Phrenic Nerve Palsy After Interscalene Block

Phrenic nerve palsy after interscalene block is block (or injury) of the phrenic nerve (C3 to C5) causing ipsilateral hemidiaphragm paralysis. It is nearly universal after usual volume interscalene block and usually transient.

Epidemiology. Hemidiaphragmatic paresis affects 90% to 100% of patients after interscalene block with 20 mL of local anaesthetic, 25% to 50% with low volumes (5 to 10 mL), and about 10% to 20% after supraclavicular block. Symptomatic respiratory compromise occurs in patients with limited reserve. Persistent palsy beyond 3 to 6 months is rare.

Risk factors. Obesity, chronic obstructive pulmonary disease, restrictive lung disease, contralateral diaphragm weakness, neuromuscular disease, high volume or concentration, and sitting position.

Pathophysiology.

The phrenic nerve lies on the anterior scalene muscle, just anterior to the interscalene groove. Local anaesthetic spreads anteriorly to the nerve, reducing ipsilateral diaphragm function by 25% in forced vital capacity and forced expiratory volume in 1 second.

Dyspnoea, shallow breathing, desaturation, a sense of suffocation, and paradoxical movement. Many are asymptomatic. Onset occurs within 30 minutes and lasts the duration of the block (6 to 24 hours).

Red flags.

• Saturation under 90% or severe dyspnoea.
• Contralateral lung disease.
• Dyspnoea persisting after block resolution.

Ultrasound of diaphragm excursion (M-mode: excursion under 10 mm in quiet breathing, or paradoxical motion on sniff). Chest radiograph shows an elevated hemidiaphragm. Spirometry shows 25% fall in forced vital capacity. Fluoroscopy (sniff test) if needed.

Differential diagnosis.

• Pneumothorax.
• Pulmonary embolism.
• Atelectasis.
• Phrenic nerve injury from surgery.
• Local anaesthetic toxicity.
• Aspiration.

Sit the patient upright, give supplemental oxygen to saturation of 94% to 98% (88% to 92% if hypercapnic risk), encourage deep breathing, and avoid sedatives. Use non-invasive ventilation (bilevel positive airway pressure) for hypercapnia or fatigue.

Avoid interscalene block in severe lung disease or contralateral phrenic weakness. Alternative blocks: superior trunk, suprascapular with axillary nerve, or low volume interscalene (5 to 10 mL), which reduce phrenic involvement.

Analgesic plan for opioid sparing: paracetamol 1 g orally every 6 hours, maximum 4 g daily (reduce in hepatic impairment or under 50 kg).

Prevention. Select a diaphragm sparing block, use low volumes of dilute local anaesthetic, and screen for respiratory reserve.

Monitoring. Respiratory rate, saturation and work of breathing for 24 hours (continuous oximetry in high risk patients).

Referral and escalation. Respiratory and critical care review for failure. Diaphragm surgery review for persistent paralysis.