Hypotonic Labor

Etiology

The cause of hypotonic labor is uterine inertia, also known as hypotonic or hypocontractile uterine dysfunction. Though the etiology of the inertia is unknown, these conditions are commonly associated with hypocontractile uterine dysfunction:

• Uterine overdistension and overuse as seen in multifetal gestation, fetal macrosomia, polyhydramnios, and grand-multiparity
• Mechanical disruption of myometrial function from myoma or distension of the bladder or bowel
• Malpositioning and malpresentation of the fetus, where there is an absent reflex in uterine contraction, due to inadequate contact of the presenting part onto the lower uterine segment
• Abnormal uterine axis as seen in a pendulous abdomen. (There is an exaggerated anteversion of the uterus.)
• Uterine deformities or myometrial disorganization as seen with developmental uterine hypoplasia and extensive myomectomy
• Prematurity below 30 weeks gestation where oxytocin receptors are not fairly established
• Other general/systemic causes may include maternal anemia, maternal exhaustion, and improper use of analgesia in labor

Epidemiology

Most labor abnormalities frequently occur in nulliparous women (25%) than in the multiparous group (10%), including those caused by hypocontractile uterine action. The incidence of labor abnormalities is also noted to be higher in elderly nulliparous women. Uterine inertia is a common indication for primary Cesarean section.[2] Many cesarean deliveries are found to be a result of poor labor progress.[3] Results from studies have found the most common cause of labor progress abnormality to be hypotonic uterine contractions.[4] In 2017, the United States and the United Kingdom had a cesarean section rate of 37% and 27.3% respectively. This increasing trend in cesarean deliveries, especially in developed countries, has drawn attention to assessing the indications for primary Cesarean sections.

Pathophysiology

Normal uterine contractions are generated in a coordinated fashion by two uterine pacemakers located at the cornua portion of the uterine fundus (fundal dominance) and propagated in a synchronous order towards the lower uterine segment. The downward coordinated path of contraction explains the role of contraction in the downward migration of the fetus and cervical changes. In the normal active phase, the contraction frequency is usually 3 to 4 within 10 minutes with a pressure of 30 to 50 mm Hg above the resting tone. The minimum characteristics required to describe a contraction as effective are:

• Minimum frequent contractions with intervals less than 5 minutes apart 
• Intensity of contractions greater than 25 mm Hg

The duration of each contraction also increases from 30 seconds in early labor to between 60 to 90 seconds in the latter part of labor.[5] Friedman established the accepted pattern for the normal progression of labor in the 1950s. The World Health Organization adopted the Friedman curve on the partograph, a graphical labor monitoring tool. Based on his data on nulliparous and multiparous women, he described that the transition from the latent to the active phase of labor occurs at 3 to 4 cm cervical dilatation. He also described that for most women, the minimum rate of cervical dilatation in the active phase is 1.2 cm/hour in nulliparous and faster in multiparous women at 1.5 cm cervical dilatation/ hour.

In hypocontractile uterine dysfunction, although the uterine basal tone is within the normal range (less than 10 mm Hg), the peak/active pressure does not rise higher than 25 mm Hg (normal pressure is about 60 mm Hg or 8 kPa). The hypotonic contractions are described with 4 properties: intensity, duration, relaxation, and interval. Characteristically, there is diminished intensity lower than 10 mm Hg, the duration is shortened by less than 20 to 30 seconds, and there is good relaxation between contractions, but the interval between contractions is increased. The frequency of contractions is often less than 2 to 3 within 10 minutes.

History and Physical

An assessment would confirm that ‘true’ labor has been established, with appropriate documentation of onset time. The patient feels less pain with every contraction, with less frequency and increasing intervals between the contractions. Maternal exhaustion is unusual but appears later as anxiety increases. The fetus is also usually not affected early on. The common clinical finding is inadequate cervical dilatation or fetal descent. With ineffective labor, there may be observed slow progression called protracted labor, absence of progress called the arrest of labor or ineffective expulsive effort. The woman must be in the active phase with cervical dilatation up to 4 cm to be diagnosed with any of these dysfunctions.[6]

