Muscle Stiffness due to Neuromuscular Hyperexcitability

Neuromuscular hyperexcitability disorders often manifest as muscle stiffness, spasms, or cramps. These uncommon disorders may be due to central or peripheral nervous system dysfunctions and be either subacute in onset or chronic. Their diagnosis requires a high index of suspicion and is often delayed. Stiff-person syndrome, the prototype of central hyperexcitability disorders, is an immune-mediated disorder that is often associated with high titer glutamic acid decarboxylase antibodies and is amenable to symptomatic treatment and immunotherapy. Peripheral nerve hyperexcitability syndromes, including Isaacs syndrome and Morvan syndrome, are the most common peripheral nerve disorders associated with muscle stiffness. Their diagnosis is usually confirmed by identifying after-discharges that follow the motor response during motor nerve conduction studies and by neuromyotonic and myokymic discharges on needle electromyography (EMG). They are often associated with voltage-gated potassium channel-complex antibodies, specifically toward contact-associated protein-like 2 protein and, less commonly, against leucine rich glioma inactivated 1 protein. These potassium channelopathies overlap with limbic encephalitis, the second most common autoimmune encephalitis. Myotonic disorders, including myotonia congenita and myotonic dystrophies, are the most common muscle disorders that manifest in muscle stiffness and spasms. Rippling muscle disease (RMD) and Brody disease are extremely rare nonprogressive myopathies associated with electrical silence on needle EMG during muscle stiffness and delayed muscle relaxation. RMD has hereditary and immune-mediated forms. It is due to the loss of calveolin-3 or one of its cavin-supportive rafts in muscle membrane. Brody disease is autosomal recessive myopathy due to defective pumping of calcium from the cytoplasm by sarco(endo)plasmic reticulum Ca²⁺ adenosine triphosphatase pumps.