Hajdu–Cheney syndrome, also called acroosteolysis with osteoporosis and changes in skull and mandible, arthrodentoosteodysplasia and Cheney syndrome, is an extremely rare autosomal dominant congenital disorder of the connective tissue characterized by severe and excessive bone resorption leading to osteoporosis and a wide range of other possible symptoms. Mutations in the NOTCH2 gene, identified in 2011, cause HCS. HCS is so rare that only about 50 cases have been reported worldwide since the discovery of the syndrome in 1948

Signs and symptoms
Hajdu–Cheney syndrome causes many issues with an individuals connective tissues. Some general characteristics of an individual with Hajdu–Cheney syndrome include bone flexibility and deformities, short stature, delayed acquisition of speech and motor skills, dolichocephalic skull, Wormian bone, small maxilla, hypoplastic frontal sinuses, basilar impression, joint laxity, bulbous finger tips and severe osteoporosis. Wormian bone occurs when extra bones appear between cranial sutures. Fetuses with Hajdu–Cheney syndrome often will not be seen to unclench their hands on obstetrical ultrasound. They may also have low-set ears and their eyes may be farther apart than on a usual child, called hypertelorism. Childrens heads can have some deformities in their shape and size (plagiocephaly). Early tooth loss and bone deformities, such as serpentine tibiae and fibulae, are also common in those affected. Genetics
Hajdu–Cheney syndrome is a monogenic disorder. The disorder is inherited and controlled by a single pair of genes. A single copy of the mutant gene on an autosome causes HCS.
A diagnosis of inappropriate sinus tachycardia is primarily one of exclusion, and the following may be observed:
Exclusion of all other causes of sinus tachycardia
Common forms of supraventricular tachycardia (SVT) must be excluded
Normal P wave morphology
A resting sinus tachycardia is usually (but not always) present
Nocturnal dip in heart rate
Inappropriate heart rate response on exertion
Mean heart rate in 24hrs >95 bpm
Symptoms are documented to be due to tachycardia
Hypotension is occasionally observed
Syncope (fainting) is occasionally reported

Treatment
IST has been treated both pharmacologically and invasively, with varying degrees of success. IST, in and of itself, is not indicative of higher rates of mortality, and non-treatment is an option chosen by many if they have minimal symptoms.Some types of medication tried by cardiologists and other physicians include: beta blockers, selective sinus node If channel inhibitors (such as ivabradine), calcium channel blockers, and antiarrhythmic agents. Some SSRI drugs are also occasionally tried, as are treatments more commonly used to treat postural orthostatic tachycardia syndrome, such as fludrocortisone.Invasive treatments include forms of catheter ablation such as sinus node modification (selective ablation of the sinus node), complete sinus node ablation (with associated implantation of a permanent artificial pacemaker), and AV node ablation in very resistant cases (creation of iatrogenic complete heart block, necessitating implantation of a permanent artificial pacemaker). However, invasive treatments can also make the symptoms worse. See also
Supraventricular tachycardia
Sinus tachycardia
Postural orthostatic tachycardia syndrome
Dysautonomia

References
Further reading
Yusuf, Shamil; Camm, A. John (2005). "Deciphering the sinus tachycardias". Clinical Cardiology. 28 (6): 267–76. doi:10.1002/clc.4960280603. PMC 6654702. PMID 16028460. Still, A; Raatikainen, P;  et al. (2005).