Assessment of Respiratory Muscles in Children with SMA Greg Redding, MD Pulmonary and Sleep Medicine Seattle Children s Hospital
Disclosures Pediatric Pulmonary Section Editor, UpToDate
Inspiratory Respiratory Muscle Failure Weakness Fatigability Position Excursion http://www.healthcentral.com/acid-reflux/encyclopedia/diaphragm-4011519/ Roussos C, Macklem PT. In: The Thorax (vol 29):Marcel Dekker, Inc., 1984.
Abnormal Diaphragm Position and Contour in SMA (II) Determinants of abnormal position: Scoliosis Primary Lung Disease Obesity SMA with Lung Disease Normal Contour
Abnormal Diaphragm Contours in Children with SMA
Respiratory Muscle Functional Measurements in Patients with Weakness Maximal Sniff Pressure & Max Insp Pressure Cough peak flow & Max Exp Pressure Position-dependent variations in Muscle Strength Time-Tension Index (non-invasive) Respiratory Endurance & Resistive Load Responses Phrenic nerve stimulation/ twitch pressures
Normal Values of Respiratory Muscle Strength Gender and age dependent norms in children Different norms for those over and under 18 Different norms for Psniff and MIP Different results from invasive pressures, e.g. Pdi, Pes sniff, than non-invasive pressures. Problems: Not in TLC units, especially for MEP, +/- cough peak flow MIP and Psniff can be measured at FRC vs RV # of abnormal indices to diagnose resp. muscle weakness Variability of results (and norms) across patients
FVC Trends in SMA II and III N = 7 SMA II 9 SMA III Khirani S, et al. European Journal of Paediatric Neurology 17:552-560, 2013.
Diagnosis of Respiratory Muscle Weakness in Adults: Improving Specificity? Steier J, et al. Thorax 62:975-980, 2007.
Time Tension Index: Muscle Fatigability * Pdi/Pdi max x Ti/T tot Pi**/P max x Ti/T tot Normal Value: 0.02 Risk of Fatigue: 0.15 Normal Value: 0.05 Fatigue: 0.18-0.20 Risk of Where P max = maximum pressure with airway occlusion and Pi** = P o.1sec x 5 x Ti *Relates strength/weakness to fatigability
Time Tension Index and Time to Respiratory Muscle Fatigue Bellemare F, Grassino, A. J Appl Physiol.: Respirat. Environ. Exercise Physiol 53(5):1190-1195, 1982.
T T mus In Children with Neuromuscular Weakness =Normals =Neuromuscular Weakness P i /MIP Mulreany L T, et al. J Appl Physiol 95: 931-937 2003.
Questions specific to SMA What does MIP/Psniff mean when scoliosis also exists? When parasol chest occurs? How to measure respiratory muscle strength in infants and young children? Would response to mechanical loads be of use, i.e. more sensitive? What would be the ideal measure for new therapies?
Maximum Inspiratory Pressure vs Vital Capacity in Children with Scoliosis MIP % predicted 20 40 60 80 100 120 20 40 60 80 100 120 FVC % predicted Early Onset Scoliosis Martinez-Llorens AIS Data, 2010
Threshold Elastic Load: A Test of Respiratory Muscle Endurance C max = Maximum load x 2 minutes T lim =Time of sustained breathing at a load of 80% C max (1) Flynn MG, et al. Chest 95:535-540, 1989 (2) Fiz JA, et al. Respiration 65:21-27, 1998.
Bilateral Phrenic Nerve Stimulation in Neonates and Trans-diaphragm Twitch Pressure (P TW-DIAPH ) Rafferty GF, et al. Am J Respir Crit Care Med 162:2337-2340, 2000.
Airway Twitch Pressures: Values in Normal Anesthetized Children (N=17) Mean TwPdi Newborn = 9 +/- 4cm H 2 O Adult = 28 +/- 5cm H 2 O Rafferty GF, et al. Pediatr Pulmonol 40:141-47, 2005. Rafferty GF, et al. Am J Respir Crit Care Med 162:2337-2340, 2000.
Final Thoughts Measures of respiratory muscle function should be performed with specific questions in mind. Superimposed pulmonary and spine issues will complicate the interpretation of tests of respiratory muscle function. Invasive assessments do not better describe longitudinal trends compared to non-invasive methods.
More Final Thoughts Should multiple tests of weakness be used to measure changes over time and response to new treatments? Involuntary measures, using phrenic nerve stimulation, need standardization (e.g. FRC measures, anesthesia, body position) if they are to be used in young children with SMA.