shoulder
Dystocia • Dystocia – abnormal, slow, or difficult child birth
process
• Shoulder Dystocia – delay in delivery of the infant
involving the shoulders
• Only a shoulder dystocia involving the anterior
shoulder will be observable
• NBPP can occur
• With a shoulder dystocia involving the
affected (anterior) limb
• With a shoulder dystocia involving the
contralateral limb (anterior shoulder in SD,
posterior arm NBPP)
• Without any shoulder dystocia (either arm)
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Current Scientific Agreement on NBPP
• The primary force that injures the brachial plexus during the birth process is tension (pulling) on the nerve
• Injuries can happen in the absence of clinician-applied traction
• Stretch to the brachial plexus occurs during deliveries as a result of maternal forces alone
• BPI can occur to anterior or posterior shoulders and with or without a clinical shoulder dystocia
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Current Debates on NBPP
• Is the pulling of the nerve that causes injury primarily due to clinician-applied traction or due to maternal forces?
• If maternal forces can stretch the brachial plexus when a shoulder is restrained by the mother’s pelvis, how will even normal traction add to that stretch?
• Can permanent injuries be caused by maternal forces alone?
• What is the injury threshold for the infant brachial plexus?
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How do Maternal Forces
Stretch the Brachial Plexus?
• Spinal loading: driving force from the rear
• Loading to the infant’s bottom through the
uterus will continue up spine
• Spine in compression acts as a solid rod
• Will transmit force through to cervical spine,
continuing to move head forward
• If shoulder stuck, force will still try to move
spine/neck/head forward and will widen angle
between shoulder and neck
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Important Concepts
Related to Nerve Injury
• Nerves can be injured through compression
(crushing) or tension (pulling)
• Tension or traction of nerves does not
necessarily require pulling on the human
body
• A combination of compression and tension
is more likely to cause injury than one of
these alone
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Important Concepts
Related to Nerve Injury
• Nerve injuries occur along a continuum of severity
• Neuropraxia: sustained “falling asleep” of a limb
• Partial rupture: some nerve fibers are still connected, but amount of innervation of muscles is reduced
• Complete rupture: no axons remain connected, chance of spontaneous healing is minimal
• Avulsion: rupture at the connection of the nerve to the spinal cord, which does not provide any nerve to which a graft can be connected
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Important Concepts
Related to Nerve Injury
• Nerves are a biological tissue
• There is no single value for nerve strength
• Whether a nerve will fail under a given force or stretch depends on many variables related to the individual in question
• Anatomy
• Tissue properties
• Where a nerve fails depends on the weak point of that nerve (rupture vs avulsion)
• The same injury in two individuals does not mean that the same force was applied to both nerves
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Maternal Delivery Forces –
Clinical Estimates
• Calculated based on clinical measurements of
intrauterine pressure
• Varies based on intrauterine pressure
• Up to 120 mmHg
• Depends on the cross-sectional area of
baby’s torso
• For a 50th percentile male
• 30 – 40 lbf during the 2nd stage of labor
• The level of maternal forces cannot be
compared directly to clinician-applied forces to
estimate injury risk – the key factor is how the
force stretches the brachial plexus
ACOG, 2014
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Applied Delivery Forces –
Clinical Measurements
• Force sensors on hands*
• Normal: 3.9 – 12.3 lbf
• “Difficult”: 11 – 16 lbf
• Shoulder Dystocia: 11 – 22.5 lbf
• Force plate under feet**
• Normal: 3.26 – 12.2 lbf
• Approximately 75- 100 deliveries
• 3 clinicians
• 4 shoulder dystocias
• 1 temporary BPP
*Allen, O&G, 1991; Poggi, AJOG, 2004; Poggi, AJOG, 2005;
