THE CAUSES OF ERBS PALSY - PREDISPOSING FACTORS

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William Smellie, a British obstetrician is credited with the first medical description of obstetric brachial plexus palsy. In 1861, Guillaume Benjamin Amand Duchenne coined the term ‘obstetric palsy of the brachial plexus’ after analyzing four infants with paralysis of identical muscles in the arm and shoulder. In 1874, Wilhelm Heinrich Erb concluded his thesis on adult brachial plexus injuries that associated palsies of the deltoid, biceps and subscapularis are derived from a radicular lesion at the level of the 5th and 6th cervical nerve roots rather than isolated peripheral nerve lesions. Erb-Duchenne palsy therefore is named after both doctors whose research widened the existing knowledge about the condition.

Erb’s Palsy or Erb-Duchenne Palsy is relatively rare in the United Kingdom occurring in about 1 in 2,000 births. In terms of frequency of occurrence, Erb’s palsy occurs more frequently than Down’s syndrome, muscular dystrophy and spina bifida.

The nerves of the brachial plexus originate from the 5th, 6th, 7th, 8th cervical and the 1st thoracic nerve roots and are distributed across the entire upper extremity. Erb’s palsy is the result of an injury or damage to the 5th and 6th cervical nerve roots with occasional involvement of the 4th and 7th cervical nerve roots.

Causes of Erb’s Palsy

Erb’s palsy most commonly occurs as a result of injury before or during delivery. Factors known to predispose the foetus to brachial plexus injury (Erb’s palsy) include:

Maternal factors

    • Uterine abnormalities including fibroids, bicornuate uterus.
    • Diabetes mellitus – poorly controlled maternal diabetes mellitus has been known to be associated with foetal macrosomia, which could become complicated by shoulder dystocia resulting in an injury to the upper roots of the brachial plexus during delivery.
    • Maternal body proportions – passage of a baby through a small maternal pelvis could be traumatic enough to cause injury to the roots of the plexus.

Foetal factors

    • Foetal macrosomia.
    • Presence of a cervical rib – a cervical rib is an extra rib that arises from the 7th cervical vertebra. It results in a thoracic outlet syndrome characterized by compression of the roots of the brachial plexus as well as the subclavian artery.
    • Transverse lie – The lie of a foetus is the relationship between the long axis of the foetus and the long axis of the maternal spine; a longitudinal lie is the most favorable for spontaneous vaginal delivery. A transverse lie at term could result in brachial plexus injury because it is often associated with difficult labour; obstetric manipulations performed in an effort to relieve this difficulty could result in an injury to the plexus.
    • Poor muscle tone.
    • 5 minute APGAR score of less than 5.

Intrapartum events

    • Mechanical forces of labour.
    • Vaginal breech delivery.
    • Prolonged labour.
    • Operative vaginal delivery.
    • Shoulder dystocia or clavicular fracture.
    • Precipitous delivery.
    • Prolonged head to body interval at delivery.

Neonatal conditions after delivery

    • Neoplasm.
    • Infection.
    • Extrinsic compression and edema.

The neonatal conditions after delivery mentioned above could cause compression of or injury (in the case of an infection) to the roots of the brachial plexus.

Clinical features

The clinical features of Erb’s palsy are summed up below:

    • Affected nerves – They include the dorsal scapula, suprascapular, lateral pectoral, long thoracic, musculocutaneous, radial, median and phrenic nerves.
    • Sensory deficits – There is sensory deficit on the radial side of the deltoid, forearm and hand. The medial side of the hand remains unaffected.
    • Patterns of muscle weakness – The following groups of muscles are affected
        • External rotators and abductors of the shoulder/arm.
        • Flexors of the forearm.
        • Extensors of the fingers (if the 7th cervical nerve root is involved).
        • Diaphragm descent (if the 4th cervical nerve root is involved)/
    • Clinical presentation: The classic picture is the ‘waiter’s tip’ posture of the hand. The shoulder is adducted and internally rotated, the forearm is extended and pronated and the fingers are flexed. There is paralysis of the diaphragm on the affected side if there is involvement of the 4th cervical nerve root.
    • Associated defects: These include ‘winging of the scapula’ from shoulder dislocation, bone deformity, disparity in the length of the limbs and deformity in the forearm.

Prognosis

Majority of brachial plexus injuries resolve in the first few months of life; 5 to 25% do not fully recover and result in life-long disability. In the latter, early surgical intervention may be required in the first year of life.

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