Muscular Dystrophy: This is a group of inherited disorders characterized by progressive muscle weakness and degeneration. The inability to hold the head up and frequent falls are common symptoms, especially in Duchenne muscular dystrophy, which often presents in early childhood.

Myasthenia Gravis: An autoimmune disease that leads to fluctuating muscle weakness, which can worsen with activity and improve with rest. Symptoms can include difficulty holding the head up, weakness in the arms and legs, and frequent falls due to muscle fatigue.
Spinal Muscular Atrophy (SMA): A genetic disorder that affects the nerve cells responsible for controlling voluntary muscle movement. SMA can lead to muscle weakness and wasting, with symptoms including difficulty holding the head up, swallowing, and breathing, as well as frequent falls.
Guillain-Barré Syndrome: An autoimmune disorder where the body's immune system mistakenly attacks part of its peripheral nervous system, leading to muscle weakness, and sometimes paralysis. While it often starts in the legs and ascends, it can lead to significant weakness affecting the ability to hold the head up and causing falls.

Botulism: A rare but serious illness caused by a toxin produced by the bacteria Clostridium botulinum. Symptoms can include muscle weakness, drooping eyelids, difficulty swallowing, and shortness of breath. If not promptly treated, botulism can lead to respiratory failure, making it critical not to miss this diagnosis.
Tetanus: Caused by the toxin of Clostridium tetani, tetanus leads to muscle stiffness and spasms, which can be severe enough to cause fractures or respiratory failure. Early symptoms can include muscle weakness and stiffness, which might be mistaken for other conditions.
Stroke or Spinal Cord Injury: While these conditions might present with sudden onset of symptoms, they are critical to identify early due to the potential for significant and permanent damage if not promptly addressed.

Lambert-Eaton Myasthenic Syndrome (LEMS): A rare autoimmune disorder that affects the nerve-muscle connection, leading to muscle weakness, fatigue, and sometimes paralysis. It is often associated with small cell lung cancer.
Mitochondrial Myopathies: A group of disorders caused by mitochondrial dysfunction, leading to muscle weakness, exercise intolerance, and other systemic symptoms. These conditions are rare and often have a complex presentation.
Congenital Myopathies: A group of muscle disorders (such as nemaline myopathy, central core disease, and others) present at birth, characterized by muscle weakness and hypotonia. While rare, they are important to consider in the differential diagnosis of muscle weakness in infants and children.