** Peisner, AJOG, 2011.
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Delivery Forces and NBPP
– Clinical Measurements
• Single, large scale prospective study by Mollberg
• 31,000 deliveries -- 18 permanent BPI
• Clinicians asked to mark on a scale from 0 to 100
• 0 – no force
• 100 – ”greatest force you would apply”
• No attempt to equate with actual force
• Permanent BPI
• More likely to have force greater than 50% of
“greatest force you would apply”
• 17 of 18 permanent BPI had fundal pressure
applied after the head delivered
• 18 of 18 permanent BPI had at least 3
attempts at pushing after head delivered
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Computer Modeling:
BP Stretch and Delivery Force Predictions
0
2
4
6
8
10
12
14
16
18
20
0
5
10
15
20
25
30
MaternalForces -StandardPosition
MaternalForces -
McRobertsPosition
MaternalForces -
LithotomyPosition
(NoDelivery)
ClinicianForces -StandardPosition(Axial)
ClinicianForces -StandardPosition
(Bending)
Re
su
ltin
g B
rac
hia
l P
lex
us
Str
etc
h (
%)
Ap
pli
ed
De
live
ry F
orc
e (
lbf)
Effect of Delivery Forces in a Shoulder Dystocia
DeliveryForce
BrachialPlexusStretch
Gonik, AJOG, 189:1168, 2003
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Computer Modeling:
BP Stretch and Delivery Force Predictions
Gonik, AJOG, 2003 & 2010
0
2
4
6
8
10
12
14
16
0
2
4
6
8
10
12
14
16
18
ClinicanForces -
LithotomyPosition
ClinicanForces -
McRoberts(30 deg)
ClinicanForces -
McRoberts(20 deg)
ClinicanForces - 80 NSuprapubicPressure
(Lithotomy)
ClinicanForces -Oblique
Positioning
ClinicanForces -
Posterior ArmDelivery
Re
su
ltin
g B
rac
hia
l P
lex
us
Str
etc
h (
%)
Ap
pli
ed
Fo
rce
(lb
f)
Effect of Clinican Maneuvers in a Shoulder Dystocia
DeliveryForce
BrachialPlexusStretch
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Physical Modeling:
BP Stretch and Delivery Force Predictions
0
5
10
15
20
25
30
35
0
2
4
6
8
10
12
14
16
18
ClinicianForces -
McRobertsPosition
ClinicianForces -PosteriorRubins
ClinicianForces -AnteriorRubins
MaternalForces - NoSD (BD =11.9 cm)
MaternalForces -
Unilateral SD(BD = 12.4
cm)
MaternalForces -
Bilateral SD(BD = 12.9
cm)
Bra
ch
ial P
lex
us
Str
etc
h
De
live
ry F
orc
e (
lbf)
Effect of Force Type and Maneuvers
DeliveryForce (N)
Anterior BPStretch (mm)
Posterior BPStretch (mm)
Anterior BPStretch (%)
Posterior BPStretch (%)
AJOG: Gurewitsch, 2005; Allen, 2007
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Physical Modeling: Delivery
Force Predictions
• Delivery forces measured in clinical simulations
of shoulder dystocia before any new training
• Delivery of posterior arm required to relieve
SD
• 113 clinicians
• Maximum traction applied if no delivery:
• 1.35 – 53 lbf
• 10 – 260 seconds after start of sim
• Maximum traction applied if delivered:
• 10.3 – 56 lbf
• 50 – 250 seconds after start of sim
Crofts, AJOG, 2007
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Forces and BP Stretch -
Summary
• Maximum clinician-applied force measured in a
clinical delivery: 22.5 lbf
• Maximum clinician-applied force measured in a
simulator that would not deliver without a
tertiary maneuver: 56 lbf
• Maximum stretch predicted due to clinician-
applied forces
• 16 lbf traction in McR – 30% (phys model)
• Early physical model
• 18 lbf bending – 18.2% (computer model)
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Forces and BP Stretch -
Summary
• Maximum stretch predicted due to maternal
forces during shoulder impaction
• 22.5 lbf in Lithotomy (shoulder remains
stuck) – 18% (computer model)
• 28 lbf in Lithotomy (shoulder cleared
spontaneously) – 15.7% (computer model)
• Lithotomy (anterior SD – physical model)
• Anterior: 10.0 +/- 3.3%
• Posterior: 14.5 +/- 4.5%
• Lithotomy (bilateral SD – physical model)
• Anterior: 10.4 +/- 6.6%
• Posterior: 15.3 +/- 3.4%
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Susceptibility to Brachial
Plexus Injuries
• How much an a neonatal brachial plexus
stretch before it is injured?