Evaluation

Palpation and monitoring tools like an external tocodynamometer can be used to qualitatively evaluate uterine activity. This assessment is subjective since the diagnosis of hypocontractility is based on the perception that contractions are infrequent and weak, with less than 3 to 4 contractions/10 minutes and duration of less than 45 seconds. Another qualitative evaluation of progression is the cervical assessment by digital examination. This tracks the changes in cervical dilatation, effacement, and fetal station performed at standard timed intervals from admission time through the various stages of labor. These changes may be charted on a partogram, which aids in comparison with the expected lower limit of normal labor progress. A deviation to the right of the normal labor curve on the partograph denotes slow progress.[7] A protraction in the active phase would refer to cervical dilation slower than 1 to 2 cm/hour, whereas active phase arrest is diagnosed where there is no cervical change for >6 hours with inadequate contractions.[8] The internal pressure catheter is a device that measures the pressure generated by each contraction, recorded quantitatively in Montevideo units (MVU). Uterine activity less than 200 to 250 MVU is considered inadequate and unlikely to affect expected cervical dilation and fetal descent. This may be a more objective assessment than external monitoring in very obese women.

Treatment / Management

Assessing the mother and fetus to exclude other causes of abnormal labor progression, such as cephalopelvic disproportion (CPD) and fetal malpositioning. Management can be discussed as supportive measures and active measures.

Supportive Measures

• Continuous reassurance to keep the mother calm. Maternal stress increases endogenous adrenaline, which can inhibit uterine contractions.
• Encourage ambulation and avoid the supine position. Although these are not proven to improve contractions or prolonged labor due to hypocontractility, they may improve the comfort of the parturient.[9][10]
• Empty bladder, consider catheterization.
• Maintain adequate hydration.
• Adequate pain relief.
(A1)

Active Measures

Medical management: These are interventions that improve the quality of uterine contractions.

• Amniotomy

Membrane rupture (amniotomy) stimulates contractions by releasing prostaglandins and reflex stimulation of the uterus when the presenting part becomes closely applied to the lower uterine segment. Amniotomy should be attempted when vaginal delivery is probable; where cervical dilatation >4 cm, there is adequate fetal descent (station -2 or lower), and the presenting part is well-applied to the lower uterine segment.

• Oxytocin

Provided there are no contraindications. Oxytocin is the medication of choice for augmenting contractions. The dosage regimen should be titrated to effect for achieving adequate uterine contractions. However, dosing generally does not exceed 30 milliunits/minute.[11] The usual protocol is 5 units of oxytocin in 500 mL of 5% dextrose intravenous infusion, starting with 10 drops/min and gradually titrating the rate to achieve a contraction rate of at least 3 per minute. When instituted early in the active phase, a combination of amniotomy and oxytocin augmentation is more effective in managing hypocontractile labor than amniotomy alone.[12](A1)

Surgical management:

• Assisted vaginal delivery may be performed using forceps, vacuum, or breach extraction, provided the cervix is fully dilated and vaginal delivery is indicated and probable.
• Operative delivery by cesarean section should be considered early when the assessment indicates a CPD or fetal malpositioning/malpresentation. However, in the absence of an early indication, cesarean section is performed if all other measures have failed to stimulate the uterine contractions; when oxytocin is contraindicated (including cephalopelvic disproportion), if there is maternal exhaustion, fetal distress (category III fetal heart tracing), or before full cervical dilation.[13]

Differential Diagnosis

Differential diagnosis of hypotonic labor include:

• Braxton-Hicks contractions
• Amnionitis
• Malpresentation/malpositioning
• Uterine rupture

Prognosis

The prognosis is good. The maternal and fetal outcomes are favorable when hypotonic labor is recognized early and timely interventions instituted while ensuring close monitoring. The risk of a surgical intervention increases when decreased uterine activity occurs in the presence of a cephalopelvic disproportion, fetal malpositioning, or fetal distress. The early decision for cesarean section improves both maternal and fetal outcomes.

Complications

Complications of hypotonic labor may be maternal, fetal, or both, and can include:

• Arrest of labor
• Maternal anxiety and exhaustion
• Postpartum hemorrhage due to uterine atony
• Retained placenta due to ineffective myometrial retraction
• Increased risk of instrumental delivery and possible injuries to mother and baby
• Cesarean section risk with the attending surgical and anesthetic complications
• Fetal distress and birth asphyxia