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Duchenne muscular dystrophy

Duchenne Muscular Dystrophy

Harish Hosalkar MD, MBMS (Orth), FCPS (Orth), DNB (Orth)


Introduction

      Duchenne muscular dystrophy (DMD) is the most common form of muscular dystrophy. It is a X-linked recessive myopathy generally affecting only males. Its incidence ranges from 13 to 33 per 100,000 males, of which 1/3rd arise from new mutations.  The dystrophic process of skeletal muscle associated with DMD is relentlessly progressive leading to a breakdown of the muscle fibers and replacement of the muscle sarcoplasm with fibrofatty infiltrate.  This process causes progressive weakness despite an incongruous increase in the bulk of the muscle mass referred to as "pseudohypertrophy*" *which is a hallmark for this disease.   Typically, proximal muscles such as those around the shoulder and hip girdles are involved initially, followed by more distally placed muscles.  The result is weakness and an inability to resist gravity leading to frequent falls with difficulty in rising.  Again, this is a great hindrance in use of walking aids.  

          Contractures: In DMD (and many other neuromuscular disorders) joint contractures typically evolve as weakness progresses beyond the grade 4/5 stage of strength (less than active resistance).  Contractures are especially common in non-ambulatory patients. Typically, patients with DMD develop flexed, abducted and externally rotated deformities of the hips, which eventually become fixed in position. These patients develop flexion contractures of the knees approaching 900 and stiff equinovarus contractures of both ankles.  Often, one type of contracture may influence body positioning and results in the development of other contractures. For example, a patient with flexion deformity of both the hips and knees, when lying supine in bed, then develops tightness in the iliotibial band.  The hip abductors shorten and the patient's hips become fixed in flexion and abduction - the so-called "frog-leg" position. This may make it difficult for the abducted patient to sit comfortably in a wheelchair as the sides of the chair force adduction.

    

         Gait and Posture: With progressive weakness around the hip girdle, walking becomes difficult and could result in frequent falls. The muscles responsible for standing upright, particularly the extensors of the hips and knees become too weak to sustain an upright stance.  Initially, most patients manage an upright stance by a compensatory posture, marked by hyperlordosis of the spine. This extreme lordotic posture shifts the patient's weight distribution so that the weight reaction line runs posterior to the center of rotation of the hips and anterior to the center of rotation for the knees. This compensation is tenuous at best, as it is disturbed easily leading to falls and a feeling of instability.  Equinus of the ankles is actually helpful to the maintenance of the compensatory posture.  Unless there is particularly rapid evolution of Achilles tendon contractures, while the patient is still very active and not lordotic, it is not advisable in the usual case to surgically correct the equinus because this causes the patient to fall forward leading to flexion of both hips and knees, failure of compensation and falling.  At this stage, a hindfoot release only allows standing with long leg braces.  Accordingly, heel cord lengthening or release is usually postponed to a later stage of the disease.

   

        Spinal Deformity: Spinal curvature results from progressive weakness of the paraspinal muscles and is a common component of many neuromuscular conditions. Scoliosis in boys with DMD can usually be observed approximately two years after the patient becomes wheelchair dependent.  Initially, the curve is small and may only be appreciated by a radiograph; however, the deformity invariably progresses with time.  Because there is some variability in the severity of the disease from patient to patient; and because the severity of the spinal deformity varies with the observed weakness, there is a conflicting prevalence and natural history for scoliosis reported for this population. These differences can be explained by the different time of followup reported time in these studies.   However, if observed long enough, virtually all children with DMD will develop a progressive spinal deformity, usually kyphosing scoliosis. The annualized progression averages approximately 100. Invariably, the rate of progression usually accelerates with time, such that the annualized rate will reach 300 or more.  This phenomenon is frequently referred to as spinal collapse.  Spinal collapse reflects the state of the patient's weakness and course of disease.  Over time, pulmonary and cardiac status deteriorates and the spinal deformity becomes larger and more rigid.  Accordingly, surgical correction after the stage of spinal collapse carries a higher risk and greater difficulties than surgery performed prior to collapse.  Additionally, the reported blood loss with surgery after spinal collapse is higher than most other types of neuromuscular scoliosis, although this varies with the stage of the disease at time of surgery.

