Duchenne Muscular Dystrophy - A Case Study

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Abstract

A fictional case study for an 8 year old boy with Duchenne Muscular Dystrophy (DMD) during the late ambulatory stage will be presented. The purpose of the case study will be to provide the reader with the typical presentation of DMD as well as assessment tools and intervention strategies used during this phase. The patient presented with bilateral proximal lower extremity weakness and decreased range of motion. There were also concerns with balance and mobility as indicated with specific outcome measures. Treatment focused on maintaining range of motion, balance, an introduction to respiratory training and education about orthotics. As the patient entered the non-ambulatory stage of the disease there was a loss of ambulation and function.

Introduction

The following is a fictional case study of an 8 year old boy diagnosed with Duchenne's Muscular Dystrophy (DMD). In Canada, the disease has a prevalence of 10.3 per 100,000 males.[1] The disease is caused by a mutation in the dystrophin gene that results in an absence or a decrease in dystrophin within muscle fibers.[2] DMD is a progressive neuromuscular disease characterized by muscle weakness, associated motor delays, loss of ambulation, respiratory impairment and cardiomyopathy.[2] Muscle weakness begins in the lower limbs and effects proximal more than distal muscles. The disease typically progresses through 5 stages: presymptomatic, early ambulatory, late ambulatory, early non-ambulatory, and late non-ambulatory.[3] Diagnosis typically occurs during the early ambulatory stage around age 5 when the initial symptoms are seen including frequent falls, a gower's sign, and trouble with running and climbing stairs.[3] Loss of ambulation occurs around age 12 and is associated with a decrease in health related quality of life (HRQOL) and increase in economic burden.[4][5] High quality multidisciplinary care can slow disease progression, prolong functional independence and prolong life expectancy.[3] Those that are diagnosed with DMD are living longer due to earlier detection of the disease, better clinical practice guidelines and increased ventilator use. A study in France showed that the mean life expectancy increased from 25.77 years for those born between 1955 and 1969 to 40.95 years for those born between 1970 and 1994.[6]

A multidisciplinary team approach is recommended to provide anticipatory and preventative care in order to maintain function for as long as possible.[3] Our case study will focus on the physical therapy assessment and intervention strategies for an 8 year old boy during the late ambulation phase as he progresses towards the early non-ambulatory phase. The goal of this case study will be to provide the reader with the typical presentation of DMD during the late ambulatory phase, and recommended assessment tools and intervention strategies during this phase.[2]

Client characteristics

J.D. is an 8 year old boy who was diagnosed with DMD when he was 4 years old. J.D.'s mother took her son to her family doctor, after noticing signs of weakness, clumsiness and difficulty climbing stairs and was referred to a neuromuscular specialist. After diagnosis of DMD he was referred to us by his neuromuscular specialist and has been receiving physiotherapy once per week since diagnosis. He started glucocorticoids when he was 6 years old. J.D. is in grade 3 and is able to participate fully in school and all activities that he enjoys. He enjoys building Star Wars lego sets and playing with them. J.D. has been doing well and is enthusiastic with his treatment. However, his mother has indicated that J.D. is falling more often at home, and has received multiple calls from the school regarding falling during recess and scraping his knee.

Examination findings

Subjective

J.D. reports that he is falling during recess more often. He says that this often happens while playing tag with his friends. He is often sent to the office by the recess monitor. He expresses frustration and embarrassment of being injured in front of his friends. Because of this he fears that he will not be asked to play and will be left out. His mother reports that she has started to notice that it is taking J.D. longer to walk up and down the stairs of their house. She also notes that when her and J.D. are walking the family dog that J.D. gets tired about halfway through their walk and wants to turn back to go home.

The PedsQL represents the child's and parent's perception of the impact of the disease on their own functioning and well-being. It has been recommended that both the PedsQL 3.0 Neuromuscular Model and PedsQL 4.0 Generic Core Scales are used to allow for a more comprehensive assessment for children with neuromuscular disorders.[7] Because of their different perspectives, both the child's self-report and the parent proxy-report should be evaluated in each measure.[7] The greatest concern from these measures are the low ratings by both the parent and child in the physical and social functioning scales. The parent also self-reports concern about her son's communication issues as well as concerns about their family resources. In both measures, the mother has reported lower scores. This could indicate that J.D. is not aware of his physical limitations, however his mother is able to see that he is at a lower functional level than his peers.