• Not directly measured in infant BP
• Surrogate studies are required
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Susceptibility to Brachial
Plexus Injuries
• Kalmin (1995)*:
• Russian study of elastic and failure
properties of neonatal/fetal C3 and C4
nerves
• Measured up to 50% stretch before
failure
• C3 and C4 responded differently
• Did not follow modern practices for
measuring stretch in the nerves
Kalmin, Morfologiia, 111:39, 1997
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Susceptibility to Brachial
Plexus Injuries
Original
Length
New Length 1
Stretch of the
Nerve
New Length 2
Slip of Nerve
in Grips
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Susceptibility to Brachial
Plexus Injuries
• Singh et al. (2006)*:
• Spinal nerve roots of rats fail at a wide range of
strains
• 29+/- 9% failure strain - what does that mean?
• 2/3 of nerves in the population will fail
between 20 and 38% stretch
• 1/6 of nerves in the population will fail
between 11 and 20% stretch
Singh, J Biomech, 39:1669, 2006
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Why Don’t More Injuries
Occur? • To be injured, an infant’s shoulders must
be restrained as forces move the infant’s
head and neck forward
• Nominally 1-2% of deliveries
• Out of those 1-2% of infants, what is the
overlap with the population that is most
susceptible to injury?
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Population Statistics
High injury risk
Shoulder dystocia
All births
No PBPP due to
maternal forces
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Population Statistics
High injury risk
Shoulder dystocia
All births
10-13% of SD
result in PBPP
due to maternal
forces
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Population Statistics
High injury risk
Shoulder dystocia
All births
Some portion of
SD result in
PBPP due to
maternal forces
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Susceptibility to Brachial Plexus Injuries
• What makes one baby more susceptible
to injuries than another?
• $64,000 question
• NOTE: All of these are generalities!
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Susceptibility to Brachial Plexus Injuries
• Surrounding Tissue Properties: the less stiff the shoulder and neck, the greater the amount of force and stretch that will be experienced by the nerve
• As muscle tone goes down, the stiffness of the shoulder and neck will go down
• 1 minute Apgar score lower than 7 significantly increases risk of injury*
• Increased risk even higher if 5 minute Apgar is less than 7
McFarland, Obst Gyn, 68:784, 1986
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Susceptibility to Brachial Plexus Injuries
• Anatomy: smaller babies will experience more stretch to their tissues (including BP) for the same amount of applied force
• Smaller structures are less stiff and will stretch more
• Force Applied: within the same infant a larger applied force will cause a larger stretch and increase the risk of injury
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Susceptibility to Brachial Plexus Injuries
• Properties of the nerve itself
• Stiffness - less stiff nerves will stretch more, but will not necessarily fail earlier
• Failure strength or strain - how much force or stretch the nerve can take before it fails
• These will all vary between individuals and may depend on some factors in yet unknown ways
• Effect of diabetes?
• Effect of in utero positioning, compression, or development?
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Conclusions • Infant brachial plexus can be stretched
significantly
• Due to maternal forces
• Due to clinician applied forces
• In both anterior and posterior shoulder
• Stretch due to bending is greatest
• Stretch due to maternal forces is higher than
caused by axial traction in lithotomy or during
maneuvers
• Most recent data on nerve injury thresholds
indicates that some infants will sustain a
permanent injury at stretch levels that occur due
to maternal forces
• All infants are different – the pattern of injury
cannot be used to determine the amount of force
applied