Management

Limbs
-         The main goal of limb surgery is to prolong safe ambulation. The surgeon should try to correct the posture while maintaining all the compensatory stabilizing biomechanical factors that are present in individual cases. Maintaining upright weight bearing also preserves bone mineralization and allows for assisting with transfers even when non-ambulant. However, controversy exists as to the value of tendon release and transfer procedures in DMD. This may destroy compensatory posture and adversely effect locomotion post-operatively requiring added bracing and support.  Shoulder girdle weakness prohibits the effective use of walking aids and patients are unable get up from a chair without help.  Further, patients are often afraid of falling and may prefer the stability provided by a wheelchair.  Weaker patients with a more severe disease course seldom achieve much benefit from aggressive surgery.  Hence, limb surgery should be reserved for Duchenne patients with a less progressive course and for patients with Becker MD who will derive more benefit from surgery.Foot and Ankle Surgery
-               Equinovarus deformity of the foot and ankle could be bothersome in some wheel-chair bound cases; their feet get caught in the wheel chair platforms; they have difficulty with shoe wear; or they find the deformity cosmetically displeasing.  Ankle and hind foot releases can very helpful in these cases.

-               Frequently, severe contractures can be partially controlled by the prophylactic nighttime use of modified ankle-foot orthoses (MAFOs). These light-weight plastic braces help overcome the plantar-directed force of gravity.  Although, these devises do not eliminate the deformity, they can temporarily limit the progression. 

-               A simple tenotomy of both Achilles tendons performed early can correct the ankle enough that the foot can be positioned flat or plantar grade and shoe wear is facilitated. Later, with disease progression, a more aggressive approach may be needed to correct severe equinovarus contracture with release of all of the tight tissues in the posterior and medial aspects of the foot and ankle. In both situations postoperative bracing is required to prevent recurrent contractures.  However, in ambulatory patients with DMD, the equinus position of the ankle is helpful in maintaining balance with compensatory posture. Contracture release in these patients prohibits this compensatory mechanism and the patient are unable to achieve a balanced stance in the absence of braces that extend from the toes to the upper thighs.  This is an important consideration in timing corrective surgery. 

-               For ambulatory patients, particularly those with the Becker MD, the bilateral posteromedial release of the ankles is combined with a transfer of the tibialis posterior tendon from the medial side of the foot and ankle.  The tibialis posterior tendon is rerouted through the interosseus membrane that runs between the tibia and fibula above the ankle, and attached to the midtarsus at the dorsum of the foot. This provides for correction, a balanced foot and some protection against recurrent deformity. 

 Hip and Knee Surgery
-               The release of contractures at the hip and knee are less frequently indicated because they occur later in the disease course once the goals for the patient have shifted from maintaining an upright stand to achieving and maintaining balanced sitting.  However, patients who are unable to sit comfortably due to contracted hip abductors may be treated with release of the tensor fascia lata and the anterior fibers of the gluteus medius.  Generally a simple bilateral release may easily address this problem. Percutaneous tendon lengthening procedures are typically well tolerated and effective.

 

Scoliosis

-               The progressive nature of neuromuscular disorders leads to rigid and unbalanced spinal deformities, which eventually interferes with comfortable positioning in a wheelchair, reduces sitting tolerance to a few hours at a time and causes difficulty achieving comfortable positioning for sleep. Restrictive lung disease is accentuated on the concave side of the curve due to mechanical restriction in lung expansion. Surgical correction and stabilization with spinal instrumentation and arthrodesis, particularly if done before the curve becomes severe and the disease advanced, is the best way to handle this situation.

-               The use of a scoliosis brace is never definitive and does not avoid surgery, although it may possibly slow the curve progression and may gain some time to allow for somatic growth. Unfortunately, this is not a benefit in conditions like DMD because the brace only serves to mask the problem while the disease continues to progress. When bracing is used, the seriousness of the situation becomes apparent later when the patient has become weaker and the risks involved with surgery are greater. Further, brace wear is often uncomfortable for the child and not well-tolerated.

-               It is essential for the spinal surgery procedure to occur prior to spinal collapse. Because the natural history for scoliosis with DMD is so predictable, optimal timing for treatment of the deformity can be achieved by obtaining radiographic exams every six months, beginning approximately two years following the use of a wheelchair full-time. It is wise to recommend spinal surgery when the curve reaches 300 and certainly by 600  Delaying surgery risks spinal collapse, which further complicates DMD with numerous associated health problems.

Preoperative Preparation

The minimum preparation includes: -         Cardiac consult to identify the presence of cardiomyopathy, arrhythmia, and to assess the risk for cardiac failure.  An echocardiogram and an electrocardiogram should be included with this investigation.
-         Pulmonary evaluation to determine the pulmonary function, the risk for pulmonary infection and the potential ventilation needs. This includes a chest radiograph and pulmonary function tests, including lung volumes, maximal inspiratory and expiratory pressures (MIP-MEP) and blood gases.
-         Because of the tendency to bleed more than most other children, testing for an underlying coagulopathy should be performed preoperatively and adequate units of blood should be on hand for surgery.
-         Experienced anesthesiologists and critical care physicians are required to help manage the patient during and after surgery. These children are at high risk for cardiac and pulmonary complications and should only be managed in tertiary care centers.