The Faces Pain Scale was used to monitor J.D.'s perceived level of pain.[8] We also asked J.D. to rate his pain currently, after playing outside during recess and over the past 24 hours. The results indicate an increased level of pain with increased levels of activity. This should be monitored moving forward to determine if it is necessary to decrease the intensity of J.D.'s activities.

PedsQL 4.0 - Generic Core Scales

Scale Parent Proxy-Report Child Self-Report
Total 61% 70%
Physical Functioning 50% 56%
Emotional Functioning 70% 80%
Social Functioning 60% 75%
School Functioning 70% 75%

PedsQL 3.0 - Neuromuscular Model

Scale Parent Proxy-Report Child Self-Report
Total 70% 80%
About My Neuromuscular Disorder 72% 81%
Communication 67% 75%
About our Family Resources 65% 80%

Faces Pain Scale - Revised (FPS-R)

  • Current = 2/10
  • After playing during recess = 6/10
  • Over the past day = 4/10

Objective

To avoid fatigue of the patient, objective measures were collected over 3 separate appointments. 

Muscle strength and joint range of motion (ROM) was quantified for each major muscle group using the isometric break test and active range of motion. The results showed generalized muscle weakness affecting the lower limb bilaterally, with some weakness starting to develop in the upper limbs. Joint range of motion was good for most movements except for ankle dorsiflexion and the 90-90 SLR. 

Posture and balance analysis was conducted in sitting and standing. On observation in standing, a hyper-lordotic curve was evident in the lumbar spine accompanied with an anterior pelvic tilt and a wide base of support. The patients heels occasionally came off the ground when standing, which indicated that his center of mass was shifted anteriorly. The hyper-lordotic curve disappeared in sitting and was replaced with an obvious increased thoracic kyphosis. The patient had bilateral thick calf muscles as well as winging and anterior tilting of both scapulae. 

The patient was able to sit without handheld support. When moving outside of his base of support, the patient experienced the most difficulty when moving anteriorly. The patient was able to accept mild external perturbations, but needed to put a hand down when moderate forces were applied. In standing the patient needed to take a step when shifting his weight anteriorly. When shifting laterally, the contralateral foot lifted off the ground however was able to recover. A step was required when mild external perturbations were applied in all directions when standing. 

During gait analysis J.D. walked with a wide base of support and in-toeing was evident during the stance phase. During initial contact there was a decreased heel strike with more weight being placed on the midfoot. A bilateral trendelenburg gait was seen which made it look like he was waddling when walking. J.D also had an increased arm swing with the arms coming in front of the body. As J.D. was asked to walk faster, he walked on his toes and had difficulty walking in a straight line.

The North Star Ambulatory Assessment (NSAA), and Six Minute Walk Test (6MWT) were used to assess mobility of the patient. The NSAA measures gross motor ability (i.e getting up from the ground) and is a highly reliable tool.[9] J.D scored 22/34 on the NSAA. The items that were most challenging for him included getting up from the ground, standing on one leg and jumping. Treatment with glucocorticoids has been shown to increase NSAA scores by an average of 1.3 points until age 7.[10] After the age of 7 an average decline of 4 points is seen on the NSAA. A score of 9 or less has been associated with greater functional decline in ambulation over the following year. [10]

The 6MWT is highly reliable, feasible and sensitive in detecting change in children who are declining with ambulation.[11] This tool, measuring ambulatory function, can be affected from biomechanics inefficiencies, decreased endurance and decreased lower extremity strength.[11] The average change in distance over 1 year is -53.67.[12] A score below the threshold of 325m places the individual at higher risk of ambulatory decline in the following year.[12][13]

The Motor Function Measure (MFM) is a tool that can be used for most neuromuscular conditions and is applicable to all degrees of severity in ambulant and non-ambulant patients.[14] The MFM contains 32 items divided into 3 domains. J.D. scored lowest in the standing and transfers domain and scored highest in the distal limb motor function domain. This was to be expected as DMD typically affects the proximal lower limb muscles before progressing to the upper extremities and distal muscles. It can predicted that a patient will lose the ability to walk within a year when the standing and transfer score is less than 40% or the total score is less than 70%.[15] J.D.'s current scores are above these values. These scores should be monitored during reassessment to predict when the loss of ambulation will occur.  

