How often should Negative Inspiratory Force (NIF) be tested in a patient experiencing a myasthenia gravis (MG) crisis?

Frequency of NIF Testing in Myasthenia Gravis Crisis

In myasthenia gravis crisis, perform NIF measurements frequently throughout the day as part of continuous respiratory monitoring, with formal pulmonary function testing (NIF and vital capacity) conducted at minimum every 6 hours during the acute phase, and more often if clinical deterioration is suspected. 1, 2

Initial Assessment Upon Crisis Recognition

• Perform immediate pulmonary function assessment with both NIF and vital capacity (VC) measurements as soon as myasthenic crisis is suspected 1
• Use the "20/30/40 rule" to identify imminent respiratory failure: vital capacity <20 ml/kg, maximum inspiratory pressure (NIF) <30 cmH₂O, or maximum expiratory pressure <40 cmH₂O 1, 2
• Admit to ICU immediately if NIF <30 cmH₂O or FVC <20 ml/kg, and prepare for elective intubation before emergent respiratory arrest occurs 2

Continuous Monitoring During Crisis

• Conduct daily neurological evaluation with frequent pulmonary function assessments throughout the crisis period 1, 2
• Perform frequent respiratory function monitoring for patients with moderate to severe generalized weakness (MGFA class III-V) 1
• Supplement formal NIF measurements with the single breath count test (patient takes deep breath and counts at rate of two numbers per second while exhaling; counting to ≥25 correlates with normal respiratory muscle function) 1, 2

Practical Monitoring Approach

The key distinction is between "frequent" monitoring during acute crisis versus scheduled monitoring in stable disease. During active myasthenic crisis requiring ICU admission, NIF and VC should be measured multiple times daily—typically every 4-6 hours or more frequently if clinical status changes 1, 2. This contrasts sharply with stable outpatients who require testing only every 6 months 1.

• Monitor for minimum 24 hours in ICU, HDU, or recovery unit even after apparent stabilization 1
• Recognize that respiratory insufficiency may develop without obvious dyspnea symptoms, necessitating objective measurements rather than relying on clinical signs alone 1
• Note that pulse oximetry and arterial blood gases might not be reliable early indicators of emerging respiratory failure; NIF and VC are superior early warning parameters 1

Common Pitfalls to Avoid

• Do not wait for clinical signs of dyspnea before measuring respiratory function—weakness can progress rapidly and respiratory compromise may be subclinical 1
• Avoid relying solely on pulse oximetry or ABGs as these lag behind actual respiratory muscle weakness 1
• Ensure testing is performed by adequately trained practitioners familiar with assessing individuals with neuromuscular disorders 1

Transition to Stable Monitoring

• Once crisis resolves and patient stabilizes, transition to respiratory function assessments every 6 months 1
• Consider polysomnography when concern exists that pulmonary function tests and clinical evaluation are not capturing complications such as hypoventilation 1