Most Common causes:
-Idiopathic tinnitus (cause not found),
-Sensorineural hearing loss (the most common identified cause)

Tinnitus originating from within the auditory system
Vascular
Stroke
Vascular malformations
Small vessel disease
Hypercoagulable states
Hypercholesterolemia
Diabetic vasculopathy
Hypertension
Pseudotumor cerebri
Sickle cell anemia
Other anemia
Vascular: arterial bruit, venous hum, arteriovenous malformation, vascular tumors, carotid atherosclerosis, dissection, or tortuosity
Infections (peripheral & central)
Viral, bacterial, fungal
Rubella
Neurosyphilis
Lyme disease
Meningitis
Measles
Cytomegalovirus
Chronic otitis media
Neoplasm
Tumor
Vestibular schwannoma (Acoustic neuroma)
Glomus tumor
Drugs
Drugs that injure the cochlear: Salicylates; Antibiotics; Loop diuretics; chemotherapy
Substance use.
Idiopathic/Iatrogenic
Idiopathic tinnitus
Congenital: Congenital hearing loss
Autoimmune:
Autoimmune inner ear disease (not always associated with systemic autoimmune disease)
Systemic lupus erythematosus
Sarcoidosis
Rheumatoid arthritis
Trauma:
Cochlear trauma, barotrauma
Cerumen removal
Endocrine / Metabolic disorders
Thyroid disease
Diabetes mellitus
Hyperparathyroidism
hyperlipidemia,
Chronic renal failure
Deficiencies:
Vitamin B12 deficiency
Musculoskeletal
Paget’s disease
Osteogenesis imperfecta
Otosclerosis
Fibrous dysplasia
TMJ dysfunction (see below)
Head or neck injury
Environmental: Noise-induced hearing loss
Neuropsychiatric:
Multiple sclerosis
Meniere syndrome (caused by injury to the cochlear)
Vestibular migraine
Idiopathic intracranial hypertension
Spontaneous intracranial hypotension
Type I Chiari malformation
Sensorineural hearing loss
Idiopathic
Presbycusis
Late-onset congenital hearing loss
Noise-induced hearing loss
Ototoxicity
Neurologic: palatal myoclonus, idiopathic stapedial or tensor tympani muscle spasm

Tinnitus originating from outside the auditory system
Vascular disorders (Pulsatile tinnitus)
Arterial bruits from arteries narrowed by atherosclerosis)
Congenital AVMs may be associated with hearing loss or tinnitus. This type of tinnitus generally occurs in only one ear.
Head and neck paragangliomas are highly vascular tumors that can cause a loud pulsing tinnitus.
Head and neck tumors compressing blood vessels causing tinnitus and other symptoms.
Hypertension (systemic or intracranial) produces venous hums. 
Turbulent blood flow from narrowings or kinks in the carotid artery or jugular vein can cause tinnitus.
Neurologic disorders: pulsatile tinnitus caused by a muscle spasm one or both of the muscles within the middle ear (the tensor tympani and the stapedius muscle). They are supplied by CN V and VII respectively.
Eustachian tube dysfunction (A patulous eustachian tube)
Temporomandibular joint (TMJ) dysfunction
Whiplash injuries and other cervical-spinal disorders

 

Thinking through the differential diagnosis
Note: Tinnitus is a symptom, not a disease. It’s a symptom of an underlying problem.

See, classification of tinnitus. (Tinnitus originating from within the auditory system vs. Tinnitus originating outside the auditory system). Tinnitus originating from within the auditory system is the most common cause of tinnitus.

To come up with a differential diagnosis, the anatomic divisions we will use are 1) The auditory system, and 2) Outside of the auditory system (The rest of the head and body). The auditory system is further divided into the peripheral auditory system and central auditory system. Applying the mechanisms of disease (VINDICATED MEN) to these anatomic divisions at once gives following differential diagnosis.

Further Reading

Am Fam Physician. 2014 Jan 15;89(2):106-113. http://www.aafp.org/afp/2014/0115/p106.html

https://www.mayoclinic.org/diseases-conditions/tinnitus/symptoms-causes/syc-20350156

uptodate.com/contents/etiology-and-diagnosis-of-tinnitus

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