A detailed reassessment should be done every 6 months to monitor for any significant changes.[2]  

All results of the objective examination can be seen below:

Neurological Examination

  • Reflex Testing
    • L3 and S1/S2 = Grade 1
  • Muscle Tone
    • Palpation of quadriceps and hamstrings reveals decreased muscle tone
  • No other neurological findings

North Star Ambulatory Assessment

Six Minute Walk Test

  • J.D. walked 369m.[12]

MMT and ROM

Joint Movement Grade ROM
Shoulder Flexion 4+ 180°
Shoulder Abduction 4+ 172°
Shoulder Extension 5 47°
Elbow Flexion 5 143°
Elbow Extension 4+
Middle Trapezius 4+ N/A
Upper Trapezius 4+ N/A
Hip Flexion 5 117°
Hip Abduction 4 28°
Hip Adduction 3+ 10°
Hip Extension 3+ 10°
Knee Flexion 4+ 127°
Knee Extension 3+
Hamstring 90-90 SLR N/A 150°
Ankle Dorsiflexion 3
Ankle Plantarflexion 4 50°

Postural Analysis

  • Standing: Hyper-lorditic curve in the lumbar spine, anterior pelvic tilt, weight is shifted forward. Bilateral thick calf muscles, winging and anterior tipping of scapulae
  • Sitting: Hyper-lorditic curve disappears when sitting. Patient sits with increased thoracic kyphosis.

Gait Analysis

  • Wide base of support, decreased heel strike, decreased stride length, 'waddling' type of gait, increased arm swing, in-toeing, bilateral Trendelenburg Gait

Motor Function Measure

Scale Score
Total Score 79/96 = 82%
D1: Standing and Transfers 30/39 = 77%
D2: Axial and Proximal Limb Motor Function 29/36 = 80%
D3: Distal Limb Motor Function 20/21 = 95%

Clinical Impression

Physical Therapy Diagnosis

8 year old boy presented to physical therapy with diagnosis of Duchenne Muscular Dystrophy. Weakness and decrease in range of motion is present in proximal lower extremity muscles. Patient is experiencing an increase in falls during activities and challenges with balance and mobility. At risk of decreased participation during school activities. Patient has a good social support system with teachers and family. Patient is a good candidate for physical therapy with to goal to slow the progression of muscle weakness, joint range of motion limitations and maintain independence.

Problem List

  • Exhibits primarily lower extremity weakness
  • Decreased range of motion in dorsiflexion, hip extension, hip adduction, and knee extension. At risk of developing contracture
  • Increasing difficulty with walking and balance, at risk of falls
  • Decreased walking endurance and trouble climbing stairs and getting up off the floor
  • Potential for decreased participation with peers and possible loss of motivation

Intervention

Patient-Centered Goals

Long Term Goals

  • Maintain bilateral 90-90 SLR ROM at 150° at the 6-month reassessment as measured by a goniometer
  • Improve bilateral ankle dorsiflexion ROM to 0° at the 6-month reassessment as measured by a goniometer
  • Improve bilateral ankle dorsiflexion muscle strength to 3+ at the 6-month reassessment as measured by isometric break testing
  • Maintain NSAA score at 22/34 at the 6 month reassessment to continue his instrumental activities of daily living
  • Maintain MFM Domain 1 score above 60% one year from now
  • Increase feelings of inclusion and confidence when playing with friends during recess by increasing the Social Functioning scale on the PedsQL 4.0 - Generic Core Scales to 85% within 6 months

Short Term Goals

  • Within one month of treatment reduce the number of self-reported falls during recess from 3+ to 2 falls per week.
  • Maintain MFM domain 1 score above 70% by the 6 month reassessment
  • Prevent increased limitations in bilateral dorsiflexion and 90-90 SLR ROM as measured by a goniometer within the first month of treatment
  • Increase feelings of inclusion and confidence when playing with friends during recess by increasing the Social Functioning scale on the PedsQL 4.0 - Generic Core Scales from 75% to 80% within 1 month

Management Program

In clinic therapy sessions will be completed once per week. A home exercise program will be included for the patient to complete. The focus of treatment will be on stretching, balance and mobility.

Stretching

During the ambulatory phase, the focus will be on the preventive stretching of the lower extremities, and as the disease progress, the focus will shift to emphasize the upper extremities.During treatment sessions limitations in range of motion will be monitored in other joints.