Surgical procedures

      -         The surgical approach for spinal correction is best done through a midline posterior exposure. A segmental spinal instrumentation is preferably placed on either side of the midline of the spine. At the same time, a spinal arthrodesis or fusion is done through the facet joints by adding bone graft or graft substitute. The purchase of the implants through multiple sites of attachment provides load sharing and stability. This should ensure that there will be little loss of correction postoperatively and provides an enhanced chance of obtaining a solid arthrodesis.  A brace is not required post-operatively with the biomechanical stability afforded by the segmental spinal instrumentation construct.

      -         One topic of controversy over this procedure concerns whether the instrumentation should be extended to the sacrum and pelvis or stopped at L4. The pros and cons are enumerated. Those that champion stopping short, claim similar results in all but a few patients to those who extend the instrumentation.  Additionally, shortened instrumentation saves about an hour of surgery and therefore decreases associated blood loss. Those who instrument the whole spine claim that it better ensures no further loss of correction of the scoliosis or the pelvic obliquity, thereby reducing the risk of associated unbalanced sitting.  These surgeons also note that the patient is generally at their healthiest at the time of the index surgery.  Thus, taking the time to instrument the entire spine is necessary to ensure correction avoiding the need for revision surgery at a later time- which is not a trivial undertaking.  The authors subscribe to the notion that the initial correction should be extended to the sacrum and pelvis.

-         Equinus of the ankle plays an important role in maintaining the compensatory posture, helping to position the weight reaction line anterior to the knees.  Disruption of the fragile compensatory posture will cause the patient to fall.  Physicians are cautioned to carefully consider all benefits and risks that a given procedure may incorporate.  For example, an ill-advised release of the Achilles tendons to correct equinus disturbs this tenuous relationship of position and balance, resulting in a shift of the weight reaction line posterior to the knees.  This causes the knees to flex and the patient will fall.  The only way that that patient can maintain an upright stance following heelcord release is with a long leg brace, a knee-ankle-foot orthosis (KAFO). Thus, if bracing is to be avoided at this time, it is wise to postpone hind foot release until brace management can be accepted by the patient.

  Non-surgical Management

-         Physical therapy with passive range of motion is particularly important when there is acute weakness and evolution of contractures. In patients with slowly progressing weakness, the therapist can instruct caregivers in stretching techniques and can guide the family in appropriate activity modification (swimming, horseback riding and adapted skiing, for example). The fitting of orthotics/braces and addressing mobility options (e.g., scooter, power chair) is also a specialized role of the physiatrist and physical therapist. 

-         The occupational therapist addresses limitations in fine motor skills and related activities of daily living (dressing, bathing, toileting, food preparation, eating).  Evaluation of the home and school settings are often needed to ensure that proper adaptive equipment and safety devices are obtained.

-         The speech therapist evaluates oral-motor function relating to feeding and speech limitation.  In extreme cases, augmentative communication devices can facilitate communication when speech is not possible.

-         Constipation is a frequent complication when there is abdominal wall weakness and limited fluid intake. This needs to be treated vigorously with adequate fluid intake and the use of fiber added to the diet, fruit juices, senna-based herbal tea or granules, mineral oil preparations or an occasional suppository.

-         The risk of malignant hyperthermia in patients with certain muscle diseases undergoing anesthesia needs to be placed prominently in the patient's chart and the family should be counseled. 

-         Psychosocial support is necessary at home, at school and in the community. The clinic nurse and social worker are indispensable in inquiring about these issues and in identifying local services.  A representative of the Muscular Dystrophy Association (MDA) often attends a MDA-sponsored neuromuscular clinic and offers additional resources, such as regional summer camp programs, funding for diagnostic testing and some therapeutic support.  Several other focused support groups have evolved to provide current information on the diagnosis, treatment options and research news for their particular topic (The Parent Project, for example, in Duchenne MD).  A comprehensive listing of these disease-specific support groups can be obtained from the Exceptional Parent magazine website: www.eparent.com.  
Future Prospects for treatment of Muscular Dystrophies

 
New hypotheses as to the pathogenesis of a particular disease can be derived from an understanding of the role a particular gene plays in encoding for a specific structural protein or enzyme critical for normal muscle or nerve cell function.  Specific adaptive mechanisms within the muscle or nerve cell can then be explored and possibly enhanced with the use of medication.  The prospect of gene and stem cell therapies to correct the genetic defect and restore function is on the horizon, with the first pilot human study using a viral vector in Limb Girdle MD initiated in 1999.  Understanding acquired muscle diseases continues to increase with current research. These newer tools allow for more accurate diagnosis and assessment of disease activity, thus allowing physicians to counsel the patient and family regarding prognosis, genetic implications and appropriate treatment options.

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