  • Done actively at least 4 times per week as part of his home exercise program
    • The focus is placed on stretching his ankle dorsiflexion, knee extensors, and flexors, and well as hip flexors and extensors.
    • When done by the therapist, manual therapy including PNF stretching, joint mobilizations and myofascial release techniques may be added
Muscle group Stretch Sets Reps Intensity
Ankle plantarflexors Knee to wall 3 times per day 3 reps, 20-second hold Light pull/ mild discomfort
Knee extensors Side-lying quad stretch 3 times per day 3 reps, 20-second hold Light pull/ mild discomfort
Knee flexors Seated hamstring stretch 3 times per day 3 reps, 20-second hold Light pull/ mild discomfort
Hip flexors Half-kneeling/couch stretch 3 times per day 3 reps, 20-second hold Light pull/ mild discomfort
Hip extensor Knee to opposite shoulder 3 times per day 3 reps, 20-second hold Light pull/ mild discomfort

Balance

Balance exercises will focus on standing static and dynamic balance. The patient will participate in a variety of static balance drills in unilateral and bilateral stances, as well as encompass dynamic walking drill to address functional activities.

We will focus on his balance training during his weekly sessions at the clinic. Balance exercises will be included in his home exercise plan to be done on a daily basis. Exercises may include agility drills including change in direction and changes in cadence of gait to mimic the flow of a game of tag. We will also include walking exercises with object manipulation to replicate him playing with his Star Wars lego sets.

For J.D's home exercise balance program, we will include: Romberg Test with eyes open and closed in both unilateral and bilateral stances. He will aim to hold these stances for 20 seconds, and he or his parents will be recording his attempts, including the number of times he lost balance or opened his eyes. This will provide a method of tracking his progress towards his goal of reducing the number of falls.

Respiratory Training

The patient will begin inspiratory muscle training as well as deep diaphragmatic breathing techniques. As the disease progresses, J.D. will experience weakness of his respiratory musculature, resulting in an increased risk of lung infection and functional decline. We will begin training these muscles now, in an attempt to maintain his respiratory function and prevent decreases in total lung volume as well as forced vital capacity. Particular attention will be given to maintaining chest wall mobility in an effort to prevent the onset of scoliosis. The use of early respiratory training has been found to prolong the quality of life and decrease respiratory complications later in life.[17][18]

Introduction of gentle, low intensity and impact aerobic exercise is recommended with assistance. Settings from which this may be achieved include cycling, pool exercises and swimming.[19] Discussions will be had with his mother to monitor J.D's activity level during recess and other physical activities and to be wary of potential falls. It will be encouraged that J.D.'s mother relays this information to his teachers to monitor this during school hours. This may include a minutes limitation of certain activities in Phys Ed as well as to be monitored while on the playground. All members involved will be educated on the possibility that J.D may have decreased exercise capacity despite his level of functioning.

Orthotics

We discussed the potential use of Knee-ankle-foot orthotics with J.D.'s mother as a treatment path that can be taken, if they choose. It will help prolong the ambulatory period for J.D. as well as reduce the progression of contracture development and will support weight-bearing with the increase in muscular weakness. The conversation was also directed around the use of walkers and wheelchairs, as the conditions progresses, and J.D. inevitably loses his ability to walk. The use of standing devices, along with the knee-ankle-foot orthotics haven been found to prolong ambulation for 2-4 years, depending on the progression of muscle weakness and balance loss.[20] The use of a walker or manual mobility device can be useful if there is ever a need for traveling long distances where it will allow J.D. to take a break or even be pushed. [19]

There are several Ankle-foot orthotics that can be worn at night to facilitate stretching, as well as hand/wrist splints that can be used too. The use of them during ambulation is not recommended as they lead to compensatory movements, but should be used at night to aid in the prevention of contractures, especially in the ankle plantarflexors.[20]

Outcome

It has been 4 years since J.D.'s assessment when he was 8 years old. Due to our treatment, J.D. was able to maintain his activity and participation in school and the activities he enjoys. Unfortunately the muscle weakness in his lower limbs has progressed such that J.D. is no longer able to walk. He is now in the early non-ambulatory stage of the disease. J.D.'s PedsQL scores have lowered considerably, due to his inability to walk and the impact it is having on his functional activities. J.D.'s scores on the MFM have lowered as well. It should be noted that J.D's score on the distal limb and motor function domain is still relatively high. This is indicative of the disease affecting mostly the proximal muscles while the distal muscles in the hands are still being spared. Because of this J.D. is still able to find enjoyment with his Star Wars lego sets. Because of his decreased mobility the NSAA and 6MWT will no longer be used as an outcome measure as it may not be valid and meaningful to him anymore.

J.D. has also seen some decreases in his seated balance indicated by his decreased score on the MFM Domain 2: Axial and Proximal Limb Motor Function. While seated J.D. is only now only able to accept mild internal perturbations.

Manual muscle testing now reveals greater weakness in the proximal upper limb muscles scoring 3+ bilaterally in these muscles. Due to manual therapy and a home stretching program, ROM in the upper extremities has been spared.

Because he is now in the non-ambulatory stage, respiratory function will start to become a greater concern. J.D. has been referred to a respiratory therapist for pulmonary function testing. He has also been referred an orthopaedic surgeon to monitor for changes in scoliosis and the need for braces and possible invasive procedures. An occupational therapist has also been involved in the treatment plan for J.D. They will be recommending home modifications now that J.D. is non-ambulatory. We will continue to see J.D. for weekly physical therapy sessions to continue to slow the disease progression. Inter-professional communication with all those that are involved in his care will be important to ensure we are providing the best care for J.D.

PedsQL 4.0 - Generic Core Scales

Scale Baseline Parent Proxy Current Parent Proxy Baseline Child Self-Report Current Child Self-Report
Total 61% 46% 70% 51%
Physical Functioning 50% 25% 56% 34%
Emotional Functioning 70% 55% 80% 65%
Social Functioning 60% 50% 75% 50%
School Functioning 70% 65% 75% 65%

PedsQL 3.0 - Neuromuscular Model

Scale Baseline Parent Proxy Current Parent Proxy Baseline Child Self-Report Current Child Self-Report
Total 70% 52% 80% 62%
About My Neuromuscular Disorder 72% 50% 81% 65%
Communication 67% 58% 75% 67%
About our Family Resources 65% 55% 80% 70%

Motor Function Measure

Scale Baseline Score Current Score
Total Score 79/96 = 82% 53/96 = 55%
D1: Transfers and Standing 30/39 = 77% 15/39 = 38%
D2: Axial and Proximal Limb Motor Function 29/36 = 80% 20/36 = 56%
D3: Distal Limb Motor Function 20/21 = 95% 18/21 = 86%

Discussion

The fictional case study presented depicts the assessment and intervention strategies that were used for an 8 year old boy who was diagnosed with DMD. Our case study focuses on the progression from the late ambulatory to early non-ambulatory stage of the disease. Our assessment measured body function and structure impairments through a neurological examination, ROM and manual muscle testing. Limitations in activity were measured through a gait and balance assessment and objective outcome measures (NSAA, 6MWT, MFM). Participation restrictions were measured through two PedsQL self-report measures. These assessment tools gave us a comprehensive look into how the disease was affecting J.D. This allowed us to develop meaningful and functional goals for in order to slow functional decline. The long term goals that were developed were aimed at maintaining or improving outcome measures that were reassessed every 6 months. The interventions that were used focused on balance, ROM, introduction to respiratory training, and education about orthotics. As J.D. progressed towards the non-ambulatory phase, his performance on outcome measures declined and some measures were no longer applicable due to the loss of his functional mobility. At this stage, referral to other health professionals was needed for more detailed assessment and other intervention strategies as they would be addressed better by those professionals. Weekly physical therapy sessions will be continued to slow the disease progression.

The unexpected challenges when working with patients in this population is dealing with the progressive nature of the disease as well as the early onset of the condition. A holistic approach to treatment needs to be used when dealing with families of this disease. It can be emotionally draining on the family, so it is important to address those feelings and worries. Ensuring independency for as long as possible can help increase social wellbeing and maintain high levels of overall quality of life for a longer period of time. Goal setting is an additional challenge that clinicians face when dealing with this population. Due to the progressive decline with the condition, it can be difficult to make goals that improve function that may motivate the patient to achieve them. This results in interventions that focus more on compensation of function rather than recovery of function. However, it is important to work with the patient and family to develop interventions that target compensation of function that can unveil abilities that the patient and their family may not have thought were possible given their condition. Health professionals should not settle to just maintain function but should aim to improve function through various intervention strategies. Continued monitoring of patient's status is required throughout the condition as declines in the body systems may be gradual and may go unnoticed unless frequent reassessment occurs.[2] When these changes unfold the treatment plan must be adjusted accordingly and new goals may need to be developed as previous ones may no longer apply or be realistic. This stresses the importance of using outcome measures that are able to address changes in function across all stages of the disease. The MFM is able to accomplish this and should be used in conjunction with other tools such as the NSAA, 6MWT or Vignos Scale; these measures may not be applicable as the patient progresses into different stages and will need to be replaced.[14]

DMD can be a devastating condition to the family, however improvements in research and high quality multidisciplinary care has prolonged life expectancy and can maintain participation in activities that the patient feels are important for a longer period of time.[4][6]

Self-study Questions

What characteristic would likely be seen upon observation of someone with Duchenne Muscular Dystrophy?

  1. Bilateral hypertrophied calf muscles
  2. Large Head
  3. Thin lips
  4. Slap foot during gait

What is the typical sign that appears in the early ambulatory stage of Duchenne Muscular Dystrophy?

  1. Trendelenburg sign
  2. Gower's sign
  3. Popeye's sign
  4. Adson's sign

What outcome measure would be most appropriate to monitor changes in function across all stages of Duchenne Muscular Dystrophy?

  1. North Star Ambulatory Assessment
  2. 6 Minute Walk Test
  3. Motor Function Measure
  4. Berg Balance Scale

Answers: 1) A 2) B 3) C

Additional Resources

  1. For an overview of the diagnosis and management of Duchenne Muscular Dystrophy or to provide as a resource for families, Muscular Dystrophy Canada has developed a guide for families. A link to the PDF can be found here
  2. A 3-part care considerations has been developed for Duchenne Muscular Dystrophy. The guidelines are supported by the US Centers for Disease Control and Prevention with involvement of the TREAT-NMD network for neuromuscular diseases, the Muscular Dystrophy Association, and Parent Project Muscular Dystrophy. Links to the 2018, 3-part series can be found here: Part 1, Part 2, Part 3
  3. A PDF copy of the Motor Function Measure User's Manual can be found here

References

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  2. 2.0 2.1 2.2 2.3 2.4 Birnkrant DJ, Bushby K, Bann CM, Apkon SD, Blackwell A, Brumbaugh D, et al. Diagnosis and management of Duchenne muscular dystrophy, part 1: diagnosis, and neuromuscular, rehabilitation, endocrine, and gastrointestinal and nutritional management. The Lancet Neurology. 2018;17(3):251-67.
  3. 3.0 3.1 3.2 3.3 Bushby K, Finkel R, Birnkrant DJ, Case LE, Clemens PR, Cripe L, et al. Diagnosis and management of Duchenne muscular dystrophy, part 1: diagnosis, and pharmacological and psychosocial management. Lancet Neurology. 2010;9(1):77-93.
  4. 4.0 4.1 Schreiber-Katz O, Klug C, Thiele S, Schorling E, Zowe J, Reilich P, et al. Comparative cost of illness analysis and assessment of health care burden of Duchenne and Becker muscular dystrophies in Germany. Orphanet Journal of Rare Diseases. 2014;9(1).
  5. Ryder S, Leadley R, Armstrong N, Westwood M, de Kock S, Butt T, et al. The burden, epidemiology, costs and treatment for Duchenne muscular dystrophy: an evidence review. Orphanet Journal of Rare Diseases. 2017;12(1).
  6. 6.0 6.1 Kieny P, Chollet S, Delalande P, Le Fort M, Magot A, Pereon Y, et al. Evolution of life expectancy of patients with Duchenne muscular dystrophy at AFM Yolaine de Kepper centre between 1981 and 2011. Annals of Physical and Rehabilitation Medicine. 2013;56(6):443-54.
  7. 7.0 7.1 Davis SE, Hynan LS, Limbers CA, Andersen CM, Greene MC, Varni JW, et al. The PedsQL in pediatric patients with Duchenne muscular dystrophy: feasibility, reliability, and validity of the Pediatric Quality of Life Inventory Neuromuscular Module and Generic Core Scales. Journal of clinical neuromuscular disease. 2010;11(3):97-109.
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  11. 11.0 11.1 McDonald CM, Henricson EK, Abresch RT, Florence J, Eagle M, Gappmaier E, Glanzman AM, PTC124‐GD‐007‐DMD Study Group, Spiegel R, Barth J, Elfring G. The 6‐minute walk test and other clinical endpoints in Duchenne muscular dystrophy: reliability, concurrent validity, and minimal clinically important differences from a multicenter study. Muscle & nerve. 2013 Sep;48(3):357-68. 
  12. 12.0 12.1 12.2 Henricson E, Abresch R, Han JJ, Nicorici A, Keller EG, de Bie E, McDonald CM. The 6-minute walk test and person-reported outcomes in boys with duchenne muscular dystrophy and typically developing controls: longitudinal comparisons and clinically-meaningful changes over one year. PLoS currents. 2013 Apr 21;5.
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